Add buildings above floors and a building structure page

Projects with several buildings could not be structured: floors and rooms hung directly off the project. Floors can now belong to a building (migration 0009: table buildings, nullable floors.building_id). Rooms and distribution boards keep their floor; their building follows from it. Existing floors stay without a building. Buildings use the location-structure command store with project-building.insert/.update/.delete, inverses and persistent Undo/Redo; deleting a building with assigned floors is rejected. Floor commands carry buildingId since command version 2. Version 1 floor commands from existing history predate buildings and are upgraded with buildingId = null before they are executed and persisted, so forward and inverse keep one version. Project snapshot schema 7 stores buildings and floor.buildingId; versions 1-6 are upgraded without buildings. Restore and portable transfer include buildings, and the transfer remaps building ids. Buildings, floors and rooms move from the project overview to their own page /projects/:id/struktur with three columns (Gebäude -> Etagen -> Räume) that filter each other, and a sidebar entry. Floor selects in the board and room dialogs are grouped by building. Unchanged dialogs close without a command. Details in docs/building-structure.md; see issue #6. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
2026-09-26 12:52:34 +02:00 · 2026-09-26 12:52:34 +02:00 · cd89c4a499
commit cd89c4a499
parent 7692699ee2
42 changed files with 5883 additions and 373 deletions
--- a/AGENTS.md
+++ b/AGENTS.md
@ -250,11 +250,14 @@ not change the project revision or undo/redo stacks. Restoring a server-stored
 snapshot verifies its checksum and the current-state hash, replaces supported
 project data atomically and records a new `restore` revision with a complete
 inverse command. Restore can therefore be undone and redone after a restart.
-Current snapshot schema version 6 additionally stores the distribution-board
-info fields `boardNumber`, `location`, `fedFrom` and `remark` and the project
-cover fields `siteAddress`, `workStage`, `projectLead` and `projectEditor`;
-all older supported versions are upgraded with `null` for these fields.
-Stored named and automatic snapshots of every supported version (1 to 6)
+Current snapshot schema version 7 additionally stores the project buildings
+and `floors[].buildingId` (see `docs/building-structure.md`); versions 1 to 6
+are upgraded with no buildings and `buildingId = null`. Version 6 stores the
+distribution-board info fields `boardNumber`, `location`, `fedFrom` and
+`remark` and the project cover fields `siteAddress`, `workStage`,
+`projectLead` and `projectEditor`; all older supported versions are upgraded
+with `null` for these fields.
+Stored named and automatic snapshots of every supported version (1 to 7)
 remain restorable; they are upgraded while parsing. Version 5 stores
 the manual quantity share of every CircuitDeviceRow. Version 4 snapshots are upgraded with
 `manualQuantity = quantity`; they already contain the complete confirmed
@ -270,7 +273,7 @@ project graph.
 Project-wide Revit CSV configuration uses
 `external-csv-configuration.update`. Its complete expected/target snapshot is
 persisted through the shared project-command transaction, so create, update,
-delete, Undo and Redo are revision-safe. Snapshot schema 5/6, restore and portable
+delete, Undo and Redo are revision-safe. Snapshot schema 5-7, restore and portable
 project duplication preserve the configuration; duplication remaps its internal
 id and project link.
 The project page exposes the Revit CSV modal. It edits the complete
--- a/docs/README.md
+++ b/docs/README.md
@ -14,6 +14,8 @@ ausdrücklich getrennt und dürfen nicht als bereits implementiert verstanden we
  Drag-and-drop, Sortierung, Filterung und Undo/Redo
 - [Seitenansicht und PDF-Export](print-view.md) – Deckblatt,
  Verteilerübersicht, Verteilerpläne, Büro-Logo
+- [Gebäudestruktur](building-structure.md) – Gebäude → Etagen → Räume,
+  Befehle, Snapshot-Schema 7, eigene Seite
 - [Editor-API](circuit-list-editor-api.md) – unterstützte Circuit-First-Endpunkte
 - [Bekannte Einschränkungen](circuit-list-editor-known-limitations.md) –
  bewusst noch nicht implementierte oder begrenzte Funktionen
--- a/docs/building-structure.md
+++ b/docs/building-structure.md
@ -0,0 +1,79 @@
+# Gebäudestruktur (Gebäude → Etagen → Räume)
+
+Projekte mit mehreren Gebäuden werden über eine eigene Ebene **Gebäude**
+gegliedert. Etagen können einem Gebäude zugeordnet sein, Räume und Verteilungen
+hängen wie bisher an der Etage. Das Gebäude eines Raums oder einer Verteilung
+ergibt sich also aus der Etage; eine zweite, konkurrierende Zuordnung gibt es
+nicht. Abstimmung mit Julian: Issue #6.
+
+## Datenmodell
+
+| Tabelle | Feld | Bedeutung |
+|---|---|---|
+| `buildings` | `id`, `project_id` (FK, cascade), `name`, `sort_order` | Gebäude eines Projekts |
+| `floors` | `building_id` (nullable FK → `buildings`, `ON DELETE set null`) | optionales übergeordnetes Gebäude |
+
+Migration `0009_buildings`. Bestehende Etagen behalten `building_id = null` und
+erscheinen in der Oberfläche als „Ohne Gebäude“. Es gibt keine Zwangsmigration
+und kein automatisch angelegtes Standardgebäude.
+
+## Befehle, Undo/Redo
+
+Alle Änderungen laufen über die bestehende Orts-Struktur
+(`ProjectLocationStructureProjectCommandRepository`) mit Revision, Inverse und
+persistentem Undo/Redo:
+
+- `project-building.insert` / `project-building.delete` mit vollständigem
+  Gebäude-Snapshot (Schema-Version 1)
+- `project-building.update`: ändert nur den Namen
+- Löschen eines Gebäudes wird abgelehnt, solange Etagen zugeordnet sind
+  („Das Gebäude kann nicht gelöscht werden, solange Etagen zugeordnet sind.“)
+- Etagenbefehle `project-floor.insert/.update/.delete` haben seit
+  Schema-Version **2** das Feld `buildingId`. `project-floor.update` darf Name
+  und/oder Gebäude ändern, nicht aber `sortOrder`. Ein referenziertes Gebäude
+  muss zum Projekt gehören.
+- Version-1-Etagenbefehle aus bestehender Historie bleiben ausführbar. Sie
+  stammen aus der Zeit vor den Gebäuden und werden vor der Ausführung mit
+  `buildingId = null` auf Version 2 gehoben; auch die daraus neu gespeicherten
+  Vorwärts- und Inversbefehle sind Version 2. Das ist bei linearer Historie
+  exakt: Wird ein Gebäude später zugeordnet, liegt dieser neuere Befehl im
+  Undo-Stapel vor dem alten Befehl und wird zuerst zurückgenommen.
+- Raumbefehle bleiben unverändert (Version 1).
+
+## Snapshots und Transfer
+
+Projekt-Snapshot-Schema **7** enthält `buildings` und `floors[].buildingId`.
+Alle älteren unterstützten Versionen (1–6) werden beim Einlesen mit leerer
+Gebäudeliste und `buildingId = null` hochgestuft; gespeicherte Sicherungspunkte
+bleiben wiederherstellbar. Die Relationen werden geprüft (Gebäude gehört zum
+Projekt, `buildingId` verweist auf ein Gebäude des Snapshots). Restore löscht
+und schreibt Gebäude zusammen mit den Etagen; der Projekt-Transfer vergibt neue
+Gebäude-IDs und stellt die Etagenverweise um.
+
+## API
+
+- `GET /api/projects/:projectId/buildings` – Gebäude sortiert nach
+  `sortOrder`, Name
+- `POST /api/projects/:projectId/buildings` – `{ "name": "Haus A", "expectedRevision": 3 }`
+- `PUT /api/projects/:projectId/buildings/:buildingId` – `{ "name": "Haus B", "expectedRevision": 4 }`
+- `DELETE /api/projects/:projectId/buildings/:buildingId` – `{ "expectedRevision": 5 }`
+- `POST/PUT /api/projects/:projectId/floors[/:floorId]` akzeptieren zusätzlich
+  optional `buildingId` (`string | null`). Fehlt das Feld, bleibt beim Bearbeiten
+  das bisherige Gebäude erhalten; ältere Clients funktionieren unverändert.
+
+## Oberfläche
+
+Gebäude, Etagen und Räume werden auf der eigenen Seite
+`/projects/:projectId/struktur` („Gebäudestruktur“) gepflegt, erreichbar über
+die Seitenleiste und die Kennzahlen der Projektübersicht. Drei Spalten
+Gebäude → Etagen → Räume filtern sich gegenseitig: Klick auf ein Gebäude zeigt
+nur dessen Etagen, Klick auf eine Etage nur deren Räume, „Alle“ hebt den Filter
+auf. Hinzufügen, Bearbeiten und Löschen laufen über Icon-Buttons wie in der
+Stromkreisliste.
+
+## Offen
+
+- Revit: Das Gebäude kann später aus einem Revit-Parameter kommen und über die
+  bestehende CSV-Zuordnung gemappt werden (Frage an Julian in Issue #6).
+- Sortierung von Gebäuden per Drag-and-drop ist noch nicht umgesetzt; neue
+  Gebäude werden hinten angehängt.
--- a/docs/circuit-list-editor-api.md
+++ b/docs/circuit-list-editor-api.md
@ -257,15 +257,27 @@ returns HTTP `409` with `PROJECT_HISTORY_OPERATION_UNAVAILABLE`.
  `distribution-board.delete-subtree`. Persistent Undo/Redo restores or
  removes the same complete subtree, including protection devices.

-### Project Floors and Rooms
+### Project Buildings, Floors and Rooms

+- `GET /projects/:projectId/buildings`
+- `POST /projects/:projectId/buildings`
+  - body: `{ "name": "Haus A", "expectedRevision": 12 }`
+  - executes `project-building.insert` with a stable building id
+  - response: `{ "building": { ... }, "revision": { ... }, "history": { ... } }`
+- `PUT /projects/:projectId/buildings/:buildingId`
+  - body: `{ "name": "Haus B", "expectedRevision": 13 }` (name only)
+- `DELETE /projects/:projectId/buildings/:buildingId`
+  - body: `{ "expectedRevision": 14 }`
+  - rejects buildings with assigned floors
 - `POST /projects/:projectId/floors`
-  - body: `{ "name": "EG", "expectedRevision": 13 }`
-  - executes `project-floor.insert` with a stable floor id
+  - body: `{ "name": "EG", "buildingId": "building_1", "expectedRevision": 13 }`
+    (`buildingId` optional, `null` = no building)
+  - executes `project-floor.insert` (command version 2) with a stable floor id
  - response: `{ "floor": { ... }, "revision": { ... }, "history": { ... } }`
 - `PUT /projects/:projectId/floors/:floorId`
-  - body: `{ "name": "1. OG", "expectedRevision": 14 }`
-  - executes an exact persistent floor update
+  - body: `{ "name": "1. OG", "buildingId": null, "expectedRevision": 14 }`
+  - executes an exact persistent floor update of name and/or building; a
+    missing `buildingId` keeps the current building
 - `DELETE /projects/:projectId/floors/:floorId`
  - body: `{ "expectedRevision": 15 }`
  - rejects floors with assigned rooms or distribution boards
--- a/docs/current-architecture.md
+++ b/docs/current-architecture.md
@ -246,9 +246,11 @@ Werte, Revision und Historienstapel gemeinsam. `PUT /api/projects/:projectId`
 verlangt deshalb `expectedRevision`, liefert Projekt plus aktualisierten
 Historienstand und besitzt keinen separaten direkten Settings-Schreibweg mehr.
 Kommando- und Snapshot-Payloads müssen dem aktuellen Baseline-Schema entsprechen.
-Die Projektseite zeigt Verteilungen, Etagen und Räume als kompakte
-Bestandsübersichten; ihre versionierten Erstellwege öffnen beschriftete Modals
-statt dauerhafter Eingabezeilen. Projektgeräte werden als durchsuchbare,
+Die Projektseite zeigt Verteilungen als kompakte Bestandsübersicht; ihre
+versionierten Erstellwege öffnen beschriftete Modals statt dauerhafter
+Eingabezeilen. Gebäude, Etagen und Räume werden auf der eigenen Seite
+„Gebäudestruktur“ (`/projects/:projectId/struktur`) gepflegt; Etagen können
+einem Gebäude zugeordnet sein (`docs/building-structure.md`). Projektgeräte werden als durchsuchbare,
 fünfspaltige Übersicht dargestellt. Manuelle Anlage, Übernahme aus der globalen
 Bibliothek und vollständige Bearbeitung erfolgen in einem gemeinsamen Modal.
 Die anschließende Vorschau verknüpfter Stromkreiszeilen bleibt davon getrennt,
@ -422,6 +424,8 @@ Derselbe Befehl versioniert die Felder der Verteiler-Infokarte
 Infokarte liegt im Bearbeiten-Dialog der Verteilung; die Werte sind lokale
 Planungswerte und sollen später über den Revit-Abgleich nur nach Vorschau und
 ausdrücklicher Bestätigung vorbelegt werden. `fedFrom` ist vorerst Freitext.
+Snapshot-Schema 7 enthält zusätzlich die Gebäude und die Gebäudezuordnung
+der Etagen; ältere Versionen werden ohne Gebäude hochgestuft.
 Snapshot-Schema 6 enthält diese Felder und die Deckblattangaben des
 Projekts; ältere Versionen werden mit `null`
 hochgestuft. Snapshot-Schema 5 enthält zusätzlich den manuellen Mengenanteil jeder
--- a/docs/project-history-and-external-model-architecture.md
+++ b/docs/project-history-and-external-model-architecture.md
@ -275,6 +275,10 @@ Completed foundation:
 - `project-floor.insert` and `project-room.insert` persist complete location
  records with stable ids; their inverses reject changed or referenced records
  so Undo never clears room assignments or device-row links implicitly
+- `project-building.insert/.update/.delete` persist complete building records;
+  deleting a building with assigned floors is rejected. Floor commands carry
+  `buildingId` since command version 2; version 1 floor commands from older
+  history are upgraded with `buildingId = null` before execution
 - the floor and room POST routes require the current expected revision and the
  project page tracks the returned history state

--- a/scripts/db-verify-circuit-schema.js
+++ b/scripts/db-verify-circuit-schema.js
@ -5,6 +5,7 @@ const dbPath = path.resolve("data", "leistungsbilanz.db");
 const db = new Database(dbPath, { readonly: true });

 const requiredTables = [
+  "buildings",
  "circuit_sections",
  "circuits",
  "circuit_device_rows",
@ -100,6 +101,13 @@ const projectColumns = new Set(
 const missingProjectColumns = requiredProjectColumns.filter(
  (name) => !projectColumns.has(name)
 );
+const requiredFloorColumns = ["building_id"];
+const floorColumns = new Set(
+  db.prepare("PRAGMA table_info(floors)").all().map((column) => column.name)
+);
+const missingFloorColumns = requiredFloorColumns.filter(
+  (name) => !floorColumns.has(name)
+);
 const requiredDistributionBoardColumns = [
  "floor_id",
  "supply_type",
@ -155,6 +163,7 @@ console.log(
  requiredCircuitSectionColumns.join(", ")
 );
 console.log("Required project columns:", requiredProjectColumns.join(", "));
+console.log("Required floor columns:", requiredFloorColumns.join(", "));
 console.log(
  "Required distribution-board columns:",
  requiredDistributionBoardColumns.join(", ")
@ -171,6 +180,11 @@ if (missing.length > 0) {
  process.exit(1);
 }

+if (missingFloorColumns.length > 0) {
+  console.error("Missing floor columns:", missingFloorColumns.join(", "));
+  process.exit(1);
+}
+
 if (missingProjectDeviceColumns.length > 0) {
  console.error("Missing project-device columns:", missingProjectDeviceColumns.join(", "));
  process.exit(1);
--- a/src/app/globals.css
+++ b/src/app/globals.css
@ -1461,3 +1461,144 @@ a.kpi:hover {
 .sidebar-collapsed .sidebar-label {
  display: none;
 }
+
+/* ── Gebäudestruktur (Gebäude → Etagen → Räume) ─────────────────────── */
+
+.structure-columns {
+  display: grid;
+  grid-template-columns: minmax(0, 1fr);
+  gap: var(--space-md);
+  align-items: start;
+}
+
+@media (min-width: 992px) {
+  .structure-columns {
+    grid-template-columns: repeat(3, minmax(0, 1fr));
+  }
+}
+
+.structure-column-title {
+  font-size: 1rem;
+  font-weight: 700;
+  margin: 0;
+  display: flex;
+  align-items: center;
+}
+
+.structure-icon-button {
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  padding: 0.25rem 0.4rem;
+  line-height: 1;
+}
+
+.structure-back-link {
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  padding: 0.35rem 0.45rem;
+  margin-top: 0.1rem;
+}
+
+.structure-list {
+  max-height: 70vh;
+  overflow-y: auto;
+}
+
+.structure-entry {
+  display: flex;
+  align-items: center;
+  gap: var(--space-sm);
+  padding: 0;
+}
+
+.structure-entry-main {
+  flex: 1 1 auto;
+  min-width: 0;
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: var(--space-sm);
+  padding: 0.55rem 0.75rem;
+  border: 0;
+  background: transparent;
+  color: inherit;
+  text-align: left;
+  font: inherit;
+}
+
+button.structure-entry-main {
+  cursor: pointer;
+}
+
+button.structure-entry-main:hover {
+  background: var(--surface-hover);
+}
+
+button.structure-entry-main:focus-visible {
+  outline: 2px solid var(--accent-blue);
+  outline-offset: -2px;
+}
+
+.structure-entry.is-selected {
+  background: var(--surface-selected);
+  box-shadow: inset 3px 0 0 var(--color-signal);
+}
+
+.structure-entry.is-selected .structure-entry-label {
+  font-weight: 600;
+}
+
+.structure-entry.is-selected button.structure-entry-main:hover {
+  background: transparent;
+}
+
+.structure-entry-text {
+  min-width: 0;
+  display: flex;
+  flex-direction: column;
+}
+
+.structure-entry-label {
+  overflow-wrap: anywhere;
+}
+
+.structure-entry-hint {
+  font-size: 0.78rem;
+  color: var(--color-ink-soft);
+}
+
+.structure-entry-meta {
+  flex: 0 0 auto;
+  font-size: 0.78rem;
+  color: var(--color-ink-soft);
+  white-space: nowrap;
+}
+
+.structure-entry-actions {
+  flex: 0 0 auto;
+  display: inline-flex;
+  gap: 4px;
+  padding-right: 0.75rem;
+}
+
+.structure-room-number {
+  font-weight: 600;
+  font-variant-numeric: tabular-nums;
+}
+
+.structure-group-label {
+  font-size: 0.66rem;
+  letter-spacing: 0.08em;
+  text-transform: uppercase;
+  font-weight: 600;
+  color: var(--color-accent);
+  background: var(--panel-bg-subtle);
+  padding: 0.35rem 0.75rem;
+}
+
+.structure-empty {
+  color: var(--color-ink-soft);
+  font-size: 0.9rem;
+}
--- a/src/app/projects/[projectId]/page.tsx
+++ b/src/app/projects/[projectId]/page.tsx
@ -8,17 +8,15 @@ import {
  copyDistributionBoard,
  copyProjectDeviceToGlobal,
  createDistributionBoard,
-  createFloor,
  createProjectDevice,
  createRoom,
-  deleteFloor,
  deleteProjectDevice,
-  deleteRoom,
  deleteDistributionBoard,
  disconnectProjectDeviceRows,
  exportProjectTransfer,
  getProject,
  getProjectDeviceSyncPreview,
+  listBuildings,
  listCircuitLists,
  listDistributionBoards,
  listFloors,
@ -28,12 +26,11 @@ import {
  importProjectTransfer,
  synchronizeProjectDeviceRows,
  updateDistributionBoard,
-  updateFloor,
  updateProjectDevice,
  updateProjectSettings,
-  updateRoom,
 } from "../../../frontend/utils/api";
 import type {
+  BuildingDto,
  CircuitListDto,
  CreateRoomInput,
  CreateProjectDeviceInput,
@ -67,7 +64,7 @@ import {
 import { FormModal } from "../../../frontend/components/form-modal";
 import { ProjectDeviceModal } from "../../../frontend/components/project-device-modal";
 import { RevitCsvModal } from "../../../frontend/components/revit-csv-modal";
-import { RoomModal } from "../../../frontend/components/room-modal";
+import { FloorSelectOptions } from "../../../frontend/components/floor-select-options";

 const projectDeviceSyncFieldLabels: Record<ProjectDeviceSyncField, string> = {
  name: "Technischer Name",
@ -89,6 +86,7 @@ export default function ProjectDetailPage() {
  const [project, setProject] = useState<ProjectDto | null>(null);
  const [boards, setBoards] = useState<DistributionBoardDto[]>([]);
  const [circuitLists, setCircuitLists] = useState<CircuitListDto[]>([]);
+  const [buildings, setBuildings] = useState<BuildingDto[]>([]);
  const [floors, setFloors] = useState<FloorDto[]>([]);
  const [rooms, setRooms] = useState<RoomDto[]>([]);
  const [projectDevices, setProjectDevices] = useState<ProjectDeviceDto[]>([]);
@ -116,14 +114,9 @@ export default function ProjectDetailPage() {
        remark: null,
      })
    );
-  const [floorName, setFloorName] = useState("");
-  const [editingFloor, setEditingFloor] = useState<FloorDto | null>(null);
-  const [editingRoom, setEditingRoom] = useState<RoomDto | null>(null);
  const [isProjectSettingsOpen, setIsProjectSettingsOpen] = useState(false);
  const [isRevitCsvOpen, setIsRevitCsvOpen] = useState(false);
-  const [structureModal, setStructureModal] = useState<
-    "board" | "floor" | "room" | null
-  >(null);
+  const [structureModal, setStructureModal] = useState<"board" | null>(null);
  const [isProjectDeviceModalOpen, setIsProjectDeviceModalOpen] =
    useState(false);
  const [editingProjectDevice, setEditingProjectDevice] =
@ -147,6 +140,7 @@ export default function ProjectDetailPage() {
      getProject(projectId),
      listDistributionBoards(projectId),
      listCircuitLists(projectId),
+      listBuildings(projectId),
      listFloors(projectId),
      listRooms(projectId),
      listProjectDevices(projectId),
@ -156,6 +150,7 @@ export default function ProjectDetailPage() {
        currentProject,
        distributionBoards,
        loadedCircuitLists,
+        loadedBuildings,
        loadedFloors,
        loadedRooms,
        loadedProjectDevices,
@ -164,6 +159,7 @@ export default function ProjectDetailPage() {
        setProject(currentProject);
        setBoards(distributionBoards);
        setCircuitLists(loadedCircuitLists);
+        setBuildings(loadedBuildings);
        setFloors(loadedFloors);
        setRooms(loadedRooms);
        setProjectDevices(loadedProjectDevices);
@ -176,6 +172,7 @@ export default function ProjectDetailPage() {
  }, [projectId]);

  const boardCount = useMemo(() => boards.length, [boards.length]);
+  const buildingCount = useMemo(() => buildings.length, [buildings.length]);
  const floorCount = useMemo(() => floors.length, [floors.length]);
  const roomCount = useMemo(() => rooms.length, [rooms.length]);
  const projectDeviceCount = useMemo(() => projectDevices.length, [projectDevices.length]);
@ -260,74 +257,6 @@ export default function ProjectDetailPage() {
    }
  }

-  async function handleSaveFloor(event: FormEvent<HTMLFormElement>) {
-    event.preventDefault();
-    if (!projectId || !project || !floorName.trim()) {
-      return;
-    }
-    setIsSaving(true);
-    setError(null);
-    try {
-      const result = editingFloor
-        ? await updateFloor(
-            projectId,
-            editingFloor.id,
-            { name: floorName.trim() },
-            project.currentRevision
-          )
-        : await createFloor(
-            projectId,
-            { name: floorName.trim() },
-            project.currentRevision
-          );
-      setFloors((current) =>
-        editingFloor
-          ? current.map((floor) =>
-              floor.id === result.floor.id ? result.floor : floor
-            )
-          : [...current, result.floor]
-      );
-      applyProjectRevision(result.history.currentRevision);
-      setFloorName("");
-      setEditingFloor(null);
-      setStructureModal(null);
-    } catch (err) {
-      setError(err instanceof Error ? err.message : "Etage konnte nicht gespeichert werden.");
-    } finally {
-      setIsSaving(false);
-    }
-  }
-
-  async function handleSaveRoom(input: CreateRoomInput) {
-    if (!projectId || !project) return;
-    setIsSaving(true);
-    setError(null);
-    try {
-      const result = editingRoom
-        ? await updateRoom(
-            projectId,
-            editingRoom.id,
-            input,
-            project.currentRevision
-          )
-        : await createRoom(projectId, input, project.currentRevision);
-      setRooms((current) =>
-        editingRoom
-          ? current.map((room) =>
-              room.id === result.room.id ? result.room : room
-            )
-          : [...current, result.room]
-      );
-      applyProjectRevision(result.history.currentRevision);
-      setEditingRoom(null);
-      setStructureModal(null);
-    } catch (err) {
-      setError(err instanceof Error ? err.message : "Raum konnte nicht gespeichert werden.");
-    } finally {
-      setIsSaving(false);
-    }
-  }
-
  async function handleSaveProjectSettings(input: ProjectSettingsInput) {
    if (!projectId || !project) {
      return;
@ -515,65 +444,6 @@ export default function ProjectDetailPage() {
    return result;
  }

-  function openCreateFloor() {
-    setEditingFloor(null);
-    setFloorName("");
-    setStructureModal("floor");
-  }
-
-  function openEditFloor(floor: FloorDto) {
-    setEditingFloor(floor);
-    setFloorName(floor.name);
-    setStructureModal("floor");
-  }
-
-  function openCreateRoom() {
-    setEditingRoom(null);
-    setStructureModal("room");
-  }
-
-  function openEditRoom(room: RoomDto) {
-    setEditingRoom(room);
-    setStructureModal("room");
-  }
-
-  async function handleDeleteFloor() {
-    if (!projectId || !project || !editingFloor) return;
-    if (!window.confirm(`Etage „${editingFloor.name}“ wirklich löschen?`)) return;
-    setIsSaving(true);
-    setError(null);
-    try {
-      const result = await deleteFloor(projectId, editingFloor.id, project.currentRevision);
-      setFloors((current) => current.filter((floor) => floor.id !== editingFloor.id));
-      applyProjectRevision(result.history.currentRevision);
-      setEditingFloor(null);
-      setFloorName("");
-      setStructureModal(null);
-    } catch (err) {
-      setError(err instanceof Error ? err.message : "Etage konnte nicht gelöscht werden.");
-    } finally {
-      setIsSaving(false);
-    }
-  }
-
-  async function handleDeleteRoom() {
-    if (!projectId || !project || !editingRoom) return;
-    if (!window.confirm(`Raum „${editingRoom.roomNumber} ${editingRoom.roomName}“ wirklich löschen?`)) return;
-    setIsSaving(true);
-    setError(null);
-    try {
-      const result = await deleteRoom(projectId, editingRoom.id, project.currentRevision);
-      setRooms((current) => current.filter((room) => room.id !== editingRoom.id));
-      applyProjectRevision(result.history.currentRevision);
-      setEditingRoom(null);
-      setStructureModal(null);
-    } catch (err) {
-      setError(err instanceof Error ? err.message : "Raum konnte nicht gelöscht werden.");
-    } finally {
-      setIsSaving(false);
-    }
-  }
-
  async function handleCopyBoard() {
    if (
      !projectId ||
@ -902,14 +772,18 @@ export default function ProjectDetailPage() {
          <div className="kpi-label">Verteilungen</div>
          <div className="kpi-value">{boardCount}</div>
        </a>
-        <a className="kpi" href="#etagen">
+        <Link className="kpi" href={`/projects/${projectId}/struktur`}>
+          <div className="kpi-label">Gebäude</div>
+          <div className="kpi-value">{buildingCount}</div>
+        </Link>
+        <Link className="kpi" href={`/projects/${projectId}/struktur`}>
          <div className="kpi-label">Etagen</div>
          <div className="kpi-value">{floorCount}</div>
-        </a>
-        <a className="kpi" href="#raeume">
+        </Link>
+        <Link className="kpi" href={`/projects/${projectId}/struktur`}>
          <div className="kpi-label">Räume</div>
          <div className="kpi-value">{roomCount}</div>
-        </a>
+        </Link>
        <a className="kpi" href="#projektgeraete">
          <div className="kpi-label">Projektgeräte</div>
          <div className="kpi-value">{projectDeviceCount}</div>
@ -1025,88 +899,6 @@ export default function ProjectDetailPage() {
            </div>
          </div>
        </section>
-
-        <section className="col-12 col-lg-5" id="etagen">
-          <div className="card shadow-sm h-100">
-            <div className="card-header d-flex justify-content-between align-items-center">
-              <div>
-                <span>Etagen</span>
-                <span className="badge text-bg-secondary ms-2">{floorCount}</span>
-              </div>
-              <button
-                className="btn btn-sm btn-outline-primary"
-                onClick={openCreateFloor}
-                type="button"
-              >
-                Hinzufügen
-              </button>
-            </div>
-            <ul className="list-group list-group-flush">
-              {floors.map((floor) => (
-                <li className="list-group-item d-flex justify-content-between align-items-center gap-2" key={floor.id}>
-                  <span>{floor.name}</span>
-                  <button className="btn btn-sm btn-outline-secondary" onClick={() => openEditFloor(floor)} type="button">
-                    Bearbeiten
-                  </button>
-                </li>
-              ))}
-              {!floors.length ? (
-                <li className="list-group-item text-secondary">Noch keine Etagen vorhanden.</li>
-              ) : null}
-            </ul>
-          </div>
-        </section>
-
-        <section className="col-12 col-lg-7" id="raeume">
-          <div className="card shadow-sm">
-            <div className="card-header d-flex justify-content-between align-items-center">
-              <div>
-                <span>Räume</span>
-                <span className="badge text-bg-secondary ms-2">{roomCount}</span>
-              </div>
-              <button
-                className="btn btn-sm btn-outline-primary"
-                onClick={openCreateRoom}
-                type="button"
-              >
-                Hinzufügen
-              </button>
-            </div>
-            <div className="table-responsive">
-              <table className="table table-hover align-middle mb-0">
-                <thead>
-                  <tr>
-                    <th>Raumnummer</th>
-                    <th>Raumname</th>
-                    <th>Etage</th>
-                    <th className="text-end">Aktionen</th>
-                  </tr>
-                </thead>
-                <tbody>
-                  {rooms.map((room) => (
-                    <tr key={room.id}>
-                      <td>{room.roomNumber}</td>
-                      <td>{room.roomName}</td>
-                      <td>{room.floorId ? floorById.get(room.floorId)?.name ?? "-" : "-"}</td>
-                      <td className="text-end">
-                        <button className="btn btn-sm btn-outline-secondary" onClick={() => openEditRoom(room)} type="button">
-                          Bearbeiten
-                        </button>
-                      </td>
-                    </tr>
-                  ))}
-                  {!rooms.length ? (
-                    <tr>
-                      <td colSpan={4} className="text-center text-secondary py-4">
-                        Noch keine Räume vorhanden.
-                      </td>
-                    </tr>
-                  ) : null}
-                </tbody>
-              </table>
-            </div>
-          </div>
-        </section>
      </div>

      <section className="card shadow-sm mt-4" id="projektgeraete">
@ -1366,11 +1158,7 @@ export default function ProjectDetailPage() {
                value={boardFloorId}
              >
                <option value="">Ohne Etage</option>
-                {floors.map((floor) => (
-                  <option key={floor.id} value={floor.id}>
-                    {floor.name}
-                  </option>
-                ))}
+                <FloorSelectOptions buildings={buildings} floors={floors} />
              </select>
            </div>
            <div className="col-12 col-md-6">
@ -1436,11 +1224,7 @@ export default function ProjectDetailPage() {
                value={editingBoardFloorId}
              >
                <option value="">Ohne Etage</option>
-                {floors.map((floor) => (
-                  <option key={floor.id} value={floor.id}>
-                    {floor.name}
-                  </option>
-                ))}
+                <FloorSelectOptions buildings={buildings} floors={floors} />
              </select>
            </div>
            <div className="col-12 col-md-6">
@ -1572,57 +1356,6 @@ export default function ProjectDetailPage() {
        </FormModal>
      ) : null}

-      {structureModal === "floor" ? (
-        <FormModal
-          isSaving={isSaving}
-          onClose={() => {
-            setStructureModal(null);
-            setEditingFloor(null);
-          }}
-          onSubmit={handleSaveFloor}
-          submitDisabled={!floorName.trim()}
-          submitLabel={editingFloor ? "Änderungen speichern" : "Etage hinzufügen"}
-          title={editingFloor ? "Etage bearbeiten" : "Etage hinzufügen"}
-        >
-          <label className="form-label" htmlFor="floor-name">
-            Bezeichnung
-          </label>
-          <input
-            autoFocus
-            className="form-control"
-            id="floor-name"
-            onChange={(event) => setFloorName(event.target.value)}
-            placeholder="z. B. EG"
-            required
-            value={floorName}
-          />
-          {editingFloor ? (
-            <div className="border-top mt-4 pt-3">
-              <p className="text-secondary small mb-2">
-                Etagen mit zugeordneten Räumen oder Verteilungen können nicht gelöscht werden.
-              </p>
-              <button className="btn btn-outline-danger" disabled={isSaving} onClick={() => void handleDeleteFloor()} type="button">
-                Etage löschen
-              </button>
-            </div>
-          ) : null}
-        </FormModal>
-      ) : null}
-
-      {structureModal === "room" ? (
-        <RoomModal
-          floors={floors}
-          initialRoom={editingRoom ?? undefined}
-          isSaving={isSaving}
-          onClose={() => {
-            setStructureModal(null);
-            setEditingRoom(null);
-          }}
-          onDelete={editingRoom ? () => void handleDeleteRoom() : undefined}
-          onSave={handleSaveRoom}
-        />
-      ) : null}
-
      {isProjectDeviceModalOpen ? (
        <ProjectDeviceModal
          globalDevices={globalDevices}
--- a/src/app/projects/[projectId]/struktur/page.tsx
+++ b/src/app/projects/[projectId]/struktur/page.tsx
@ -0,0 +1,792 @@
+"use client";
+
+import Link from "next/link";
+import { useParams } from "next/navigation";
+import {
+  type FormEvent,
+  type ReactNode,
+  useCallback,
+  useEffect,
+  useMemo,
+  useState,
+} from "react";
+import { ArrowLeft, Pencil, Plus, Trash2 } from "lucide-react";
+import {
+  createBuilding,
+  createFloor,
+  createRoom,
+  deleteBuilding,
+  deleteFloor,
+  deleteRoom,
+  getProject,
+  listBuildings,
+  listFloors,
+  listRooms,
+  updateBuilding,
+  updateFloor,
+  updateRoom,
+} from "../../../../frontend/utils/api";
+import type {
+  BuildingDto,
+  CreateRoomInput,
+  FloorDto,
+  ProjectCommandResultDto,
+  ProjectDto,
+  RoomDto,
+} from "../../../../frontend/types";
+import {
+  ALL_SELECTION,
+  UNASSIGNED_BUILDING_LABEL,
+  UNASSIGNED_FLOOR_LABEL,
+  UNASSIGNED_SELECTION,
+  countFloorsByBuilding,
+  countRoomsByFloor,
+  defaultBuildingIdForNewFloor,
+  defaultFloorIdForNewRoom,
+  describeRoomLocation,
+  filterFloorsByBuilding,
+  filterRooms,
+  groupFloorsByBuilding,
+  hasFloorsWithoutBuilding,
+  normalizeBuildingSelection,
+  normalizeFloorSelection,
+  showsRoomsWithoutFloorEntry,
+  type StructureSelection,
+} from "../../../../frontend/utils/building-structure";
+import { FormModal } from "../../../../frontend/components/form-modal";
+import { RoomModal } from "../../../../frontend/components/room-modal";
+
+const BUILDING_HAS_FLOORS_MESSAGE =
+  "Das Gebäude kann nicht gelöscht werden, solange Etagen zugeordnet sind.";
+
+type StructureDialog =
+  | { kind: "building"; building: BuildingDto | null }
+  | { kind: "floor"; floor: FloorDto | null }
+  | { kind: "room"; room: RoomDto | null; defaultFloorId: string };
+
+function formatCount(count: number, singular: string, plural: string) {
+  return `${count} ${count === 1 ? singular : plural}`;
+}
+
+function compareRooms(left: RoomDto, right: RoomDto) {
+  return (
+    left.roomNumber.localeCompare(right.roomNumber, "de", { numeric: true }) ||
+    left.roomName.localeCompare(right.roomName, "de")
+  );
+}
+
+interface StructureColumnProps {
+  addLabel: string;
+  children: ReactNode;
+  count: number;
+  headingId: string;
+  isAddDisabled: boolean;
+  onAdd: () => void;
+  title: string;
+}
+
+function StructureColumn({
+  addLabel,
+  children,
+  count,
+  headingId,
+  isAddDisabled,
+  onAdd,
+  title,
+}: StructureColumnProps) {
+  return (
+    <section aria-labelledby={headingId} className="card shadow-sm structure-column">
+      <div className="card-header d-flex justify-content-between align-items-center gap-2">
+        <h2 className="structure-column-title" id={headingId}>
+          {title}
+          <span className="badge text-bg-secondary ms-2">{count}</span>
+        </h2>
+        <button
+          aria-label={addLabel}
+          className="btn btn-sm btn-outline-primary structure-icon-button"
+          disabled={isAddDisabled}
+          onClick={onAdd}
+          title={addLabel}
+          type="button"
+        >
+          <Plus aria-hidden="true" size={16} />
+        </button>
+      </div>
+      <ul className="list-group list-group-flush structure-list">{children}</ul>
+    </section>
+  );
+}
+
+interface StructureEntryProps {
+  actions?: ReactNode;
+  hint?: string;
+  isSelected?: boolean;
+  label: ReactNode;
+  meta?: string;
+  onSelect?: () => void;
+}
+
+function StructureEntry({
+  actions,
+  hint,
+  isSelected = false,
+  label,
+  meta,
+  onSelect,
+}: StructureEntryProps) {
+  const content = (
+    <>
+      <span className="structure-entry-text">
+        <span className="structure-entry-label">{label}</span>
+        {hint ? <span className="structure-entry-hint">{hint}</span> : null}
+      </span>
+      {meta ? <span className="structure-entry-meta">{meta}</span> : null}
+    </>
+  );
+  return (
+    <li
+      className={
+        isSelected
+          ? "list-group-item structure-entry is-selected"
+          : "list-group-item structure-entry"
+      }
+    >
+      {onSelect ? (
+        <button
+          aria-pressed={isSelected}
+          className="structure-entry-main"
+          onClick={onSelect}
+          type="button"
+        >
+          {content}
+        </button>
+      ) : (
+        <div className="structure-entry-main">{content}</div>
+      )}
+      {actions ? <div className="structure-entry-actions">{actions}</div> : null}
+    </li>
+  );
+}
+
+interface EntryActionsProps {
+  deleteLabel: string;
+  editLabel: string;
+  isDisabled: boolean;
+  onDelete: () => void;
+  onEdit: () => void;
+}
+
+function EntryActions({
+  deleteLabel,
+  editLabel,
+  isDisabled,
+  onDelete,
+  onEdit,
+}: EntryActionsProps) {
+  return (
+    <>
+      <button
+        aria-label={editLabel}
+        className="btn btn-sm btn-outline-secondary structure-icon-button"
+        disabled={isDisabled}
+        onClick={onEdit}
+        title={editLabel}
+        type="button"
+      >
+        <Pencil aria-hidden="true" size={14} />
+      </button>
+      <button
+        aria-label={deleteLabel}
+        className="btn btn-sm btn-outline-danger structure-icon-button"
+        disabled={isDisabled}
+        onClick={onDelete}
+        title={deleteLabel}
+        type="button"
+      >
+        <Trash2 aria-hidden="true" size={14} />
+      </button>
+    </>
+  );
+}
+
+export default function BuildingStructurePage() {
+  const params = useParams<{ projectId: string }>();
+  const projectId = params.projectId;
+  const [project, setProject] = useState<ProjectDto | null>(null);
+  const [buildings, setBuildings] = useState<BuildingDto[]>([]);
+  const [floors, setFloors] = useState<FloorDto[]>([]);
+  const [rooms, setRooms] = useState<RoomDto[]>([]);
+  const [buildingSelection, setBuildingSelection] =
+    useState<StructureSelection>(ALL_SELECTION);
+  const [floorSelection, setFloorSelection] =
+    useState<StructureSelection>(ALL_SELECTION);
+  const [dialog, setDialog] = useState<StructureDialog | null>(null);
+  const [dialogName, setDialogName] = useState("");
+  const [dialogBuildingId, setDialogBuildingId] = useState("");
+  const [error, setError] = useState<string | null>(null);
+  const [isLoading, setIsLoading] = useState(true);
+  const [isSaving, setIsSaving] = useState(false);
+
+  const reloadStructure = useCallback(async () => {
+    const [currentProject, loadedBuildings, loadedFloors, loadedRooms] =
+      await Promise.all([
+        getProject(projectId),
+        listBuildings(projectId),
+        listFloors(projectId),
+        listRooms(projectId),
+      ]);
+    setProject(currentProject);
+    setBuildings(loadedBuildings);
+    setFloors(loadedFloors);
+    setRooms(loadedRooms);
+  }, [projectId]);
+
+  useEffect(() => {
+    if (!projectId) {
+      return;
+    }
+    setIsLoading(true);
+    reloadStructure()
+      .then(() => setError(null))
+      .catch((err: unknown) =>
+        setError(
+          err instanceof Error
+            ? err.message
+            : "Gebäudestruktur konnte nicht geladen werden."
+        )
+      )
+      .finally(() => setIsLoading(false));
+  }, [projectId, reloadStructure]);
+
+  const effectiveBuildingSelection = normalizeBuildingSelection(
+    buildingSelection,
+    floors,
+    buildings
+  );
+  const effectiveFloorSelection = normalizeFloorSelection(
+    floorSelection,
+    rooms,
+    floors,
+    buildings,
+    effectiveBuildingSelection
+  );
+
+  const visibleFloors = useMemo(
+    () => filterFloorsByBuilding(floors, buildings, effectiveBuildingSelection),
+    [buildings, effectiveBuildingSelection, floors]
+  );
+  const visibleFloorGroups = useMemo(
+    () =>
+      effectiveBuildingSelection === ALL_SELECTION && buildings.length
+        ? groupFloorsByBuilding(visibleFloors, buildings)
+        : [{ buildingId: null, label: "", floors: visibleFloors }],
+    [buildings, effectiveBuildingSelection, visibleFloors]
+  );
+  const visibleRooms = useMemo(
+    () =>
+      filterRooms(
+        rooms,
+        floors,
+        buildings,
+        effectiveBuildingSelection,
+        effectiveFloorSelection
+      ).sort(compareRooms),
+    [buildings, effectiveBuildingSelection, effectiveFloorSelection, floors, rooms]
+  );
+  const roomsInBuildingSelection = useMemo(
+    () =>
+      filterRooms(rooms, floors, buildings, effectiveBuildingSelection, ALL_SELECTION)
+        .length,
+    [buildings, effectiveBuildingSelection, floors, rooms]
+  );
+  const floorCountByBuilding = useMemo(
+    () => countFloorsByBuilding(floors, buildings),
+    [buildings, floors]
+  );
+  const roomCountByFloor = useMemo(
+    () => countRoomsByFloor(rooms, floors),
+    [floors, rooms]
+  );
+  const showUnassignedBuilding = hasFloorsWithoutBuilding(floors, buildings);
+  const showUnassignedFloor = showsRoomsWithoutFloorEntry(
+    rooms,
+    floors,
+    effectiveBuildingSelection
+  );
+  const isBusy = isSaving || !project;
+
+  function selectBuilding(selection: StructureSelection) {
+    setBuildingSelection(selection);
+    setFloorSelection(ALL_SELECTION);
+  }
+
+  async function runCommand<TResult extends ProjectCommandResultDto>(
+    execute: (expectedRevision: number) => Promise<TResult>,
+    fallbackMessage: string
+  ): Promise<boolean> {
+    if (!project) {
+      return false;
+    }
+    setIsSaving(true);
+    setError(null);
+    try {
+      const result = await execute(project.currentRevision);
+      setProject((current) =>
+        current
+          ? { ...current, currentRevision: result.history.currentRevision }
+          : current
+      );
+      await reloadStructure();
+      return true;
+    } catch (err) {
+      setError(err instanceof Error ? err.message : fallbackMessage);
+      // Refresh the revision and lists so a stale state does not block the
+      // next attempt; the command error stays visible.
+      await reloadStructure().catch(() => undefined);
+      return false;
+    } finally {
+      setIsSaving(false);
+    }
+  }
+
+  function openBuildingDialog(building: BuildingDto | null) {
+    setDialogName(building?.name ?? "");
+    setDialog({ kind: "building", building });
+  }
+
+  function openFloorDialog(floor: FloorDto | null) {
+    setDialogName(floor?.name ?? "");
+    setDialogBuildingId(
+      floor
+        ? floor.buildingId ?? ""
+        : defaultBuildingIdForNewFloor(effectiveBuildingSelection)
+    );
+    setDialog({ kind: "floor", floor });
+  }
+
+  function openRoomDialog(room: RoomDto | null) {
+    setDialog({
+      kind: "room",
+      room,
+      defaultFloorId: defaultFloorIdForNewRoom(effectiveFloorSelection),
+    });
+  }
+
+  async function handleSaveBuilding(event: FormEvent<HTMLFormElement>) {
+    event.preventDefault();
+    if (dialog?.kind !== "building" || !dialogName.trim()) {
+      return;
+    }
+    const editingBuilding = dialog.building;
+    const name = dialogName.trim();
+    // An unchanged dialog is not a command; the backend rejects no-op updates.
+    if (editingBuilding && editingBuilding.name === name) {
+      setDialog(null);
+      return;
+    }
+    const saved = await runCommand(
+      (expectedRevision) =>
+        editingBuilding
+          ? updateBuilding(projectId, editingBuilding.id, { name }, expectedRevision)
+          : createBuilding(projectId, { name }, expectedRevision),
+      "Gebäude konnte nicht gespeichert werden."
+    );
+    if (saved) {
+      setDialog(null);
+    }
+  }
+
+  async function handleSaveFloor(event: FormEvent<HTMLFormElement>) {
+    event.preventDefault();
+    if (dialog?.kind !== "floor" || !dialogName.trim()) {
+      return;
+    }
+    const editingFloor = dialog.floor;
+    const input = {
+      name: dialogName.trim(),
+      buildingId: dialogBuildingId || null,
+    };
+    if (
+      editingFloor &&
+      editingFloor.name === input.name &&
+      editingFloor.buildingId === input.buildingId
+    ) {
+      setDialog(null);
+      return;
+    }
+    const saved = await runCommand(
+      (expectedRevision) =>
+        editingFloor
+          ? updateFloor(projectId, editingFloor.id, input, expectedRevision)
+          : createFloor(projectId, input, expectedRevision),
+      "Etage konnte nicht gespeichert werden."
+    );
+    if (saved) {
+      setDialog(null);
+    }
+  }
+
+  async function handleSaveRoom(input: CreateRoomInput) {
+    if (dialog?.kind !== "room") {
+      return;
+    }
+    const editingRoom = dialog.room;
+    if (
+      editingRoom &&
+      editingRoom.roomNumber === input.roomNumber.trim() &&
+      editingRoom.roomName === input.roomName.trim() &&
+      editingRoom.floorId === (input.floorId || null)
+    ) {
+      setDialog(null);
+      return;
+    }
+    const saved = await runCommand(
+      (expectedRevision) =>
+        editingRoom
+          ? updateRoom(projectId, editingRoom.id, input, expectedRevision)
+          : createRoom(projectId, input, expectedRevision),
+      "Raum konnte nicht gespeichert werden."
+    );
+    if (saved) {
+      setDialog(null);
+    }
+  }
+
+  async function handleDeleteBuilding(building: BuildingDto) {
+    if ((floorCountByBuilding.get(building.id) ?? 0) > 0) {
+      setError(BUILDING_HAS_FLOORS_MESSAGE);
+      return;
+    }
+    if (!window.confirm(`Gebäude „${building.name}“ wirklich löschen?`)) {
+      return;
+    }
+    await runCommand(
+      (expectedRevision) =>
+        deleteBuilding(projectId, building.id, expectedRevision),
+      "Gebäude konnte nicht gelöscht werden."
+    );
+  }
+
+  async function handleDeleteFloor(floor: FloorDto) {
+    if (!window.confirm(`Etage „${floor.name}“ wirklich löschen?`)) {
+      return;
+    }
+    await runCommand(
+      (expectedRevision) => deleteFloor(projectId, floor.id, expectedRevision),
+      "Etage konnte nicht gelöscht werden."
+    );
+  }
+
+  async function handleDeleteRoom(room: RoomDto) {
+    if (
+      !window.confirm(
+        `Raum „${room.roomNumber} ${room.roomName}“ wirklich löschen?`
+      )
+    ) {
+      return;
+    }
+    await runCommand(
+      (expectedRevision) => deleteRoom(projectId, room.id, expectedRevision),
+      "Raum konnte nicht gelöscht werden."
+    );
+  }
+
+  return (
+    <main className="container py-4">
+      <div className="page-header">
+        <div className="d-flex align-items-start gap-3">
+          <Link
+            aria-label="Zurück zur Projektübersicht"
+            className="btn btn-outline-secondary btn-sm structure-back-link"
+            href={`/projects/${projectId}`}
+            title="Zurück zur Projektübersicht"
+          >
+            <ArrowLeft aria-hidden="true" size={18} />
+          </Link>
+          <div>
+            <div className="kicker">{project?.name ?? "Projekt"}</div>
+            <h1>Gebäudestruktur</h1>
+            <p className="text-secondary mb-0">
+              Gebäude, Etagen und Räume des Projekts gliedern und pflegen.
+            </p>
+          </div>
+        </div>
+      </div>
+
+      {error ? (
+        <div className="alert alert-warning" role="alert">
+          {error}
+        </div>
+      ) : null}
+      {isLoading ? (
+        <p className="text-secondary" role="status">
+          Gebäudestruktur wird geladen …
+        </p>
+      ) : null}
+
+      <div className="structure-columns">
+        <StructureColumn
+          addLabel="Gebäude hinzufügen"
+          count={buildings.length}
+          headingId="structure-buildings-heading"
+          isAddDisabled={isBusy}
+          onAdd={() => openBuildingDialog(null)}
+          title="Gebäude"
+        >
+          <StructureEntry
+            isSelected={effectiveBuildingSelection === ALL_SELECTION}
+            label="Alle"
+            meta={formatCount(floors.length, "Etage", "Etagen")}
+            onSelect={() => selectBuilding(ALL_SELECTION)}
+          />
+          {buildings.map((building) => (
+            <StructureEntry
+              actions={
+                <EntryActions
+                  deleteLabel={`Gebäude „${building.name}“ löschen`}
+                  editLabel={`Gebäude „${building.name}“ bearbeiten`}
+                  isDisabled={isBusy}
+                  onDelete={() => void handleDeleteBuilding(building)}
+                  onEdit={() => openBuildingDialog(building)}
+                />
+              }
+              isSelected={effectiveBuildingSelection === building.id}
+              key={building.id}
+              label={building.name}
+              meta={formatCount(
+                floorCountByBuilding.get(building.id) ?? 0,
+                "Etage",
+                "Etagen"
+              )}
+              onSelect={() => selectBuilding(building.id)}
+            />
+          ))}
+          {showUnassignedBuilding ? (
+            <StructureEntry
+              isSelected={effectiveBuildingSelection === UNASSIGNED_SELECTION}
+              label={<em>{UNASSIGNED_BUILDING_LABEL}</em>}
+              meta={formatCount(
+                floorCountByBuilding.get(null) ?? 0,
+                "Etage",
+                "Etagen"
+              )}
+              onSelect={() => selectBuilding(UNASSIGNED_SELECTION)}
+            />
+          ) : null}
+          {!buildings.length && !isLoading ? (
+            <li className="list-group-item structure-empty">
+              Noch keine Gebäude vorhanden.
+            </li>
+          ) : null}
+        </StructureColumn>
+
+        <StructureColumn
+          addLabel="Etage hinzufügen"
+          count={visibleFloors.length}
+          headingId="structure-floors-heading"
+          isAddDisabled={isBusy}
+          onAdd={() => openFloorDialog(null)}
+          title="Etagen"
+        >
+          <StructureEntry
+            isSelected={effectiveFloorSelection === ALL_SELECTION}
+            label="Alle"
+            meta={formatCount(roomsInBuildingSelection, "Raum", "Räume")}
+            onSelect={() => setFloorSelection(ALL_SELECTION)}
+          />
+          {visibleFloorGroups.map((group) => (
+            <FloorGroupEntries
+              key={group.buildingId ?? "unassigned"}
+              label={group.label}
+            >
+              {group.floors.map((floor) => (
+                <StructureEntry
+                  actions={
+                    <EntryActions
+                      deleteLabel={`Etage „${floor.name}“ löschen`}
+                      editLabel={`Etage „${floor.name}“ bearbeiten`}
+                      isDisabled={isBusy}
+                      onDelete={() => void handleDeleteFloor(floor)}
+                      onEdit={() => openFloorDialog(floor)}
+                    />
+                  }
+                  isSelected={effectiveFloorSelection === floor.id}
+                  key={floor.id}
+                  label={floor.name}
+                  meta={formatCount(
+                    roomCountByFloor.get(floor.id) ?? 0,
+                    "Raum",
+                    "Räume"
+                  )}
+                  onSelect={() => setFloorSelection(floor.id)}
+                />
+              ))}
+            </FloorGroupEntries>
+          ))}
+          {showUnassignedFloor ? (
+            <StructureEntry
+              isSelected={effectiveFloorSelection === UNASSIGNED_SELECTION}
+              label={<em>{UNASSIGNED_FLOOR_LABEL}</em>}
+              meta={formatCount(roomCountByFloor.get(null) ?? 0, "Raum", "Räume")}
+              onSelect={() => setFloorSelection(UNASSIGNED_SELECTION)}
+            />
+          ) : null}
+          {!visibleFloors.length && !isLoading ? (
+            <li className="list-group-item structure-empty">
+              {floors.length
+                ? "Keine Etagen in dieser Auswahl."
+                : "Noch keine Etagen vorhanden."}
+            </li>
+          ) : null}
+        </StructureColumn>
+
+        <StructureColumn
+          addLabel="Raum hinzufügen"
+          count={visibleRooms.length}
+          headingId="structure-rooms-heading"
+          isAddDisabled={isBusy}
+          onAdd={() => openRoomDialog(null)}
+          title="Räume"
+        >
+          {visibleRooms.map((room) => (
+            <StructureEntry
+              actions={
+                <EntryActions
+                  deleteLabel={`Raum „${room.roomNumber} ${room.roomName}“ löschen`}
+                  editLabel={`Raum „${room.roomNumber} ${room.roomName}“ bearbeiten`}
+                  isDisabled={isBusy}
+                  onDelete={() => void handleDeleteRoom(room)}
+                  onEdit={() => openRoomDialog(room)}
+                />
+              }
+              hint={
+                effectiveFloorSelection === ALL_SELECTION
+                  ? describeRoomLocation(room, floors, buildings)
+                  : undefined
+              }
+              key={room.id}
+              label={
+                <>
+                  <span className="structure-room-number">{room.roomNumber}</span>{" "}
+                  {room.roomName}
+                </>
+              }
+            />
+          ))}
+          {!visibleRooms.length && !isLoading ? (
+            <li className="list-group-item structure-empty">
+              {rooms.length
+                ? "Keine Räume in dieser Auswahl."
+                : "Noch keine Räume vorhanden."}
+            </li>
+          ) : null}
+        </StructureColumn>
+      </div>
+
+      {dialog?.kind === "building" ? (
+        <FormModal
+          isSaving={isSaving}
+          onClose={() => setDialog(null)}
+          onSubmit={handleSaveBuilding}
+          submitDisabled={!dialogName.trim()}
+          submitLabel={dialog.building ? "Änderungen speichern" : "Gebäude hinzufügen"}
+          title={dialog.building ? "Gebäude bearbeiten" : "Gebäude hinzufügen"}
+        >
+          <label className="form-label" htmlFor="building-name">
+            Bezeichnung
+          </label>
+          <input
+            autoFocus
+            className="form-control"
+            id="building-name"
+            maxLength={200}
+            onChange={(event) => setDialogName(event.target.value)}
+            placeholder="z. B. Haus A"
+            required
+            value={dialogName}
+          />
+        </FormModal>
+      ) : null}
+
+      {dialog?.kind === "floor" ? (
+        <FormModal
+          isSaving={isSaving}
+          onClose={() => setDialog(null)}
+          onSubmit={handleSaveFloor}
+          submitDisabled={!dialogName.trim()}
+          submitLabel={dialog.floor ? "Änderungen speichern" : "Etage hinzufügen"}
+          title={dialog.floor ? "Etage bearbeiten" : "Etage hinzufügen"}
+        >
+          <div className="row g-3">
+            <div className="col-12 col-md-6">
+              <label className="form-label" htmlFor="floor-name">
+                Bezeichnung
+              </label>
+              <input
+                autoFocus
+                className="form-control"
+                id="floor-name"
+                maxLength={200}
+                onChange={(event) => setDialogName(event.target.value)}
+                placeholder="z. B. EG"
+                required
+                value={dialogName}
+              />
+            </div>
+            <div className="col-12 col-md-6">
+              <label className="form-label" htmlFor="floor-building">
+                Gebäude
+              </label>
+              <select
+                className="form-select"
+                id="floor-building"
+                onChange={(event) => setDialogBuildingId(event.target.value)}
+                value={dialogBuildingId}
+              >
+                <option value="">{UNASSIGNED_BUILDING_LABEL}</option>
+                {buildings.map((building) => (
+                  <option key={building.id} value={building.id}>
+                    {building.name}
+                  </option>
+                ))}
+              </select>
+            </div>
+          </div>
+        </FormModal>
+      ) : null}
+
+      {dialog?.kind === "room" ? (
+        <RoomModal
+          buildings={buildings}
+          floors={floors}
+          initialRoom={dialog.room ?? undefined}
+          initialValues={
+            dialog.defaultFloorId ? { floorId: dialog.defaultFloorId } : undefined
+          }
+          isSaving={isSaving}
+          onClose={() => setDialog(null)}
+          onSave={handleSaveRoom}
+        />
+      ) : null}
+    </main>
+  );
+}
+
+function FloorGroupEntries({
+  children,
+  label,
+}: {
+  children: ReactNode;
+  label: string;
+}) {
+  return (
+    <>
+      {label ? (
+        <li className="list-group-item structure-group-label">
+          {label}
+        </li>
+      ) : null}
+      {children}
+    </>
+  );
+}
--- a/src/db/migrations/0009_buildings.sql
+++ b/src/db/migrations/0009_buildings.sql
@ -0,0 +1,9 @@
+CREATE TABLE `buildings` (
+	`id` text PRIMARY KEY NOT NULL,
+	`project_id` text NOT NULL,
+	`name` text NOT NULL,
+	`sort_order` integer DEFAULT 0 NOT NULL,
+	FOREIGN KEY (`project_id`) REFERENCES `projects`(`id`) ON UPDATE no action ON DELETE cascade
+);
+--> statement-breakpoint
+ALTER TABLE `floors` ADD `building_id` text REFERENCES buildings(id) ON DELETE set null;
--- a/src/db/migrations/meta/0009_snapshot.json
+++ b/src/db/migrations/meta/0009_snapshot.json
--- a/src/db/migrations/meta/_journal.json
+++ b/src/db/migrations/meta/_journal.json
@ -64,6 +64,13 @@
      "when": 1790411560953,
      "tag": "0008_project_cover_and_office_profile",
      "breakpoints": true
+    },
+    {
+      "idx": 9,
+      "version": "6",
+      "when": 1790419185882,
+      "tag": "0009_buildings",
+      "breakpoints": true
    }
  ]
 }
--- a/src/db/repositories/building.repository.ts
+++ b/src/db/repositories/building.repository.ts
@ -0,0 +1,27 @@
+import { and, asc, eq } from "drizzle-orm";
+import type { AppDatabase } from "../database-context.js";
+import { buildings } from "../schema/buildings.js";
+
+export class BuildingRepository {
+  constructor(private readonly database: AppDatabase) {}
+
+  async listByProject(projectId: string) {
+    return this.database
+      .select()
+      .from(buildings)
+      .where(eq(buildings.projectId, projectId))
+      .orderBy(asc(buildings.sortOrder), asc(buildings.name));
+  }
+
+  async findById(projectId: string, buildingId: string) {
+    return (
+      this.database
+        .select()
+        .from(buildings)
+        .where(
+          and(eq(buildings.projectId, projectId), eq(buildings.id, buildingId))
+        )
+        .get() ?? null
+    );
+  }
+}
--- a/src/db/repositories/project-location-structure-project-command.repository.ts
+++ b/src/db/repositories/project-location-structure-project-command.repository.ts
@ -1,23 +1,34 @@
 import { and, eq } from "drizzle-orm";
 import {
+  assertProjectBuildingDeleteProjectCommand,
+  assertProjectBuildingInsertProjectCommand,
+  assertProjectBuildingUpdateProjectCommand,
  assertProjectFloorDeleteProjectCommand,
  assertProjectFloorInsertProjectCommand,
  assertProjectFloorUpdateProjectCommand,
  assertProjectRoomDeleteProjectCommand,
  assertProjectRoomInsertProjectCommand,
  assertProjectRoomUpdateProjectCommand,
+  createProjectBuildingDeleteProjectCommand,
+  createProjectBuildingInsertProjectCommand,
+  createProjectBuildingUpdateProjectCommand,
  createProjectFloorDeleteProjectCommand,
  createProjectFloorInsertProjectCommand,
  createProjectFloorUpdateProjectCommand,
  createProjectRoomDeleteProjectCommand,
  createProjectRoomInsertProjectCommand,
  createProjectRoomUpdateProjectCommand,
+  projectBuildingDeleteCommandType,
+  projectBuildingInsertCommandType,
+  projectBuildingUpdateCommandType,
  projectFloorDeleteCommandType,
  projectFloorInsertCommandType,
  projectFloorUpdateCommandType,
  projectRoomDeleteCommandType,
  projectRoomInsertCommandType,
  projectRoomUpdateCommandType,
+  upgradeProjectFloorCommand,
+  type ProjectBuildingSnapshot,
  type ProjectFloorSnapshot,
  type ProjectLocationStructureProjectCommand,
  type ProjectRoomSnapshot,
@ -27,6 +38,7 @@ import type {
  ProjectLocationStructureProjectCommandStore,
 } from "../../domain/ports/project-location-structure-project-command.store.js";
 import type { AppDatabase } from "../database-context.js";
+import { buildings } from "../schema/buildings.js";
 import { circuitDeviceRows } from "../schema/circuit-device-rows.js";
 import { floors } from "../schema/floors.js";
 import { distributionBoards } from "../schema/distribution-boards.js";
@ -40,11 +52,14 @@ export class ProjectLocationStructureProjectCommandRepository
  constructor(private readonly database: AppDatabase) {}

  execute(input: ExecuteProjectLocationStructureCommandInput) {
+    // Forward and inverse are persisted with one schema version, so a
+    // version 1 floor command from older history is upgraded first.
+    const command = toCurrentCommandVersion(input.command);
    return executeProjectCommandTransaction(
      this.database,
-      input,
+      { ...input, command },
      (tx) =>
-        this.applyCommand(tx, input.projectId, input.command)
+        this.applyCommand(tx, input.projectId, command)
    );
  }

@ -54,23 +69,43 @@ export class ProjectLocationStructureProjectCommandRepository
    command: ProjectLocationStructureProjectCommand
  ): ProjectLocationStructureProjectCommand {
    switch (command.type) {
-      case projectFloorInsertCommandType:
+      case projectBuildingInsertCommandType:
+        assertProjectBuildingInsertProjectCommand(command);
+        return this.insertBuilding(database, projectId, command.payload.building);
+      case projectBuildingDeleteCommandType:
+        assertProjectBuildingDeleteProjectCommand(command);
+        return this.deleteBuilding(database, projectId, command.payload.building);
+      case projectBuildingUpdateCommandType:
+        assertProjectBuildingUpdateProjectCommand(command);
+        return this.updateBuilding(
+          database,
+          projectId,
+          command.payload.expected,
+          command.payload.target
+        );
+      case projectFloorInsertCommandType: {
        assertProjectFloorInsertProjectCommand(command);
+        const current = upgradeProjectFloorCommand(command);
        return this.insertFloor(
          database,
          projectId,
-          command.payload.floor
+          current.payload.floor
        );
-      case projectFloorDeleteCommandType:
+      }
+      case projectFloorDeleteCommandType: {
        assertProjectFloorDeleteProjectCommand(command);
+        const current = upgradeProjectFloorCommand(command);
        return this.deleteFloor(
          database,
          projectId,
-          command.payload.floor
+          current.payload.floor
        );
-      case projectFloorUpdateCommandType:
+      }
+      case projectFloorUpdateCommandType: {
        assertProjectFloorUpdateProjectCommand(command);
-        return this.updateFloor(database, projectId, command.payload.expected, command.payload.target);
+        const current = upgradeProjectFloorCommand(command);
+        return this.updateFloor(database, projectId, current.payload.expected, current.payload.target);
+      }
      case projectRoomInsertCommandType:
        assertProjectRoomInsertProjectCommand(command);
        return this.insertRoom(
@ -100,6 +135,7 @@ export class ProjectLocationStructureProjectCommandRepository
    if (floor.projectId !== projectId) {
      throw new Error("Project-floor snapshot does not belong to project.");
    }
+    this.assertBuildingInProject(database, projectId, floor.buildingId);
    const existing = database
      .select({ id: floors.id })
      .from(floors)
@ -167,6 +203,104 @@ export class ProjectLocationStructureProjectCommandRepository
    return createProjectFloorInsertProjectCommand(expected);
  }

+  private insertBuilding(
+    database: AppDatabase,
+    projectId: string,
+    building: ProjectBuildingSnapshot
+  ) {
+    this.assertProjectExists(database, projectId);
+    if (building.projectId !== projectId) {
+      throw new Error("Project-building snapshot does not belong to project.");
+    }
+    const existing = database
+      .select({ id: buildings.id })
+      .from(buildings)
+      .where(eq(buildings.id, building.id))
+      .get();
+    if (existing) {
+      throw new Error("Project-building id already exists.");
+    }
+    database.insert(buildings).values(building).run();
+    return createProjectBuildingDeleteProjectCommand(building);
+  }
+
+  private deleteBuilding(
+    database: AppDatabase,
+    projectId: string,
+    expected: ProjectBuildingSnapshot
+  ) {
+    const persisted = database
+      .select()
+      .from(buildings)
+      .where(and(eq(buildings.id, expected.id), eq(buildings.projectId, projectId)))
+      .get();
+    if (!persisted || !sameRecord(expected, persisted)) {
+      throw new Error("Project building changed before deletion.");
+    }
+    const referencedFloor = database
+      .select({ id: floors.id })
+      .from(floors)
+      .where(eq(floors.buildingId, expected.id))
+      .limit(1)
+      .get();
+    if (referencedFloor) {
+      throw new Error(
+        "Das Gebäude kann nicht gelöscht werden, solange Etagen zugeordnet sind."
+      );
+    }
+    const deleted = database
+      .delete(buildings)
+      .where(and(eq(buildings.id, expected.id), eq(buildings.projectId, projectId)))
+      .run();
+    if (deleted.changes !== 1) {
+      throw new Error("Project building could not be deleted.");
+    }
+    return createProjectBuildingInsertProjectCommand(expected);
+  }
+
+  private updateBuilding(
+    database: AppDatabase,
+    projectId: string,
+    expected: ProjectBuildingSnapshot,
+    target: ProjectBuildingSnapshot
+  ) {
+    const persisted = database
+      .select()
+      .from(buildings)
+      .where(and(eq(buildings.id, expected.id), eq(buildings.projectId, projectId)))
+      .get();
+    if (!persisted || !sameRecord(expected, persisted)) {
+      throw new Error("Project building changed before update.");
+    }
+    const updated = database
+      .update(buildings)
+      .set({ name: target.name })
+      .where(and(eq(buildings.id, expected.id), eq(buildings.projectId, projectId)))
+      .run();
+    if (updated.changes !== 1) {
+      throw new Error("Das Gebäude konnte nicht aktualisiert werden.");
+    }
+    return createProjectBuildingUpdateProjectCommand(target, expected);
+  }
+
+  private assertBuildingInProject(
+    database: AppDatabase,
+    projectId: string,
+    buildingId: string | null
+  ) {
+    if (buildingId === null) {
+      return;
+    }
+    const building = database
+      .select({ id: buildings.id })
+      .from(buildings)
+      .where(and(eq(buildings.id, buildingId), eq(buildings.projectId, projectId)))
+      .get();
+    if (!building) {
+      throw new Error("Project floor references a foreign building.");
+    }
+  }
+
  private insertRoom(
    database: AppDatabase,
    projectId: string,
@ -217,9 +351,10 @@ export class ProjectLocationStructureProjectCommandRepository
    if (!persisted || !sameRecord(expected, persisted)) {
      throw new Error("Project floor changed before update.");
    }
+    this.assertBuildingInProject(database, projectId, target.buildingId);
    const updated = database
      .update(floors)
-      .set({ name: target.name })
+      .set({ name: target.name, buildingId: target.buildingId })
      .where(and(eq(floors.id, expected.id), eq(floors.projectId, projectId)))
      .run();
    if (updated.changes !== 1) {
@ -326,6 +461,24 @@ export class ProjectLocationStructureProjectCommandRepository
  }
 }

+function toCurrentCommandVersion(
+  command: ProjectLocationStructureProjectCommand
+): ProjectLocationStructureProjectCommand {
+  switch (command.type) {
+    case projectFloorInsertCommandType:
+      assertProjectFloorInsertProjectCommand(command);
+      return upgradeProjectFloorCommand(command);
+    case projectFloorDeleteCommandType:
+      assertProjectFloorDeleteProjectCommand(command);
+      return upgradeProjectFloorCommand(command);
+    case projectFloorUpdateCommandType:
+      assertProjectFloorUpdateProjectCommand(command);
+      return upgradeProjectFloorCommand(command);
+    default:
+      return command;
+  }
+}
+
 function sameRecord(
  expected: object,
  actual: object
--- a/src/db/repositories/project-state-restore-command.repository.ts
+++ b/src/db/repositories/project-state-restore-command.repository.ts
@ -21,6 +21,7 @@ import { circuits } from "../schema/circuits.js";
 import { distributionBoardComponentProtectionDevices } from "../schema/distribution-board-component-protection-devices.js";
 import { distributionBoardComponents } from "../schema/distribution-board-components.js";
 import { distributionBoards } from "../schema/distribution-boards.js";
+import { buildings } from "../schema/buildings.js";
 import { floors } from "../schema/floors.js";
 import { projectDevices } from "../schema/project-devices.js";
 import { projectChangeSets } from "../schema/project-change-sets.js";
@ -164,6 +165,7 @@ function canonicalProjectState(state: ProjectStateSnapshot) {
      }))
    ),
    projectDevices: byId(state.projectDevices),
+    buildings: byId(state.buildings),
    floors: byId(state.floors),
    rooms: byId(state.rooms),
    externalCsvConfiguration: state.externalCsvConfiguration,
@ -219,6 +221,10 @@ function replaceProjectState(
    .delete(floors)
    .where(eq(floors.projectId, state.project.id))
    .run();
+  database
+    .delete(buildings)
+    .where(eq(buildings.projectId, state.project.id))
+    .run();

  database
    .update(projects)
@ -241,6 +247,7 @@ function replaceProjectState(
    .where(eq(projects.id, state.project.id))
    .run();

+  insertMany(database, buildings, state.buildings);
  insertMany(database, floors, state.floors);
  if (state.externalCsvConfiguration !== null) {
    database
--- a/src/db/repositories/project-state-snapshot.persistence.ts
+++ b/src/db/repositories/project-state-snapshot.persistence.ts
@ -15,6 +15,7 @@ import { circuits } from "../schema/circuits.js";
 import { distributionBoardComponentProtectionDevices } from "../schema/distribution-board-component-protection-devices.js";
 import { distributionBoardComponents } from "../schema/distribution-board-components.js";
 import { distributionBoards } from "../schema/distribution-boards.js";
+import { buildings } from "../schema/buildings.js";
 import { floors } from "../schema/floors.js";
 import { projectDevices } from "../schema/project-devices.js";
 import { projects } from "../schema/projects.js";
@ -219,6 +220,12 @@ export function readProjectStateSnapshot(
    .where(eq(projectDevices.projectId, projectId))
    .orderBy(asc(projectDevices.displayName), asc(projectDevices.id))
    .all();
+  const buildingRows = database
+    .select()
+    .from(buildings)
+    .where(eq(buildings.projectId, projectId))
+    .orderBy(asc(buildings.sortOrder), asc(buildings.id))
+    .all();
  const floorRows = database
    .select()
    .from(floors)
@ -273,6 +280,7 @@ export function readProjectStateSnapshot(
      deviceRows: deviceRowsByCircuit.get(circuit.id) ?? [],
    })),
    projectDevices: projectDeviceRows,
+    buildings: buildingRows,
    floors: floorRows,
    rooms: roomRows,
  });
--- a/src/db/repositories/project-transfer.repository.ts
+++ b/src/db/repositories/project-transfer.repository.ts
@ -16,6 +16,7 @@ import { circuits } from "../schema/circuits.js";
 import { distributionBoardComponentProtectionDevices } from "../schema/distribution-board-component-protection-devices.js";
 import { distributionBoardComponents } from "../schema/distribution-board-components.js";
 import { distributionBoards } from "../schema/distribution-boards.js";
+import { buildings } from "../schema/buildings.js";
 import { floors } from "../schema/floors.js";
 import { projectDevices } from "../schema/project-devices.js";
 import { projects } from "../schema/projects.js";
@ -127,6 +128,7 @@ function insertProjectState(
      .values(state.externalCsvConfiguration)
      .run();
  }
+  insertMany(database, buildings, state.buildings);
  insertMany(database, floors, state.floors);
  insertMany(database, rooms, state.rooms);
  insertMany(database, projectDevices, state.projectDevices);
--- a/src/db/schema/buildings.ts
+++ b/src/db/schema/buildings.ts
@ -0,0 +1,11 @@
+import { integer, sqliteTable, text } from "drizzle-orm/sqlite-core";
+import { projects } from "./projects.js";
+
+export const buildings = sqliteTable("buildings", {
+  id: text("id").primaryKey(),
+  projectId: text("project_id")
+    .notNull()
+    .references(() => projects.id, { onDelete: "cascade" }),
+  name: text("name").notNull(),
+  sortOrder: integer("sort_order").notNull().default(0),
+});
--- a/src/db/schema/floors.ts
+++ b/src/db/schema/floors.ts
@ -1,4 +1,5 @@
 import { integer, sqliteTable, text } from "drizzle-orm/sqlite-core";
+import { buildings } from "./buildings.js";
 import { projects } from "./projects.js";

 export const floors = sqliteTable("floors", {
@ -8,4 +9,9 @@ export const floors = sqliteTable("floors", {
    .references(() => projects.id, { onDelete: "cascade" }),
  name: text("name").notNull(),
  sortOrder: integer("sort_order").notNull().default(0),
+  // Optional parent building. Floors created before buildings existed stay
+  // without one and are shown as "Ohne Gebäude".
+  buildingId: text("building_id").references(() => buildings.id, {
+    onDelete: "set null",
+  }),
 });
--- a/src/domain/models/project-location-structure-project-command.model.ts
+++ b/src/domain/models/project-location-structure-project-command.model.ts
@ -1,21 +1,42 @@
 import crypto from "node:crypto";
 import type { SerializedProjectCommand } from "./project-command.model.js";

+export const projectBuildingInsertCommandType = "project-building.insert" as const;
+export const projectBuildingDeleteCommandType = "project-building.delete" as const;
+export const projectBuildingUpdateCommandType = "project-building.update" as const;
 export const projectFloorInsertCommandType = "project-floor.insert" as const;
 export const projectFloorDeleteCommandType = "project-floor.delete" as const;
 export const projectFloorUpdateCommandType = "project-floor.update" as const;
 export const projectRoomInsertCommandType = "project-room.insert" as const;
 export const projectRoomDeleteCommandType = "project-room.delete" as const;
 export const projectRoomUpdateCommandType = "project-room.update" as const;
+/** Room commands, building commands and legacy floor commands. */
 export const projectLocationStructureCommandSchemaVersion = 1 as const;
+/**
+ * Floor commands carry `buildingId` since version 2. Version 1 floor
+ * commands stay executable from persisted history: they were recorded before
+ * buildings existed, so they are upgraded with `buildingId = null`.
+ */
+export const projectFloorCommandSchemaVersion = 2 as const;
+const legacyProjectFloorCommandSchemaVersion = 1 as const;
+
+export interface ProjectBuildingSnapshot {
+  id: string;
+  projectId: string;
+  name: string;
+  sortOrder: number;
+}

 export interface ProjectFloorSnapshot {
  id: string;
  projectId: string;
  name: string;
  sortOrder: number;
+  buildingId: string | null;
 }

+export type LegacyProjectFloorSnapshot = Omit<ProjectFloorSnapshot, "buildingId">;
+
 export interface ProjectRoomSnapshot {
  id: string;
  projectId: string;
@ -24,39 +45,93 @@ export interface ProjectRoomSnapshot {
  roomName: string;
 }

+interface ProjectBuildingStructureCommandPayload {
+  building: ProjectBuildingSnapshot;
+}
+
 interface ProjectFloorStructureCommandPayload {
  floor: ProjectFloorSnapshot;
 }

+interface LegacyProjectFloorStructureCommandPayload {
+  floor: LegacyProjectFloorSnapshot;
+}
+
 interface ProjectRoomStructureCommandPayload {
  room: ProjectRoomSnapshot;
 }

+interface ProjectBuildingUpdateCommandPayload {
+  expected: ProjectBuildingSnapshot;
+  target: ProjectBuildingSnapshot;
+}
+
 interface ProjectFloorUpdateCommandPayload {
  expected: ProjectFloorSnapshot;
  target: ProjectFloorSnapshot;
 }

+interface LegacyProjectFloorUpdateCommandPayload {
+  expected: LegacyProjectFloorSnapshot;
+  target: LegacyProjectFloorSnapshot;
+}
+
 interface ProjectRoomUpdateCommandPayload {
  expected: ProjectRoomSnapshot;
  target: ProjectRoomSnapshot;
 }

+export interface ProjectBuildingInsertProjectCommand
+  extends SerializedProjectCommand<ProjectBuildingStructureCommandPayload> {
+  schemaVersion: typeof projectLocationStructureCommandSchemaVersion;
+  type: typeof projectBuildingInsertCommandType;
+}
+
+export interface ProjectBuildingDeleteProjectCommand
+  extends SerializedProjectCommand<ProjectBuildingStructureCommandPayload> {
+  schemaVersion: typeof projectLocationStructureCommandSchemaVersion;
+  type: typeof projectBuildingDeleteCommandType;
+}
+
+export interface ProjectBuildingUpdateProjectCommand
+  extends SerializedProjectCommand<ProjectBuildingUpdateCommandPayload> {
+  schemaVersion: typeof projectLocationStructureCommandSchemaVersion;
+  type: typeof projectBuildingUpdateCommandType;
+}
+
 export interface ProjectFloorInsertProjectCommand
  extends SerializedProjectCommand<ProjectFloorStructureCommandPayload> {
-  schemaVersion: typeof projectLocationStructureCommandSchemaVersion;
+  schemaVersion: typeof projectFloorCommandSchemaVersion;
  type: typeof projectFloorInsertCommandType;
 }

 export interface ProjectFloorDeleteProjectCommand
  extends SerializedProjectCommand<ProjectFloorStructureCommandPayload> {
-  schemaVersion: typeof projectLocationStructureCommandSchemaVersion;
+  schemaVersion: typeof projectFloorCommandSchemaVersion;
  type: typeof projectFloorDeleteCommandType;
 }

 export interface ProjectFloorUpdateProjectCommand
  extends SerializedProjectCommand<ProjectFloorUpdateCommandPayload> {
-  schemaVersion: typeof projectLocationStructureCommandSchemaVersion;
+  schemaVersion: typeof projectFloorCommandSchemaVersion;
+  type: typeof projectFloorUpdateCommandType;
+}
+
+export interface LegacyProjectFloorInsertProjectCommand
+  extends SerializedProjectCommand<LegacyProjectFloorStructureCommandPayload> {
+  schemaVersion: typeof legacyProjectFloorCommandSchemaVersion;
+  type: typeof projectFloorInsertCommandType;
+}
+
+export interface LegacyProjectFloorDeleteProjectCommand
+  extends SerializedProjectCommand<LegacyProjectFloorStructureCommandPayload> {
+  schemaVersion: typeof legacyProjectFloorCommandSchemaVersion;
+  type: typeof projectFloorDeleteCommandType;
+}
+
+export interface LegacyProjectFloorUpdateProjectCommand
+  extends SerializedProjectCommand<LegacyProjectFloorUpdateCommandPayload> {
+  schemaVersion: typeof legacyProjectFloorCommandSchemaVersion;
  type: typeof projectFloorUpdateCommandType;
 }

@ -79,23 +154,46 @@ export interface ProjectRoomUpdateProjectCommand
 }

 export type ProjectLocationStructureProjectCommand =
+  | ProjectBuildingInsertProjectCommand
+  | ProjectBuildingDeleteProjectCommand
+  | ProjectBuildingUpdateProjectCommand
  | ProjectFloorInsertProjectCommand
  | ProjectFloorDeleteProjectCommand
  | ProjectFloorUpdateProjectCommand
+  | LegacyProjectFloorInsertProjectCommand
+  | LegacyProjectFloorDeleteProjectCommand
+  | LegacyProjectFloorUpdateProjectCommand
  | ProjectRoomInsertProjectCommand
  | ProjectRoomDeleteProjectCommand
  | ProjectRoomUpdateProjectCommand;

-export function createProjectFloorSnapshot(
+export function createProjectBuildingSnapshot(
  projectId: string,
  name: string,
  sortOrder: number
+): ProjectBuildingSnapshot {
+  const building: ProjectBuildingSnapshot = {
+    id: crypto.randomUUID(),
+    projectId,
+    name: name.trim(),
+    sortOrder,
+  };
+  assertProjectBuildingSnapshot(building);
+  return building;
+}
+
+export function createProjectFloorSnapshot(
+  projectId: string,
+  name: string,
+  sortOrder: number,
+  buildingId: string | null = null
 ): ProjectFloorSnapshot {
  const floor: ProjectFloorSnapshot = {
    id: crypto.randomUUID(),
    projectId,
    name: name.trim(),
    sortOrder,
+    buildingId: buildingId?.trim() || null,
  };
  assertProjectFloorSnapshot(floor);
  return floor;
@ -120,12 +218,46 @@ export function createProjectRoomSnapshot(
  return room;
 }

+export function createProjectBuildingInsertProjectCommand(
+  building: ProjectBuildingSnapshot
+): ProjectBuildingInsertProjectCommand {
+  assertProjectBuildingSnapshot(building);
+  return {
+    schemaVersion: projectLocationStructureCommandSchemaVersion,
+    type: projectBuildingInsertCommandType,
+    payload: { building },
+  };
+}
+
+export function createProjectBuildingDeleteProjectCommand(
+  building: ProjectBuildingSnapshot
+): ProjectBuildingDeleteProjectCommand {
+  assertProjectBuildingSnapshot(building);
+  return {
+    schemaVersion: projectLocationStructureCommandSchemaVersion,
+    type: projectBuildingDeleteCommandType,
+    payload: { building },
+  };
+}
+
+export function createProjectBuildingUpdateProjectCommand(
+  expected: ProjectBuildingSnapshot,
+  target: ProjectBuildingSnapshot
+): ProjectBuildingUpdateProjectCommand {
+  assertMatchingBuildingUpdate(expected, target);
+  return {
+    schemaVersion: projectLocationStructureCommandSchemaVersion,
+    type: projectBuildingUpdateCommandType,
+    payload: { expected, target },
+  };
+}
+
 export function createProjectFloorInsertProjectCommand(
  floor: ProjectFloorSnapshot
 ): ProjectFloorInsertProjectCommand {
  assertProjectFloorSnapshot(floor);
  return {
-    schemaVersion: projectLocationStructureCommandSchemaVersion,
+    schemaVersion: projectFloorCommandSchemaVersion,
    type: projectFloorInsertCommandType,
    payload: { floor },
  };
@ -136,7 +268,7 @@ export function createProjectFloorDeleteProjectCommand(
 ): ProjectFloorDeleteProjectCommand {
  assertProjectFloorSnapshot(floor);
  return {
-    schemaVersion: projectLocationStructureCommandSchemaVersion,
+    schemaVersion: projectFloorCommandSchemaVersion,
    type: projectFloorDeleteCommandType,
    payload: { floor },
  };
@ -170,7 +302,7 @@ export function createProjectFloorUpdateProjectCommand(
 ): ProjectFloorUpdateProjectCommand {
  assertMatchingFloorUpdate(expected, target);
  return {
-    schemaVersion: projectLocationStructureCommandSchemaVersion,
+    schemaVersion: projectFloorCommandSchemaVersion,
    type: projectFloorUpdateCommandType,
    payload: { expected, target },
  };
@ -188,9 +320,43 @@ export function createProjectRoomUpdateProjectCommand(
  };
 }

+export function assertProjectBuildingInsertProjectCommand(
+  command: SerializedProjectCommand<unknown>
+): asserts command is ProjectBuildingInsertProjectCommand {
+  assertProjectBuildingStructureProjectCommand(
+    command,
+    projectBuildingInsertCommandType
+  );
+}
+
+export function assertProjectBuildingDeleteProjectCommand(
+  command: SerializedProjectCommand<unknown>
+): asserts command is ProjectBuildingDeleteProjectCommand {
+  assertProjectBuildingStructureProjectCommand(
+    command,
+    projectBuildingDeleteCommandType
+  );
+}
+
+export function assertProjectBuildingUpdateProjectCommand(
+  command: SerializedProjectCommand<unknown>
+): asserts command is ProjectBuildingUpdateProjectCommand {
+  if (
+    command.schemaVersion !== projectLocationStructureCommandSchemaVersion ||
+    command.type !== projectBuildingUpdateCommandType ||
+    !isPlainObject(command.payload) ||
+    Object.keys(command.payload).length !== 2
+  ) {
+    throw new Error("Unsupported project-building update command.");
+  }
+  assertMatchingBuildingUpdate(command.payload.expected, command.payload.target);
+}
+
 export function assertProjectFloorInsertProjectCommand(
  command: SerializedProjectCommand<unknown>
-): asserts command is ProjectFloorInsertProjectCommand {
+): asserts command is
+  | ProjectFloorInsertProjectCommand
+  | LegacyProjectFloorInsertProjectCommand {
  assertProjectFloorStructureProjectCommand(
    command,
    projectFloorInsertCommandType
@ -199,7 +365,9 @@ export function assertProjectFloorInsertProjectCommand(

 export function assertProjectFloorDeleteProjectCommand(
  command: SerializedProjectCommand<unknown>
-): asserts command is ProjectFloorDeleteProjectCommand {
+): asserts command is
+  | ProjectFloorDeleteProjectCommand
+  | LegacyProjectFloorDeleteProjectCommand {
  assertProjectFloorStructureProjectCommand(
    command,
    projectFloorDeleteCommandType
@ -226,15 +394,26 @@ export function assertProjectRoomDeleteProjectCommand(

 export function assertProjectFloorUpdateProjectCommand(
  command: SerializedProjectCommand<unknown>
-): asserts command is ProjectFloorUpdateProjectCommand {
+): asserts command is
+  | ProjectFloorUpdateProjectCommand
+  | LegacyProjectFloorUpdateProjectCommand {
  if (
-    command.schemaVersion !== projectLocationStructureCommandSchemaVersion ||
    command.type !== projectFloorUpdateCommandType ||
    !isPlainObject(command.payload) ||
    Object.keys(command.payload).length !== 2
  ) {
    throw new Error("Unsupported project-floor update command.");
  }
+  if (command.schemaVersion === legacyProjectFloorCommandSchemaVersion) {
+    assertMatchingFloorUpdate(
+      upgradeLegacyFloorSnapshot(command.payload.expected),
+      upgradeLegacyFloorSnapshot(command.payload.target)
+    );
+    return;
+  }
+  if (command.schemaVersion !== projectFloorCommandSchemaVersion) {
+    throw new Error("Unsupported project-floor update command.");
+  }
  assertMatchingFloorUpdate(command.payload.expected, command.payload.target);
 }

@ -252,6 +431,68 @@ export function assertProjectRoomUpdateProjectCommand(
  assertMatchingRoomUpdate(command.payload.expected, command.payload.target);
 }

+/**
+ * Upgrades a (validated) floor command of any supported version to the
+ * current version. Version 1 commands predate buildings; their floors had no
+ * building, so `buildingId` becomes null.
+ */
+export function upgradeProjectFloorCommand(
+  command:
+    | ProjectFloorInsertProjectCommand
+    | LegacyProjectFloorInsertProjectCommand
+): ProjectFloorInsertProjectCommand;
+export function upgradeProjectFloorCommand(
+  command:
+    | ProjectFloorDeleteProjectCommand
+    | LegacyProjectFloorDeleteProjectCommand
+): ProjectFloorDeleteProjectCommand;
+export function upgradeProjectFloorCommand(
+  command:
+    | ProjectFloorUpdateProjectCommand
+    | LegacyProjectFloorUpdateProjectCommand
+): ProjectFloorUpdateProjectCommand;
+export function upgradeProjectFloorCommand(
+  command:
+    | ProjectFloorInsertProjectCommand
+    | ProjectFloorDeleteProjectCommand
+    | ProjectFloorUpdateProjectCommand
+    | LegacyProjectFloorInsertProjectCommand
+    | LegacyProjectFloorDeleteProjectCommand
+    | LegacyProjectFloorUpdateProjectCommand
+) {
+  if (command.schemaVersion === projectFloorCommandSchemaVersion) {
+    return command;
+  }
+  switch (command.type) {
+    case projectFloorInsertCommandType:
+      return createProjectFloorInsertProjectCommand(
+        upgradeLegacyFloorSnapshot(command.payload.floor)
+      );
+    case projectFloorDeleteCommandType:
+      return createProjectFloorDeleteProjectCommand(
+        upgradeLegacyFloorSnapshot(command.payload.floor)
+      );
+    case projectFloorUpdateCommandType:
+      return createProjectFloorUpdateProjectCommand(
+        upgradeLegacyFloorSnapshot(command.payload.expected),
+        upgradeLegacyFloorSnapshot(command.payload.target)
+      );
+  }
+}
+
+function assertMatchingBuildingUpdate(expected: unknown, target: unknown) {
+  assertProjectBuildingSnapshot(expected);
+  assertProjectBuildingSnapshot(target);
+  if (
+    expected.id !== target.id ||
+    expected.projectId !== target.projectId ||
+    expected.sortOrder !== target.sortOrder ||
+    expected.name === target.name
+  ) {
+    throw new Error("Project-building update must change only the name.");
+  }
+}
+
 function assertMatchingFloorUpdate(expected: unknown, target: unknown) {
  assertProjectFloorSnapshot(expected);
  assertProjectFloorSnapshot(target);
@ -259,9 +500,11 @@ function assertMatchingFloorUpdate(expected: unknown, target: unknown) {
    expected.id !== target.id ||
    expected.projectId !== target.projectId ||
    expected.sortOrder !== target.sortOrder ||
-    expected.name === target.name
+    (expected.name === target.name && expected.buildingId === target.buildingId)
  ) {
-    throw new Error("Project-floor update must change only the name.");
+    throw new Error(
+      "Project-floor update must change only the name or the building."
+    );
  }
 }

@ -277,22 +520,49 @@ function assertMatchingRoomUpdate(expected: unknown, target: unknown) {
  }
 }

+export function assertProjectBuildingSnapshot(
+  building: unknown
+): asserts building is ProjectBuildingSnapshot {
+  if (!isPlainObject(building) || Object.keys(building).length !== 4) {
+    throw new Error("Project-building snapshot is invalid.");
+  }
+  assertNormalizedNonEmptyString(building.id, "building.id");
+  assertNormalizedNonEmptyString(building.projectId, "building.projectId");
+  assertNormalizedNonEmptyString(building.name, "building.name");
+  assertSortOrder(building.sortOrder, "building.sortOrder");
+}
+
 export function assertProjectFloorSnapshot(
  floor: unknown
 ): asserts floor is ProjectFloorSnapshot {
+  if (!isPlainObject(floor) || Object.keys(floor).length !== 5) {
+    throw new Error("Project-floor snapshot is invalid.");
+  }
+  assertLegacyFloorFields(floor);
+  if (floor.buildingId !== null) {
+    assertNormalizedNonEmptyString(floor.buildingId, "floor.buildingId");
+  }
+}
+
+function assertLegacyProjectFloorSnapshot(
+  floor: unknown
+): asserts floor is LegacyProjectFloorSnapshot {
  if (!isPlainObject(floor) || Object.keys(floor).length !== 4) {
    throw new Error("Project-floor snapshot is invalid.");
  }
+  assertLegacyFloorFields(floor);
+}
+
+function assertLegacyFloorFields(floor: Record<string, unknown>) {
  assertNormalizedNonEmptyString(floor.id, "floor.id");
  assertNormalizedNonEmptyString(floor.projectId, "floor.projectId");
  assertNormalizedNonEmptyString(floor.name, "floor.name");
-  if (
-    typeof floor.sortOrder !== "number" ||
-    !Number.isSafeInteger(floor.sortOrder) ||
-    floor.sortOrder < 0
-  ) {
-    throw new Error("floor.sortOrder must be a non-negative integer.");
+  assertSortOrder(floor.sortOrder, "floor.sortOrder");
 }
+
+function upgradeLegacyFloorSnapshot(floor: unknown): ProjectFloorSnapshot {
+  assertLegacyProjectFloorSnapshot(floor);
+  return { ...floor, buildingId: null };
 }

 export function assertProjectRoomSnapshot(
@ -310,11 +580,11 @@ export function assertProjectRoomSnapshot(
  assertNormalizedNonEmptyString(room.roomName, "room.roomName");
 }

-function assertProjectFloorStructureProjectCommand(
+function assertProjectBuildingStructureProjectCommand(
  command: SerializedProjectCommand<unknown>,
  expectedType:
-    | typeof projectFloorInsertCommandType
-    | typeof projectFloorDeleteCommandType
+    | typeof projectBuildingInsertCommandType
+    | typeof projectBuildingDeleteCommandType
 ) {
  if (
    command.schemaVersion !== projectLocationStructureCommandSchemaVersion ||
@ -322,6 +592,29 @@ function assertProjectFloorStructureProjectCommand(
    !isPlainObject(command.payload) ||
    Object.keys(command.payload).length !== 1
  ) {
+    throw new Error("Unsupported project-building structure command.");
+  }
+  assertProjectBuildingSnapshot(command.payload.building);
+}
+
+function assertProjectFloorStructureProjectCommand(
+  command: SerializedProjectCommand<unknown>,
+  expectedType:
+    | typeof projectFloorInsertCommandType
+    | typeof projectFloorDeleteCommandType
+) {
+  if (
+    command.type !== expectedType ||
+    !isPlainObject(command.payload) ||
+    Object.keys(command.payload).length !== 1
+  ) {
+    throw new Error("Unsupported project-floor structure command.");
+  }
+  if (command.schemaVersion === legacyProjectFloorCommandSchemaVersion) {
+    assertLegacyProjectFloorSnapshot(command.payload.floor);
+    return;
+  }
+  if (command.schemaVersion !== projectFloorCommandSchemaVersion) {
    throw new Error("Unsupported project-floor structure command.");
  }
  assertProjectFloorSnapshot(command.payload.floor);
@ -344,6 +637,16 @@ function assertProjectRoomStructureProjectCommand(
  assertProjectRoomSnapshot(command.payload.room);
 }

+function assertSortOrder(value: unknown, field: string) {
+  if (
+    typeof value !== "number" ||
+    !Number.isSafeInteger(value) ||
+    value < 0
+  ) {
+    throw new Error(`${field} must be a non-negative integer.`);
+  }
+}
+
 function assertNormalizedNonEmptyString(
  value: unknown,
  field: string
--- a/src/domain/models/project-state-snapshot.model.ts
+++ b/src/domain/models/project-state-snapshot.model.ts
@ -35,7 +35,8 @@ import {
  type ExternalModelStateSnapshot,
 } from "../../external-model/domain/external-model-contracts.js";

-export const projectStateSnapshotSchemaVersion = 6 as const;
+export const projectStateSnapshotSchemaVersion = 7 as const;
+const coverProjectStateSnapshotSchemaVersion = 6 as const;
 const boardInfoProjectStateSnapshotSchemaVersion = 5 as const;
 const previousProjectStateSnapshotSchemaVersion = 4 as const;
 const externalModelProjectStateSnapshotSchemaVersion = 3 as const;
@ -237,7 +238,7 @@ const projectDeviceSchema = z
  })
  .strict();

-const floorSchema = z
+const buildingSchema = z
  .object({
    id: idSchema,
    projectId: idSchema,
@ -246,6 +247,19 @@ const floorSchema = z
  })
  .strict();

+const legacyFloorSchema = z
+  .object({
+    id: idSchema,
+    projectId: idSchema,
+    name: z.string().trim().min(1),
+    sortOrder: finiteNumberSchema,
+  })
+  .strict();
+
+const floorSchema = legacyFloorSchema
+  .extend({ buildingId: idSchema.nullable() })
+  .strict();
+
 const roomSchema = z
  .object({
    id: idSchema,
@ -490,11 +504,24 @@ export function parseExternalModelStateSnapshot(
  return externalModelStateSchema.parse(value);
 }

-const projectStateSnapshotContents = {
+// Versions 1-6 have floors without a building and no building list.
+const previousProjectStateSnapshotContents = {
  circuitLists: z.array(circuitListSchema),
  circuitSections: z.array(circuitSectionSchema),
  circuits: z.array(circuitSchema),
  projectDevices: z.array(projectDeviceSchema),
+  floors: z.array(legacyFloorSchema),
+  rooms: z.array(roomSchema),
+  distributionBoardComponents: z.array(distributionBoardComponentSchema),
+  circuitProtectionDevices: z.array(circuitProtectionDeviceSchema),
+  distributionBoardComponentProtectionDevices: z.array(
+    componentProtectionDeviceSchema
+  ),
+};
+
+const projectStateSnapshotContents = {
+  ...previousProjectStateSnapshotContents,
+  buildings: z.array(buildingSchema),
  floors: z.array(floorSchema),
  rooms: z.array(roomSchema),
  distributionBoardComponents: z.array(distributionBoardComponentSchema),
@ -515,6 +542,17 @@ export const projectStateSnapshotSchema = z
  })
  .strict();

+const coverProjectStateSnapshotSchema = z
+  .object({
+    schemaVersion: z.literal(coverProjectStateSnapshotSchemaVersion),
+    project: projectSchema,
+    distributionBoards: z.array(distributionBoardSchema),
+    externalCsvConfiguration: externalCsvConfigurationSnapshotSchema.nullable(),
+    externalModel: externalModelStateSchema,
+    ...previousProjectStateSnapshotContents,
+  })
+  .strict();
+
 const boardInfoProjectStateSnapshotSchema = z
  .object({
    schemaVersion: z.literal(boardInfoProjectStateSnapshotSchemaVersion),
@ -522,7 +560,7 @@ const boardInfoProjectStateSnapshotSchema = z
    distributionBoards: z.array(legacyDistributionBoardSchema),
    externalCsvConfiguration: externalCsvConfigurationSnapshotSchema.nullable(),
    externalModel: externalModelStateSchema,
-    ...projectStateSnapshotContents,
+    ...previousProjectStateSnapshotContents,
  })
  .strict();

@ -533,7 +571,7 @@ const previousProjectStateSnapshotSchema = z
    distributionBoards: z.array(legacyDistributionBoardSchema),
    externalCsvConfiguration: externalCsvConfigurationSnapshotSchema.nullable(),
    externalModel: externalModelStateSchema,
-    ...projectStateSnapshotContents,
+    ...previousProjectStateSnapshotContents,
  })
  .strict();

@ -543,7 +581,7 @@ const externalModelProjectStateSnapshotSchema = z
    project: publicBuildingProjectSchema,
    distributionBoards: z.array(legacyDistributionBoardSchema),
    externalCsvConfiguration: externalCsvConfigurationSnapshotSchema.nullable(),
-    ...projectStateSnapshotContents,
+    ...previousProjectStateSnapshotContents,
  })
  .strict();

@ -552,7 +590,7 @@ const legacyProjectStateSnapshotSchema = z
    schemaVersion: z.literal(legacyProjectStateSnapshotSchemaVersion),
    project: publicBuildingProjectSchema,
    distributionBoards: z.array(legacyDistributionBoardSchema),
-    ...projectStateSnapshotContents,
+    ...previousProjectStateSnapshotContents,
  })
  .strict();

@ -561,7 +599,7 @@ const baselineProjectStateSnapshotSchema = z
    schemaVersion: z.literal(baselineProjectStateSnapshotSchemaVersion),
    project: previousProjectSchema,
    distributionBoards: z.array(legacyDistributionBoardSchema),
-    ...projectStateSnapshotContents,
+    ...previousProjectStateSnapshotContents,
  })
  .strict();

@ -595,6 +633,28 @@ export function parseProjectStateSnapshot(
 }

 function upgradePreviousProjectStateSnapshot(value: unknown): unknown {
+  const cover = upgradeToCoverProjectStateSnapshot(value);
+  if (
+    cover === null ||
+    typeof cover !== "object" ||
+    Array.isArray(cover) ||
+    (cover as { schemaVersion?: unknown }).schemaVersion !==
+      coverProjectStateSnapshotSchemaVersion
+  ) {
+    return cover;
+  }
+  // Every supported version before 7 lacks buildings; its floors had no
+  // building.
+  const previous = coverProjectStateSnapshotSchema.parse(cover);
+  return {
+    ...previous,
+    schemaVersion: projectStateSnapshotSchemaVersion,
+    buildings: [],
+    floors: previous.floors.map((floor) => ({ ...floor, buildingId: null })),
+  };
+}
+
+function upgradeToCoverProjectStateSnapshot(value: unknown): unknown {
  if (
    value === null ||
    typeof value !== "object" ||
@ -611,7 +671,7 @@ function upgradePreviousProjectStateSnapshot(value: unknown): unknown {
    const previous = boardInfoProjectStateSnapshotSchema.parse(value);
    return {
      ...previous,
-      schemaVersion: projectStateSnapshotSchemaVersion,
+      schemaVersion: coverProjectStateSnapshotSchemaVersion,
      project: { ...previous.project, ...emptyProjectCoverValues },
      distributionBoards: withEmptyDistributionBoardInfo(
        previous.distributionBoards
@ -622,7 +682,7 @@ function upgradePreviousProjectStateSnapshot(value: unknown): unknown {
    const previous = previousProjectStateSnapshotSchema.parse(value);
    return {
      ...previous,
-      schemaVersion: projectStateSnapshotSchemaVersion,
+      schemaVersion: coverProjectStateSnapshotSchemaVersion,
      project: { ...previous.project, ...emptyProjectCoverValues },
      distributionBoards: withEmptyDistributionBoardInfo(
        previous.distributionBoards
@ -633,7 +693,7 @@ function upgradePreviousProjectStateSnapshot(value: unknown): unknown {
    const previous = externalModelProjectStateSnapshotSchema.parse(value);
    return {
      ...previous,
-      schemaVersion: projectStateSnapshotSchemaVersion,
+      schemaVersion: coverProjectStateSnapshotSchemaVersion,
      project: { ...previous.project, ...emptyProjectCoverValues },
      distributionBoards: withEmptyDistributionBoardInfo(
        previous.distributionBoards
@ -645,7 +705,7 @@ function upgradePreviousProjectStateSnapshot(value: unknown): unknown {
    const legacy = legacyProjectStateSnapshotSchema.parse(value);
    return {
      ...legacy,
-      schemaVersion: projectStateSnapshotSchemaVersion,
+      schemaVersion: coverProjectStateSnapshotSchemaVersion,
      project: { ...legacy.project, ...emptyProjectCoverValues },
      distributionBoards: withEmptyDistributionBoardInfo(
        legacy.distributionBoards
@ -658,7 +718,7 @@ function upgradePreviousProjectStateSnapshot(value: unknown): unknown {
    const baseline = baselineProjectStateSnapshotSchema.parse(value);
    return {
      ...baseline,
-      schemaVersion: projectStateSnapshotSchemaVersion,
+      schemaVersion: coverProjectStateSnapshotSchemaVersion,
      project: {
        ...baseline.project,
        isPublicBuilding: false,
@ -765,6 +825,7 @@ function assertProjectStateSnapshotRelations(
    snapshot.projectDevices,
    "project device"
  );
+  const buildingIds = uniqueIds(snapshot.buildings, "building");
  const floorIds = uniqueIds(snapshot.floors, "floor");
  const roomIds = uniqueIds(snapshot.rooms, "room");
  const circuitIds = uniqueIds(snapshot.circuits, "circuit");
@ -843,8 +904,14 @@ function assertProjectStateSnapshotRelations(
      );
    }
  }
+  for (const building of snapshot.buildings) {
+    assertProjectOwnership(building.projectId, projectId, "building");
+  }
  for (const floor of snapshot.floors) {
    assertProjectOwnership(floor.projectId, projectId, "floor");
+    if (floor.buildingId !== null) {
+      assertReference(buildingIds, floor.buildingId, "floor building");
+    }
  }
  for (const room of snapshot.rooms) {
    assertProjectOwnership(room.projectId, projectId, "room");
--- a/src/domain/models/project-transfer.model.ts
+++ b/src/domain/models/project-transfer.model.ts
@ -84,6 +84,7 @@ export function remapProjectState(
    createId
  );
  const deviceIds = idMap(source.projectDevices, createId);
+  const buildingIds = idMap(source.buildings, createId);
  const floorIds = idMap(source.floors, createId);
  const roomIds = idMap(source.rooms, createId);
  const rowIds = new Map(
@ -242,10 +243,19 @@ export function remapProjectState(
      id: requiredId(deviceIds, device.id),
      projectId: targetProjectId,
    })),
+    buildings: source.buildings.map((building) => ({
+      ...building,
+      id: requiredId(buildingIds, building.id),
+      projectId: targetProjectId,
+    })),
    floors: source.floors.map((floor) => ({
      ...floor,
      id: requiredId(floorIds, floor.id),
      projectId: targetProjectId,
+      buildingId:
+        floor.buildingId === null
+          ? null
+          : requiredId(buildingIds, floor.buildingId),
    })),
    rooms: source.rooms.map((room) => ({
      ...room,
--- a/src/domain/services/project-command.service.ts
+++ b/src/domain/services/project-command.service.ts
@ -70,12 +70,18 @@ import {
  type SerializedProjectCommand,
 } from "../models/project-command.model.js";
 import {
+  assertProjectBuildingDeleteProjectCommand,
+  assertProjectBuildingInsertProjectCommand,
+  assertProjectBuildingUpdateProjectCommand,
  assertProjectFloorDeleteProjectCommand,
  assertProjectFloorInsertProjectCommand,
  assertProjectFloorUpdateProjectCommand,
  assertProjectRoomDeleteProjectCommand,
  assertProjectRoomInsertProjectCommand,
  assertProjectRoomUpdateProjectCommand,
+  projectBuildingDeleteCommandType,
+  projectBuildingInsertCommandType,
+  projectBuildingUpdateCommandType,
  projectFloorDeleteCommandType,
  projectFloorInsertCommandType,
  projectFloorUpdateCommandType,
@ -492,6 +498,18 @@ export class ProjectCommandService implements ProjectCommandExecutor {
          command: input.command,
        }).revision;
      }
+      case projectBuildingInsertCommandType: {
+        assertProjectBuildingInsertProjectCommand(input.command);
+        return this.projectLocationStructureStore.execute({ ...input, command: input.command }).revision;
+      }
+      case projectBuildingDeleteCommandType: {
+        assertProjectBuildingDeleteProjectCommand(input.command);
+        return this.projectLocationStructureStore.execute({ ...input, command: input.command }).revision;
+      }
+      case projectBuildingUpdateCommandType: {
+        assertProjectBuildingUpdateProjectCommand(input.command);
+        return this.projectLocationStructureStore.execute({ ...input, command: input.command }).revision;
+      }
      case projectFloorInsertCommandType: {
        assertProjectFloorInsertProjectCommand(input.command);
        return this.projectLocationStructureStore.execute({
--- a/src/frontend/components/Sidebar.tsx
+++ b/src/frontend/components/Sidebar.tsx
@ -7,14 +7,21 @@ import { listProjects } from "../utils/api";
 import type { ProjectDto } from "../types";
 import { ThemeToggle } from "./ThemeToggle";

+// Anchors on the project overview page (/projects/[id]).
 const PROJECT_SECTIONS = [
  { id: "verlauf", label: "Verlauf", icon: "↺" },
  { id: "verteilungen", label: "Verteilungen", icon: "⌁" },
-  { id: "etagen", label: "Etagen", icon: "▥" },
-  { id: "raeume", label: "Räume", icon: "⌂" },
  { id: "projektgeraete", label: "Projektgeräte", icon: "⚙" },
 ] as const;

+// Separate project sub-routes (/projects/[id]/<segment>).
+const PROJECT_ROUTES = [
+  { segment: "struktur", label: "Gebäudestruktur", icon: "▥" },
+] as const;
+
+// Matches the project overview and all of its sub-routes (group 2 = rest).
+const PROJECT_PATH_PATTERN = /^\/projects\/([^/]+)(\/.*)?$/;
+
 interface SidebarProps {
  isCollapsed: boolean;
  onToggleCollapsed: () => void;
@ -22,8 +29,14 @@ interface SidebarProps {

 export function Sidebar({ isCollapsed, onToggleCollapsed }: SidebarProps) {
  const pathname = usePathname();
-  const projectMatch = pathname?.match(/^\/projects\/([^/]+)$/);
+  const projectMatch = pathname?.match(PROJECT_PATH_PATTERN);
  const activeProjectId = projectMatch ? projectMatch[1] : null;
+  const projectSubPath = projectMatch?.[2] ?? "";
+  // Section anchors only exist (and are observed) on the overview itself.
+  const overviewProjectId =
+    activeProjectId && (projectSubPath === "" || projectSubPath === "/")
+      ? activeProjectId
+      : null;

  const [projects, setProjects] = useState<ProjectDto[]>([]);
  const [activeSection, setActiveSection] = useState<string | null>(null);
@ -42,7 +55,7 @@ export function Sidebar({ isCollapsed, onToggleCollapsed }: SidebarProps) {
  }, []);

  useEffect(() => {
-    if (!activeProjectId) {
+    if (!overviewProjectId) {
      setActiveSection(null);
      return;
    }
@ -69,7 +82,7 @@ export function Sidebar({ isCollapsed, onToggleCollapsed }: SidebarProps) {

    elements.forEach((element) => observer.observe(element));
    return () => observer.disconnect();
-  }, [activeProjectId]);
+  }, [overviewProjectId]);

  return (
    <aside className={isCollapsed ? "sidebar sidebar-collapsed" : "sidebar"}>
@ -122,7 +135,8 @@ export function Sidebar({ isCollapsed, onToggleCollapsed }: SidebarProps) {
        {activeProjectId && (
          <>
            <div className="sidebar-section-label">In diesem Projekt</div>
-            {PROJECT_SECTIONS.map(({ id, label, icon }) => (
+            {PROJECT_SECTIONS.map(({ id, label, icon }) =>
+              overviewProjectId ? (
                <a
                  key={id}
                  href={`#${id}`}
@ -132,7 +146,35 @@ export function Sidebar({ isCollapsed, onToggleCollapsed }: SidebarProps) {
                  <span className="sidebar-icon">{icon}</span>
                  <span className="sidebar-label">{label}</span>
                </a>
-            ))}
+              ) : (
+                <Link
+                  key={id}
+                  href={`/projects/${activeProjectId}#${id}`}
+                  className="sidebar-link"
+                  title={label}
+                >
+                  <span className="sidebar-icon">{icon}</span>
+                  <span className="sidebar-label">{label}</span>
+                </Link>
+              )
+            )}
+            {PROJECT_ROUTES.map(({ segment, label, icon }) => {
+              const href = `/projects/${activeProjectId}/${segment}`;
+              const isActive =
+                pathname === href || Boolean(pathname?.startsWith(`${href}/`));
+              return (
+                <Link
+                  key={segment}
+                  href={href}
+                  aria-current={isActive ? "page" : undefined}
+                  className={isActive ? "sidebar-link active" : "sidebar-link"}
+                  title={label}
+                >
+                  <span className="sidebar-icon">{icon}</span>
+                  <span className="sidebar-label">{label}</span>
+                </Link>
+              );
+            })}
          </>
        )}
      </nav>
--- a/src/frontend/components/floor-select-options.tsx
+++ b/src/frontend/components/floor-select-options.tsx
@ -0,0 +1,49 @@
+"use client";
+
+import React from "react";
+import type { BuildingDto, FloorDto } from "../types";
+import { groupFloorsByBuilding } from "../utils/building-structure";
+
+export type FloorOption = Pick<FloorDto, "id" | "name"> & {
+  buildingId?: string | null;
+};
+
+interface FloorSelectOptionsProps {
+  buildings?: ReadonlyArray<Pick<BuildingDto, "id" | "name">>;
+  floors: ReadonlyArray<FloorOption>;
+}
+
+/**
+ * Floor <option>s for a <select>, grouped by building via <optgroup> as soon
+ * as the project has buildings. The caller renders its own empty option
+ * ("Ohne Etage") before these.
+ */
+export function FloorSelectOptions({
+  buildings = [],
+  floors,
+}: FloorSelectOptionsProps) {
+  if (!buildings.length) {
+    return (
+      <>
+        {floors.map((floor) => (
+          <option key={floor.id} value={floor.id}>
+            {floor.name}
+          </option>
+        ))}
+      </>
+    );
+  }
+  return (
+    <>
+      {groupFloorsByBuilding(floors, buildings).map((group) => (
+        <optgroup key={group.buildingId ?? "unassigned"} label={group.label}>
+          {group.floors.map((floor) => (
+            <option key={floor.id} value={floor.id}>
+              {floor.name}
+            </option>
+          ))}
+        </optgroup>
+      ))}
+    </>
+  );
+}
--- a/src/frontend/components/room-modal.tsx
+++ b/src/frontend/components/room-modal.tsx
@ -1,11 +1,14 @@
 "use client";

 import React, { type FormEvent, useState } from "react";
-import type { CreateRoomInput, FloorDto, RoomDto } from "../types";
+import type { BuildingDto, CreateRoomInput, RoomDto } from "../types";
+import { FloorSelectOptions, type FloorOption } from "./floor-select-options";
 import { FormModal } from "./form-modal";

 interface RoomModalProps {
-  floors: FloorDto[];
+  /** When given, the floor select groups floors by building. */
+  buildings?: ReadonlyArray<Pick<BuildingDto, "id" | "name">>;
+  floors: ReadonlyArray<FloorOption>;
  initialRoom?: RoomDto;
  initialValues?: Partial<CreateRoomInput>;
  isSaving: boolean;
@ -15,6 +18,7 @@ interface RoomModalProps {
 }

 export function RoomModal({
+  buildings,
  floors,
  initialRoom,
  initialValues,
@ -83,7 +87,7 @@ export function RoomModal({
            value={floorId}
          >
            <option value="">Ohne Etage</option>
-            {floors.map((floor) => <option key={floor.id} value={floor.id}>{floor.name}</option>)}
+            <FloorSelectOptions buildings={buildings} floors={floors} />
          </select>
        </div>
      </div>
--- a/src/frontend/types.ts
+++ b/src/frontend/types.ts
@ -226,11 +226,24 @@ export interface CircuitListDto {
  name: string;
 }

+export interface BuildingDto {
+  id: string;
+  projectId: string;
+  name: string;
+  sortOrder: number;
+}
+
+export interface ProjectBuildingCommandResultDto
+  extends ProjectCommandResultDto {
+  building: BuildingDto;
+}
+
 export interface FloorDto {
  id: string;
  projectId: string;
  name: string;
  sortOrder: number;
+  buildingId: string | null;
 }

 export interface ProjectFloorCommandResultDto
@ -288,8 +301,14 @@ export interface ProjectDeviceCommandResultDto
  projectDevice: ProjectDeviceDto;
 }

+export interface CreateBuildingInput {
+  name: string;
+}
+
 export interface CreateFloorInput {
  name: string;
+  /** Omitted = keep the current assignment on update; null = no building. */
+  buildingId?: string | null;
 }

 export interface CreateRoomInput {
--- a/src/frontend/utils/api.ts
+++ b/src/frontend/utils/api.ts
@ -1,5 +1,7 @@
 import type {
+  BuildingDto,
  CircuitListDto,
+  CreateBuildingInput,
  CreateFloorInput,
  CreateProjectDeviceInput,
  CreateRoomInput,
@ -17,6 +19,7 @@ import type {
  ProjectCommandResultDto,
  ProjectDeviceCommandResultDto,
  ProjectDeviceDto,
+  ProjectBuildingCommandResultDto,
  ProjectFloorCommandResultDto,
  ProjectHistoryStateDto,
  ProjectRevisionPageDto,
@ -881,6 +884,61 @@ export function renumberCircuitSectionCommand(
  );
 }

+export function listBuildings(projectId: string) {
+  return request<BuildingDto[]>(`/api/projects/${projectId}/buildings`);
+}
+
+export function createBuilding(
+  projectId: string,
+  input: CreateBuildingInput,
+  expectedRevision: number
+) {
+  return request<ProjectBuildingCommandResultDto>(
+    `/api/projects/${projectId}/buildings`,
+    {
+      method: "POST",
+      body: JSON.stringify({ name: input.name, expectedRevision }),
+    }
+  );
+}
+
+export function updateBuilding(
+  projectId: string,
+  buildingId: string,
+  input: CreateBuildingInput,
+  expectedRevision: number
+) {
+  return request<ProjectBuildingCommandResultDto>(
+    `/api/projects/${projectId}/buildings/${buildingId}`,
+    {
+      method: "PUT",
+      body: JSON.stringify({ name: input.name, expectedRevision }),
+    }
+  );
+}
+
+export function deleteBuilding(
+  projectId: string,
+  buildingId: string,
+  expectedRevision: number
+) {
+  return request<ProjectCommandResultDto>(
+    `/api/projects/${projectId}/buildings/${buildingId}`,
+    {
+      method: "DELETE",
+      body: JSON.stringify({ expectedRevision }),
+    }
+  );
+}
+
+// buildingId is only sent when the caller passes it (string or null);
+// an omitted buildingId keeps the current assignment on update.
+function toFloorRequestBody(input: CreateFloorInput, expectedRevision: number) {
+  return input.buildingId === undefined
+    ? { name: input.name, expectedRevision }
+    : { name: input.name, buildingId: input.buildingId, expectedRevision };
+}
+
 export function listFloors(projectId: string) {
  return request<FloorDto[]>(`/api/projects/${projectId}/floors`);
 }
@ -894,7 +952,7 @@ export function createFloor(
    `/api/projects/${projectId}/floors`,
    {
      method: "POST",
-      body: JSON.stringify({ ...input, expectedRevision }),
+      body: JSON.stringify(toFloorRequestBody(input, expectedRevision)),
    }
  );
 }
@ -907,7 +965,7 @@ export function updateFloor(
 ) {
  return request<ProjectFloorCommandResultDto>(`/api/projects/${projectId}/floors/${floorId}`, {
    method: "PUT",
-    body: JSON.stringify({ ...input, expectedRevision }),
+    body: JSON.stringify(toFloorRequestBody(input, expectedRevision)),
  });
 }

--- a/src/frontend/utils/building-structure.ts
+++ b/src/frontend/utils/building-structure.ts
@ -0,0 +1,284 @@
+// Pure selection, filtering and grouping rules for the building structure
+// view (Gebäude → Etagen → Räume) and grouped floor selects.
+import type { BuildingDto, FloorDto, RoomDto } from "../types";
+
+/** Selection key for "Alle" in the building and floor columns. */
+export const ALL_SELECTION = "__all__";
+/** Selection key for "Ohne Gebäude" / "Ohne Etage". */
+export const UNASSIGNED_SELECTION = "__unassigned__";
+
+/** ALL_SELECTION, UNASSIGNED_SELECTION or a building/floor id. */
+export type StructureSelection = string;
+
+export const UNASSIGNED_BUILDING_LABEL = "Ohne Gebäude";
+export const UNASSIGNED_FLOOR_LABEL = "Ohne Etage";
+
+type BuildingLike = Pick<BuildingDto, "id" | "name">;
+type FloorLike = Pick<FloorDto, "id" | "name"> & {
+  buildingId?: string | null;
+};
+type RoomLike = Pick<RoomDto, "floorId">;
+
+export interface FloorGroup<TFloor extends FloorLike = FloorLike> {
+  /** null for floors without (known) building. */
+  buildingId: string | null;
+  label: string;
+  floors: TFloor[];
+}
+
+/**
+ * The building a floor belongs to, or null when it has none or references a
+ * building that is not (or no longer) loaded.
+ */
+export function resolveFloorBuildingId(
+  floor: FloorLike,
+  buildingIds: ReadonlySet<string>
+): string | null {
+  const buildingId = floor.buildingId ?? null;
+  return buildingId !== null && buildingIds.has(buildingId)
+    ? buildingId
+    : null;
+}
+
+function toIdSet(entries: ReadonlyArray<{ id: string }>) {
+  return new Set(entries.map((entry) => entry.id));
+}
+
+export function hasFloorsWithoutBuilding(
+  floors: ReadonlyArray<FloorLike>,
+  buildings: ReadonlyArray<BuildingLike>
+): boolean {
+  const buildingIds = toIdSet(buildings);
+  return floors.some(
+    (floor) => resolveFloorBuildingId(floor, buildingIds) === null
+  );
+}
+
+export function hasRoomsWithoutFloor(
+  rooms: ReadonlyArray<RoomLike>,
+  floors: ReadonlyArray<FloorLike>
+): boolean {
+  const floorIds = toIdSet(floors);
+  return rooms.some(
+    (room) => room.floorId === null || !floorIds.has(room.floorId)
+  );
+}
+
+/**
+ * Groups floors by building in building order; floors without a known
+ * building come last under "Ohne Gebäude". Empty groups are omitted and the
+ * floor order inside a group is preserved.
+ */
+export function groupFloorsByBuilding<TFloor extends FloorLike>(
+  floors: ReadonlyArray<TFloor>,
+  buildings: ReadonlyArray<BuildingLike>
+): FloorGroup<TFloor>[] {
+  const buildingIds = toIdSet(buildings);
+  const floorsByBuilding = new Map<string | null, TFloor[]>();
+  for (const floor of floors) {
+    const buildingId = resolveFloorBuildingId(floor, buildingIds);
+    const group = floorsByBuilding.get(buildingId) ?? [];
+    group.push(floor);
+    floorsByBuilding.set(buildingId, group);
+  }
+  const groups: FloorGroup<TFloor>[] = [];
+  for (const building of buildings) {
+    const groupFloors = floorsByBuilding.get(building.id);
+    if (groupFloors?.length) {
+      groups.push({
+        buildingId: building.id,
+        label: building.name,
+        floors: groupFloors,
+      });
+    }
+  }
+  const unassigned = floorsByBuilding.get(null);
+  if (unassigned?.length) {
+    groups.push({
+      buildingId: null,
+      label: UNASSIGNED_BUILDING_LABEL,
+      floors: unassigned,
+    });
+  }
+  return groups;
+}
+
+/** Floors shown in the floor column for a building selection. */
+export function filterFloorsByBuilding<TFloor extends FloorLike>(
+  floors: ReadonlyArray<TFloor>,
+  buildings: ReadonlyArray<BuildingLike>,
+  buildingSelection: StructureSelection
+): TFloor[] {
+  if (buildingSelection === ALL_SELECTION) {
+    return [...floors];
+  }
+  const buildingIds = toIdSet(buildings);
+  const targetBuildingId =
+    buildingSelection === UNASSIGNED_SELECTION ? null : buildingSelection;
+  return floors.filter(
+    (floor) => resolveFloorBuildingId(floor, buildingIds) === targetBuildingId
+  );
+}
+
+/**
+ * "Ohne Etage" is offered only when rooms without floor exist and the
+ * building selection can contain them ("Alle" or "Ohne Gebäude"), because a
+ * room without floor has no building either.
+ */
+export function showsRoomsWithoutFloorEntry(
+  rooms: ReadonlyArray<RoomLike>,
+  floors: ReadonlyArray<FloorLike>,
+  buildingSelection: StructureSelection
+): boolean {
+  return (
+    (buildingSelection === ALL_SELECTION ||
+      buildingSelection === UNASSIGNED_SELECTION) &&
+    hasRoomsWithoutFloor(rooms, floors)
+  );
+}
+
+/** Rooms shown in the room column for the current selections. */
+export function filterRooms<TRoom extends RoomLike>(
+  rooms: ReadonlyArray<TRoom>,
+  floors: ReadonlyArray<FloorLike>,
+  buildings: ReadonlyArray<BuildingLike>,
+  buildingSelection: StructureSelection,
+  floorSelection: StructureSelection
+): TRoom[] {
+  const floorIds = toIdSet(floors);
+  const isWithoutFloor = (room: RoomLike) =>
+    room.floorId === null || !floorIds.has(room.floorId);
+
+  if (floorSelection === UNASSIGNED_SELECTION) {
+    return showsRoomsWithoutFloorEntry(rooms, floors, buildingSelection)
+      ? rooms.filter(isWithoutFloor)
+      : [];
+  }
+  if (floorSelection !== ALL_SELECTION) {
+    return rooms.filter((room) => room.floorId === floorSelection);
+  }
+  if (buildingSelection === ALL_SELECTION) {
+    return [...rooms];
+  }
+  const visibleFloorIds = toIdSet(
+    filterFloorsByBuilding(floors, buildings, buildingSelection)
+  );
+  return rooms.filter(
+    (room) =>
+      (room.floorId !== null && visibleFloorIds.has(room.floorId)) ||
+      (buildingSelection === UNASSIGNED_SELECTION && isWithoutFloor(room))
+  );
+}
+
+/** Falls back to "Alle" when a selected building vanished. */
+export function normalizeBuildingSelection(
+  selection: StructureSelection,
+  floors: ReadonlyArray<FloorLike>,
+  buildings: ReadonlyArray<BuildingLike>
+): StructureSelection {
+  if (selection === ALL_SELECTION) {
+    return selection;
+  }
+  if (selection === UNASSIGNED_SELECTION) {
+    return hasFloorsWithoutBuilding(floors, buildings)
+      ? selection
+      : ALL_SELECTION;
+  }
+  return buildings.some((building) => building.id === selection)
+    ? selection
+    : ALL_SELECTION;
+}
+
+/** Falls back to "Alle" when a selected floor is no longer visible. */
+export function normalizeFloorSelection(
+  selection: StructureSelection,
+  rooms: ReadonlyArray<RoomLike>,
+  floors: ReadonlyArray<FloorLike>,
+  buildings: ReadonlyArray<BuildingLike>,
+  buildingSelection: StructureSelection
+): StructureSelection {
+  if (selection === ALL_SELECTION) {
+    return selection;
+  }
+  if (selection === UNASSIGNED_SELECTION) {
+    return showsRoomsWithoutFloorEntry(rooms, floors, buildingSelection)
+      ? selection
+      : ALL_SELECTION;
+  }
+  return filterFloorsByBuilding(floors, buildings, buildingSelection).some(
+    (floor) => floor.id === selection
+  )
+    ? selection
+    : ALL_SELECTION;
+}
+
+/** Building preselected in the floor dialog ("" = Ohne Gebäude). */
+export function defaultBuildingIdForNewFloor(
+  buildingSelection: StructureSelection
+): string {
+  return buildingSelection === ALL_SELECTION ||
+    buildingSelection === UNASSIGNED_SELECTION
+    ? ""
+    : buildingSelection;
+}
+
+/** Floor preselected in the room dialog ("" = Ohne Etage). */
+export function defaultFloorIdForNewRoom(
+  floorSelection: StructureSelection
+): string {
+  return floorSelection === ALL_SELECTION ||
+    floorSelection === UNASSIGNED_SELECTION
+    ? ""
+    : floorSelection;
+}
+
+export function countFloorsByBuilding(
+  floors: ReadonlyArray<FloorLike>,
+  buildings: ReadonlyArray<BuildingLike>
+): Map<string | null, number> {
+  const buildingIds = toIdSet(buildings);
+  const counts = new Map<string | null, number>();
+  for (const floor of floors) {
+    const buildingId = resolveFloorBuildingId(floor, buildingIds);
+    counts.set(buildingId, (counts.get(buildingId) ?? 0) + 1);
+  }
+  return counts;
+}
+
+/** Room counts per floor id; rooms without known floor count under null. */
+export function countRoomsByFloor(
+  rooms: ReadonlyArray<RoomLike>,
+  floors: ReadonlyArray<FloorLike>
+): Map<string | null, number> {
+  const floorIds = toIdSet(floors);
+  const counts = new Map<string | null, number>();
+  for (const room of rooms) {
+    const floorId =
+      room.floorId !== null && floorIds.has(room.floorId)
+        ? room.floorId
+        : null;
+    counts.set(floorId, (counts.get(floorId) ?? 0) + 1);
+  }
+  return counts;
+}
+
+/** "EG · Haus A", "EG" (floor without building) or "Ohne Etage". */
+export function describeRoomLocation(
+  room: RoomLike,
+  floors: ReadonlyArray<FloorLike>,
+  buildings: ReadonlyArray<BuildingLike>
+): string {
+  const floor =
+    room.floorId === null
+      ? undefined
+      : floors.find((entry) => entry.id === room.floorId);
+  if (!floor) {
+    return UNASSIGNED_FLOOR_LABEL;
+  }
+  const buildingId = resolveFloorBuildingId(floor, toIdSet(buildings));
+  const building =
+    buildingId === null
+      ? undefined
+      : buildings.find((entry) => entry.id === buildingId);
+  return building ? `${floor.name} · ${building.name}` : floor.name;
+}
--- a/src/frontend/utils/project-version-history.ts
+++ b/src/frontend/utils/project-version-history.ts
@ -46,6 +46,9 @@ const commandTypeLabels: Record<string, string> = {
  "circuit.move-group": "Stromkreis in andere Gruppe verschoben",
  "circuit-group.delete-subtree": "Stromkreisgruppe vollständig entfernt",
  "circuit-group.restore-subtree": "Stromkreisgruppe vollständig wiederhergestellt",
+  "project-building.insert": "Gebäude angelegt",
+  "project-building.update": "Gebäude bearbeitet",
+  "project-building.delete": "Gebäude entfernt",
  "project-floor.insert": "Geschoss angelegt",
  "project-floor.update": "Geschoss bearbeitet",
  "project-floor.delete": "Geschoss entfernt",
--- a/src/server/composition/application-repositories.ts
+++ b/src/server/composition/application-repositories.ts
@ -6,6 +6,7 @@ import { CircuitRepository } from "../../db/repositories/circuit.repository.js";
 import { CircuitProtectionDeviceRepository } from "../../db/repositories/circuit-protection-device.repository.js";
 import { DistributionBoardComponentRepository } from "../../db/repositories/distribution-board-component.repository.js";
 import { DistributionBoardRepository } from "../../db/repositories/distribution-board.repository.js";
+import { BuildingRepository } from "../../db/repositories/building.repository.js";
 import { FloorRepository } from "../../db/repositories/floor.repository.js";
 import { GlobalDeviceRepository } from "../../db/repositories/global-device.repository.js";
 import { ProjectDeviceRepository } from "../../db/repositories/project-device.repository.js";
@ -26,6 +27,7 @@ export const distributionBoardComponentRepository =
  new DistributionBoardComponentRepository(db);
 export const distributionBoardRepository =
  new DistributionBoardRepository(db);
+export const buildingRepository = new BuildingRepository(db);
 export const floorRepository = new FloorRepository(db);
 export const globalDeviceRepository = new GlobalDeviceRepository(db);
 export const projectDeviceRepository = new ProjectDeviceRepository(db);
--- a/src/server/controllers/building.controller.ts
+++ b/src/server/controllers/building.controller.ts
@ -0,0 +1,111 @@
+import type { Request, Response } from "express";
+import {
+  createProjectBuildingDeleteProjectCommand,
+  createProjectBuildingInsertProjectCommand,
+  createProjectBuildingSnapshot,
+  createProjectBuildingUpdateProjectCommand,
+} from "../../domain/models/project-location-structure-project-command.model.js";
+import {
+  createBuildingSchema,
+  deleteProjectLocationSchema,
+  updateBuildingSchema,
+} from "../../shared/validation/project-structure.schemas.js";
+import { projectCommandService } from "../composition/project-command-stores.js";
+import { buildingRepository } from "../composition/application-repositories.js";
+import { respondWithProjectCommandError } from "./project-command.controller.js";
+
+export async function listBuildingsByProject(req: Request, res: Response) {
+  const { projectId } = req.params;
+  if (typeof projectId !== "string") {
+    return res.status(400).json({ error: "Invalid projectId" });
+  }
+
+  const result = await buildingRepository.listByProject(projectId);
+  return res.json(result);
+}
+
+export async function createBuilding(req: Request, res: Response) {
+  const { projectId } = req.params;
+  if (typeof projectId !== "string") {
+    return res.status(400).json({ error: "Invalid projectId" });
+  }
+
+  const parsed = createBuildingSchema.safeParse(req.body);
+  if (!parsed.success) {
+    return res.status(400).json({ error: parsed.error.flatten() });
+  }
+
+  const existingBuildings = await buildingRepository.listByProject(projectId);
+  const nextSortOrder = existingBuildings.length
+    ? Math.max(...existingBuildings.map((building) => building.sortOrder)) + 1
+    : 0;
+  const building = createProjectBuildingSnapshot(
+    projectId,
+    parsed.data.name,
+    nextSortOrder
+  );
+  try {
+    const result = projectCommandService.executeUser({
+      projectId,
+      expectedRevision: parsed.data.expectedRevision,
+      description: "Gebäude anlegen",
+      command: createProjectBuildingInsertProjectCommand(building),
+    });
+    return res.status(201).json({ ...result, building });
+  } catch (error) {
+    return respondWithProjectCommandError(error, res);
+  }
+}
+
+export async function updateBuilding(req: Request, res: Response) {
+  const { projectId, buildingId } = req.params;
+  if (typeof projectId !== "string" || typeof buildingId !== "string") {
+    return res.status(400).json({ error: "Invalid parameters" });
+  }
+  const parsed = updateBuildingSchema.safeParse(req.body);
+  if (!parsed.success) {
+    return res.status(400).json({ error: parsed.error.flatten() });
+  }
+  const current = await buildingRepository.findById(projectId, buildingId);
+  if (!current) {
+    return res.status(404).json({ error: "Building not found" });
+  }
+  const target = { ...current, name: parsed.data.name };
+  try {
+    const result = projectCommandService.executeUser({
+      projectId,
+      expectedRevision: parsed.data.expectedRevision,
+      description: "Gebäude bearbeiten",
+      command: createProjectBuildingUpdateProjectCommand(current, target),
+    });
+    return res.json({ ...result, building: target });
+  } catch (error) {
+    return respondWithProjectCommandError(error, res);
+  }
+}
+
+export async function deleteBuilding(req: Request, res: Response) {
+  const { projectId, buildingId } = req.params;
+  if (typeof projectId !== "string" || typeof buildingId !== "string") {
+    return res.status(400).json({ error: "Invalid parameters" });
+  }
+  const parsed = deleteProjectLocationSchema.safeParse(req.body);
+  if (!parsed.success) {
+    return res.status(400).json({ error: parsed.error.flatten() });
+  }
+  const current = await buildingRepository.findById(projectId, buildingId);
+  if (!current) {
+    return res.status(404).json({ error: "Building not found" });
+  }
+  try {
+    const result = projectCommandService.executeUser({
+      projectId,
+      expectedRevision: parsed.data.expectedRevision,
+      description: "Gebäude löschen",
+      command: createProjectBuildingDeleteProjectCommand(current),
+    });
+    return res.json(result);
+  } catch (error) {
+    return respondWithProjectCommandError(error, res);
+  }
+}
--- a/src/server/controllers/floor.controller.ts
+++ b/src/server/controllers/floor.controller.ts
@ -42,7 +42,8 @@ export async function createFloor(req: Request, res: Response) {
  const floor = createProjectFloorSnapshot(
    projectId,
    parsed.data.name,
-    nextSortOrder
+    nextSortOrder,
+    parsed.data.buildingId ?? null
  );
  try {
    const result = projectCommandService.executeUser({
@ -70,7 +71,14 @@ export async function updateFloor(req: Request, res: Response) {
  if (!current) {
    return res.status(404).json({ error: "Floor not found" });
  }
-  const target = { ...current, name: parsed.data.name };
+  const target = {
+    ...current,
+    name: parsed.data.name,
+    buildingId:
+      parsed.data.buildingId === undefined
+        ? current.buildingId
+        : parsed.data.buildingId,
+  };
  try {
    const result = projectCommandService.executeUser({
      projectId,
--- a/src/server/routes/project.routes.ts
+++ b/src/server/routes/project.routes.ts
@ -13,6 +13,12 @@ import {
  updateDistributionBoard,
 } from "../controllers/distribution-board.controller.js";
 import { listCircuitListsByProject } from "../controllers/circuit-list.controller.js";
+import {
+  createBuilding,
+  deleteBuilding,
+  listBuildingsByProject,
+  updateBuilding,
+} from "../controllers/building.controller.js";
 import { createFloor, deleteFloor, listFloorsByProject, updateFloor } from "../controllers/floor.controller.js";
 import { createRoom, deleteRoom, listRoomsByProject, updateRoom } from "../controllers/room.controller.js";
 import { getCircuitTree } from "../controllers/circuit-tree.controller.js";
@ -100,6 +106,10 @@ projectRouter.get(
  "/:projectId/circuit-sections/:sectionId/next-identifier",
  getNextCircuitIdentifier
 );
+projectRouter.get("/:projectId/buildings", listBuildingsByProject);
+projectRouter.post("/:projectId/buildings", createBuilding);
+projectRouter.put("/:projectId/buildings/:buildingId", updateBuilding);
+projectRouter.delete("/:projectId/buildings/:buildingId", deleteBuilding);
 projectRouter.get("/:projectId/floors", listFloorsByProject);
 projectRouter.post("/:projectId/floors", createFloor);
 projectRouter.put("/:projectId/floors/:floorId", updateFloor);
--- a/src/shared/validation/project-structure.schemas.ts
+++ b/src/shared/validation/project-structure.schemas.ts
@ -102,10 +102,22 @@ export const deleteDistributionBoardSchema = z
  })
  .strict();

+export const createBuildingSchema = z
+  .object({
+    expectedRevision: expectedProjectRevisionSchema,
+    name: z.string().trim().min(1).max(200),
+  })
+  .strict();
+
+export const updateBuildingSchema = createBuildingSchema;
+
+// A missing buildingId keeps the current value (older clients send only the
+// name); null removes the floor from its building.
 export const createFloorSchema = z
  .object({
    expectedRevision: expectedProjectRevisionSchema,
    name: z.string().trim().min(1).max(200),
+    buildingId: z.string().trim().min(1).nullable().optional(),
  })
  .strict();

--- a/tests/building-structure-frontend-api.test.ts
+++ b/tests/building-structure-frontend-api.test.ts
@ -0,0 +1,100 @@
+import assert from "node:assert/strict";
+import { describe, it } from "node:test";
+import {
+  createBuilding,
+  createFloor,
+  deleteBuilding,
+  listBuildings,
+  updateBuilding,
+  updateFloor,
+} from "../src/frontend/utils/api.js";
+
+describe("building structure frontend API", () => {
+  it("sends building commands and optional floor building assignments", async () => {
+    const requests: Array<{ url: string; method: string; body: unknown }> = [];
+    const originalFetch = globalThis.fetch;
+    globalThis.fetch = async (input, init) => {
+      requests.push({
+        url: String(input),
+        method: init?.method ?? "GET",
+        body: init?.body === undefined ? undefined : JSON.parse(String(init.body)),
+      });
+      return new Response(JSON.stringify([]), {
+        status: 200,
+        headers: { "Content-Type": "application/json" },
+      });
+    };
+    try {
+      await listBuildings("project-1");
+      await createBuilding("project-1", { name: "Haus A" }, 3);
+      await updateBuilding("project-1", "building-1", { name: "Haus B" }, 4);
+      await deleteBuilding("project-1", "building-1", 5);
+      await createFloor(
+        "project-1",
+        { name: "EG", buildingId: "building-1" },
+        6
+      );
+      await createFloor("project-1", { name: "Dach", buildingId: null }, 7);
+      await createFloor("project-1", { name: "UG", buildingId: undefined }, 8);
+      await updateFloor(
+        "project-1",
+        "floor-1",
+        { name: "1. OG", buildingId: null },
+        9
+      );
+      await updateFloor("project-1", "floor-1", { name: "2. OG" }, 10);
+    } finally {
+      globalThis.fetch = originalFetch;
+    }
+
+    assert.deepEqual(requests, [
+      {
+        url: "/api/projects/project-1/buildings",
+        method: "GET",
+        body: undefined,
+      },
+      {
+        url: "/api/projects/project-1/buildings",
+        method: "POST",
+        body: { name: "Haus A", expectedRevision: 3 },
+      },
+      {
+        url: "/api/projects/project-1/buildings/building-1",
+        method: "PUT",
+        body: { name: "Haus B", expectedRevision: 4 },
+      },
+      {
+        url: "/api/projects/project-1/buildings/building-1",
+        method: "DELETE",
+        body: { expectedRevision: 5 },
+      },
+      {
+        url: "/api/projects/project-1/floors",
+        method: "POST",
+        body: { name: "EG", buildingId: "building-1", expectedRevision: 6 },
+      },
+      {
+        url: "/api/projects/project-1/floors",
+        method: "POST",
+        body: { name: "Dach", buildingId: null, expectedRevision: 7 },
+      },
+      {
+        url: "/api/projects/project-1/floors",
+        method: "POST",
+        body: { name: "UG", expectedRevision: 8 },
+      },
+      {
+        url: "/api/projects/project-1/floors/floor-1",
+        method: "PUT",
+        body: { name: "1. OG", buildingId: null, expectedRevision: 9 },
+      },
+      {
+        url: "/api/projects/project-1/floors/floor-1",
+        method: "PUT",
+        body: { name: "2. OG", expectedRevision: 10 },
+      },
+    ]);
+    const updateWithoutBuilding = requests[8].body as Record<string, unknown>;
+    assert.equal("buildingId" in updateWithoutBuilding, false);
+  });
+});
--- a/tests/building-structure-view.test.ts
+++ b/tests/building-structure-view.test.ts
@ -0,0 +1,284 @@
+import assert from "node:assert/strict";
+import { describe, it } from "node:test";
+import type {
+  BuildingDto,
+  FloorDto,
+  RoomDto,
+} from "../src/frontend/types.js";
+import {
+  ALL_SELECTION,
+  UNASSIGNED_SELECTION,
+  countFloorsByBuilding,
+  countRoomsByFloor,
+  defaultBuildingIdForNewFloor,
+  defaultFloorIdForNewRoom,
+  describeRoomLocation,
+  filterFloorsByBuilding,
+  filterRooms,
+  groupFloorsByBuilding,
+  hasFloorsWithoutBuilding,
+  hasRoomsWithoutFloor,
+  normalizeBuildingSelection,
+  normalizeFloorSelection,
+  showsRoomsWithoutFloorEntry,
+} from "../src/frontend/utils/building-structure.js";
+
+function building(id: string, name: string, sortOrder: number): BuildingDto {
+  return { id, projectId: "project-1", name, sortOrder };
+}
+
+function floor(
+  id: string,
+  name: string,
+  buildingId: string | null,
+  sortOrder = 0
+): FloorDto {
+  return { id, projectId: "project-1", name, sortOrder, buildingId };
+}
+
+function room(id: string, floorId: string | null, roomNumber = id): RoomDto {
+  return {
+    id,
+    projectId: "project-1",
+    floorId,
+    roomNumber,
+    roomName: `Raum ${roomNumber}`,
+  };
+}
+
+const buildings = [building("b-a", "Haus A", 0), building("b-b", "Haus B", 1)];
+const floors = [
+  floor("f-free", "Außenanlage", null, 0),
+  floor("f-a-eg", "EG", "b-a", 1),
+  floor("f-b-eg", "EG", "b-b", 2),
+  floor("f-a-og", "1. OG", "b-a", 3),
+  floor("f-orphan", "Keller", "b-deleted", 4),
+];
+const rooms = [
+  room("r-a-eg", "f-a-eg"),
+  room("r-a-og", "f-a-og"),
+  room("r-b-eg", "f-b-eg"),
+  room("r-free", "f-free"),
+  room("r-orphan-floor", "f-orphan"),
+  room("r-none", null),
+  room("r-missing-floor", "f-deleted"),
+];
+
+const ids = (entries: ReadonlyArray<{ id: string }>) =>
+  entries.map((entry) => entry.id);
+
+describe("building structure view", () => {
+  it("groups floors in building order and puts unassigned floors last", () => {
+    const groups = groupFloorsByBuilding(floors, buildings);
+    assert.deepEqual(
+      groups.map((group) => ({
+        buildingId: group.buildingId,
+        label: group.label,
+        floors: ids(group.floors),
+      })),
+      [
+        { buildingId: "b-a", label: "Haus A", floors: ["f-a-eg", "f-a-og"] },
+        { buildingId: "b-b", label: "Haus B", floors: ["f-b-eg"] },
+        {
+          buildingId: null,
+          label: "Ohne Gebäude",
+          floors: ["f-free", "f-orphan"],
+        },
+      ]
+    );
+  });
+
+  it("omits empty groups", () => {
+    const groups = groupFloorsByBuilding(
+      [floor("f-1", "EG", "b-b")],
+      buildings
+    );
+    assert.deepEqual(
+      groups.map((group) => group.buildingId),
+      ["b-b"]
+    );
+    assert.deepEqual(groupFloorsByBuilding([], buildings), []);
+  });
+
+  it("filters floors by building selection", () => {
+    assert.deepEqual(
+      ids(filterFloorsByBuilding(floors, buildings, ALL_SELECTION)),
+      ids(floors)
+    );
+    assert.deepEqual(
+      ids(filterFloorsByBuilding(floors, buildings, "b-a")),
+      ["f-a-eg", "f-a-og"]
+    );
+    assert.deepEqual(
+      ids(filterFloorsByBuilding(floors, buildings, UNASSIGNED_SELECTION)),
+      ["f-free", "f-orphan"]
+    );
+  });
+
+  it("detects floors without building and rooms without floor", () => {
+    assert.equal(hasFloorsWithoutBuilding(floors, buildings), true);
+    assert.equal(
+      hasFloorsWithoutBuilding([floor("f-1", "EG", "b-a")], buildings),
+      false
+    );
+    assert.equal(hasRoomsWithoutFloor(rooms, floors), true);
+    assert.equal(hasRoomsWithoutFloor([room("r-1", "f-a-eg")], floors), false);
+  });
+
+  it("filters rooms for building and floor selections", () => {
+    assert.deepEqual(
+      ids(filterRooms(rooms, floors, buildings, ALL_SELECTION, ALL_SELECTION)),
+      ids(rooms)
+    );
+    assert.deepEqual(
+      ids(filterRooms(rooms, floors, buildings, "b-a", ALL_SELECTION)),
+      ["r-a-eg", "r-a-og"]
+    );
+    assert.deepEqual(
+      ids(filterRooms(rooms, floors, buildings, "b-a", "f-a-og")),
+      ["r-a-og"]
+    );
+    assert.deepEqual(
+      ids(filterRooms(rooms, floors, buildings, ALL_SELECTION, "f-b-eg")),
+      ["r-b-eg"]
+    );
+    assert.deepEqual(
+      ids(
+        filterRooms(
+          rooms,
+          floors,
+          buildings,
+          UNASSIGNED_SELECTION,
+          ALL_SELECTION
+        )
+      ),
+      ["r-free", "r-orphan-floor", "r-none", "r-missing-floor"]
+    );
+    assert.deepEqual(
+      ids(
+        filterRooms(
+          rooms,
+          floors,
+          buildings,
+          ALL_SELECTION,
+          UNASSIGNED_SELECTION
+        )
+      ),
+      ["r-none", "r-missing-floor"]
+    );
+    assert.deepEqual(
+      ids(filterRooms(rooms, floors, buildings, "b-a", UNASSIGNED_SELECTION)),
+      []
+    );
+  });
+
+  it("offers 'Ohne Etage' only where rooms without floor can belong", () => {
+    assert.equal(showsRoomsWithoutFloorEntry(rooms, floors, ALL_SELECTION), true);
+    assert.equal(
+      showsRoomsWithoutFloorEntry(rooms, floors, UNASSIGNED_SELECTION),
+      true
+    );
+    assert.equal(showsRoomsWithoutFloorEntry(rooms, floors, "b-a"), false);
+    assert.equal(
+      showsRoomsWithoutFloorEntry([room("r-1", "f-a-eg")], floors, ALL_SELECTION),
+      false
+    );
+  });
+
+  it("falls back to 'Alle' for vanished or hidden selections", () => {
+    assert.equal(normalizeBuildingSelection("b-a", floors, buildings), "b-a");
+    assert.equal(
+      normalizeBuildingSelection("b-deleted", floors, buildings),
+      ALL_SELECTION
+    );
+    assert.equal(
+      normalizeBuildingSelection(UNASSIGNED_SELECTION, floors, buildings),
+      UNASSIGNED_SELECTION
+    );
+    assert.equal(
+      normalizeBuildingSelection(
+        UNASSIGNED_SELECTION,
+        [floor("f-1", "EG", "b-a")],
+        buildings
+      ),
+      ALL_SELECTION
+    );
+
+    assert.equal(
+      normalizeFloorSelection("f-a-og", rooms, floors, buildings, "b-a"),
+      "f-a-og"
+    );
+    assert.equal(
+      normalizeFloorSelection("f-b-eg", rooms, floors, buildings, "b-a"),
+      ALL_SELECTION
+    );
+    assert.equal(
+      normalizeFloorSelection("f-deleted", rooms, floors, buildings, ALL_SELECTION),
+      ALL_SELECTION
+    );
+    assert.equal(
+      normalizeFloorSelection(
+        UNASSIGNED_SELECTION,
+        rooms,
+        floors,
+        buildings,
+        ALL_SELECTION
+      ),
+      UNASSIGNED_SELECTION
+    );
+    assert.equal(
+      normalizeFloorSelection(UNASSIGNED_SELECTION, rooms, floors, buildings, "b-a"),
+      ALL_SELECTION
+    );
+  });
+
+  it("preselects the selected building and floor for new entries", () => {
+    assert.equal(defaultBuildingIdForNewFloor("b-a"), "b-a");
+    assert.equal(defaultBuildingIdForNewFloor(ALL_SELECTION), "");
+    assert.equal(defaultBuildingIdForNewFloor(UNASSIGNED_SELECTION), "");
+    assert.equal(defaultFloorIdForNewRoom("f-a-eg"), "f-a-eg");
+    assert.equal(defaultFloorIdForNewRoom(ALL_SELECTION), "");
+    assert.equal(defaultFloorIdForNewRoom(UNASSIGNED_SELECTION), "");
+  });
+
+  it("counts floors per building and rooms per floor", () => {
+    assert.deepEqual(
+      [...countFloorsByBuilding(floors, buildings)],
+      [
+        [null, 2],
+        ["b-a", 2],
+        ["b-b", 1],
+      ]
+    );
+    assert.deepEqual(
+      [...countRoomsByFloor(rooms, floors)],
+      [
+        ["f-a-eg", 1],
+        ["f-a-og", 1],
+        ["f-b-eg", 1],
+        ["f-free", 1],
+        ["f-orphan", 1],
+        [null, 2],
+      ]
+    );
+  });
+
+  it("describes a room location with floor and building", () => {
+    assert.equal(
+      describeRoomLocation(room("r", "f-a-og"), floors, buildings),
+      "1. OG · Haus A"
+    );
+    assert.equal(
+      describeRoomLocation(room("r", "f-free"), floors, buildings),
+      "Außenanlage"
+    );
+    assert.equal(
+      describeRoomLocation(room("r", "f-orphan"), floors, buildings),
+      "Keller"
+    );
+    assert.equal(
+      describeRoomLocation(room("r", null), floors, buildings),
+      "Ohne Etage"
+    );
+  });
+});
--- a/tests/project-building-structure.test.ts
+++ b/tests/project-building-structure.test.ts
@ -0,0 +1,305 @@
+import path from "node:path";
+import assert from "node:assert/strict";
+import { describe, it } from "node:test";
+import { eq } from "drizzle-orm";
+import { migrate } from "drizzle-orm/better-sqlite3/migrator";
+import {
+  createDatabaseContext,
+  type DatabaseContext,
+} from "../src/db/database-context.js";
+import { ProjectHistoryRepository } from "../src/db/repositories/project-history.repository.js";
+import { ProjectLocationStructureProjectCommandRepository } from "../src/db/repositories/project-location-structure-project-command.repository.js";
+import { buildings } from "../src/db/schema/buildings.js";
+import { floors } from "../src/db/schema/floors.js";
+import { projects } from "../src/db/schema/projects.js";
+import {
+  assertProjectFloorInsertProjectCommand,
+  createProjectBuildingDeleteProjectCommand,
+  createProjectBuildingInsertProjectCommand,
+  createProjectBuildingSnapshot,
+  createProjectBuildingUpdateProjectCommand,
+  createProjectFloorInsertProjectCommand,
+  createProjectFloorSnapshot,
+  createProjectFloorUpdateProjectCommand,
+  projectFloorCommandSchemaVersion,
+  type ProjectLocationStructureProjectCommand,
+} from "../src/domain/models/project-location-structure-project-command.model.js";
+import {
+  createBuildingSchema,
+  createFloorSchema,
+  updateFloorSchema,
+} from "../src/shared/validation/project-structure.schemas.js";
+
+function createTestDatabase(): DatabaseContext {
+  const context = createDatabaseContext(":memory:");
+  migrate(context.db, {
+    migrationsFolder: path.resolve("src", "db", "migrations"),
+  });
+  context.db
+    .insert(projects)
+    .values([
+      { id: "project-1", name: "Test project" },
+      { id: "project-2", name: "Foreign project" },
+    ])
+    .run();
+  return context;
+}
+
+/** Executes a user command and returns a runner for its undo and redo. */
+function createRunner(context: DatabaseContext) {
+  const repository = new ProjectLocationStructureProjectCommandRepository(
+    context.db
+  );
+  const history = new ProjectHistoryRepository(context.db);
+  let revision = 0;
+  const run = (
+    command: ProjectLocationStructureProjectCommand,
+    source: "user" | "undo" | "redo" = "user"
+  ) => {
+    const step =
+      source === "user" ? null : history.getNextCommand("project-1", source);
+    if (source !== "user") {
+      assert.ok(step);
+    }
+    const result = repository.execute({
+      projectId: "project-1",
+      expectedRevision: revision,
+      source,
+      historyTargetChangeSetId: step?.changeSetId,
+      command,
+    });
+    revision += 1;
+    return result;
+  };
+  return { repository, run };
+}
+
+function getBuilding(context: DatabaseContext, id: string) {
+  return context.db.select().from(buildings).where(eq(buildings.id, id)).get();
+}
+
+function getFloor(context: DatabaseContext, id: string) {
+  return context.db.select().from(floors).where(eq(floors.id, id)).get();
+}
+
+describe("project building structure", () => {
+  it("creates, renames and deletes buildings with persisted undo and redo", () => {
+    const context = createTestDatabase();
+    try {
+      const { run } = createRunner(context);
+      const building = createProjectBuildingSnapshot("project-1", " Haus A ", 0);
+      assert.equal(building.name, "Haus A");
+
+      const insert = createProjectBuildingInsertProjectCommand(building);
+      const inserted = run(insert);
+      assert.deepEqual(getBuilding(context, building.id), building);
+
+      const renamed = { ...building, name: "Haus B" };
+      const update = createProjectBuildingUpdateProjectCommand(building, renamed);
+      const updated = run(update);
+      assert.equal(getBuilding(context, building.id)?.name, "Haus B");
+
+      run(updated.inverse, "undo");
+      assert.deepEqual(getBuilding(context, building.id), building);
+      run(inserted.inverse, "undo");
+      assert.equal(getBuilding(context, building.id), undefined);
+      run(insert, "redo");
+      run(update, "redo");
+      assert.deepEqual(getBuilding(context, building.id), renamed);
+
+      const deleted = run(createProjectBuildingDeleteProjectCommand(renamed));
+      assert.equal(getBuilding(context, building.id), undefined);
+      run(deleted.inverse, "undo");
+      assert.deepEqual(getBuilding(context, building.id), renamed);
+    } finally {
+      context.close();
+    }
+  });
+
+  it("assigns floors to buildings and blocks deleting a building in use", () => {
+    const context = createTestDatabase();
+    try {
+      const { run } = createRunner(context);
+      const houseA = createProjectBuildingSnapshot("project-1", "Haus A", 0);
+      const houseB = createProjectBuildingSnapshot("project-1", "Haus B", 1);
+      run(createProjectBuildingInsertProjectCommand(houseA));
+      run(createProjectBuildingInsertProjectCommand(houseB));
+
+      const floor = createProjectFloorSnapshot("project-1", "EG", 0, houseA.id);
+      const floorInsert = createProjectFloorInsertProjectCommand(floor);
+      assert.equal(floorInsert.schemaVersion, projectFloorCommandSchemaVersion);
+      run(floorInsert);
+      assert.deepEqual(getFloor(context, floor.id), floor);
+
+      const moved = { ...floor, buildingId: houseB.id };
+      const moveCommand = createProjectFloorUpdateProjectCommand(floor, moved);
+      const movedResult = run(moveCommand);
+      assert.equal(getFloor(context, floor.id)?.buildingId, houseB.id);
+
+      assert.throws(
+        () => run(createProjectBuildingDeleteProjectCommand(houseB)),
+        /solange Etagen zugeordnet/
+      );
+      assert.ok(getBuilding(context, houseB.id));
+
+      run(movedResult.inverse, "undo");
+      assert.equal(getFloor(context, floor.id)?.buildingId, houseA.id);
+      run(moveCommand, "redo");
+      assert.equal(getFloor(context, floor.id)?.buildingId, houseB.id);
+
+      const unassigned = { ...moved, buildingId: null };
+      run(createProjectFloorUpdateProjectCommand(moved, unassigned));
+      assert.equal(getFloor(context, floor.id)?.buildingId, null);
+      run(createProjectBuildingDeleteProjectCommand(houseB));
+      assert.equal(getBuilding(context, houseB.id), undefined);
+    } finally {
+      context.close();
+    }
+  });
+
+  it("rejects floors that reference a missing or foreign building", () => {
+    const context = createTestDatabase();
+    try {
+      const { run } = createRunner(context);
+      context.db
+        .insert(buildings)
+        .values({ id: "foreign", projectId: "project-2", name: "Fremd", sortOrder: 0 })
+        .run();
+      assert.throws(
+        () =>
+          run(
+            createProjectFloorInsertProjectCommand(
+              createProjectFloorSnapshot("project-1", "EG", 0, "foreign")
+            )
+          ),
+        /foreign building/
+      );
+      assert.throws(
+        () =>
+          run(
+            createProjectFloorInsertProjectCommand(
+              createProjectFloorSnapshot("project-1", "EG", 0, "missing")
+            )
+          ),
+        /foreign building/
+      );
+      assert.equal(context.db.select().from(floors).all().length, 0);
+
+      const floor = createProjectFloorSnapshot("project-1", "EG", 0);
+      run(createProjectFloorInsertProjectCommand(floor));
+      assert.throws(
+        () =>
+          run(
+            createProjectFloorUpdateProjectCommand(floor, {
+              ...floor,
+              buildingId: "foreign",
+            })
+          ),
+        /foreign building/
+      );
+      assert.throws(
+        () => createProjectFloorUpdateProjectCommand(floor, { ...floor }),
+        /only the name or the building/
+      );
+      assert.throws(
+        () =>
+          createProjectFloorUpdateProjectCommand(floor, {
+            ...floor,
+            sortOrder: 5,
+            name: "Erdgeschoss",
+          }),
+        /only the name or the building/
+      );
+    } finally {
+      context.close();
+    }
+  });
+
+  it("executes version 1 floor commands recorded before buildings existed", () => {
+    const context = createTestDatabase();
+    try {
+      const { run } = createRunner(context);
+      const legacyFloor = {
+        id: "floor-legacy",
+        projectId: "project-1",
+        name: "EG",
+        sortOrder: 0,
+      };
+      const legacyInsert = {
+        schemaVersion: 1 as const,
+        type: "project-floor.insert" as const,
+        payload: { floor: legacyFloor },
+      };
+      assertProjectFloorInsertProjectCommand(legacyInsert);
+      const inserted = run(legacyInsert);
+      assert.deepEqual(getFloor(context, legacyFloor.id), {
+        ...legacyFloor,
+        buildingId: null,
+      });
+      // The inverse is always written in the current version.
+      assert.equal(inserted.inverse.schemaVersion, projectFloorCommandSchemaVersion);
+
+      const legacyUpdate = {
+        schemaVersion: 1 as const,
+        type: "project-floor.update" as const,
+        payload: {
+          expected: legacyFloor,
+          target: { ...legacyFloor, name: "Erdgeschoss" },
+        },
+      };
+      const updated = run(legacyUpdate);
+      assert.equal(getFloor(context, legacyFloor.id)?.name, "Erdgeschoss");
+      run(updated.inverse, "undo");
+      assert.equal(getFloor(context, legacyFloor.id)?.name, "EG");
+
+      // A version 1 snapshot must not carry buildingId, a version 2 one must.
+      assert.throws(() =>
+        assertProjectFloorInsertProjectCommand({
+          schemaVersion: 1,
+          type: "project-floor.insert",
+          payload: { floor: { ...legacyFloor, buildingId: null } },
+        })
+      );
+      assert.throws(() =>
+        assertProjectFloorInsertProjectCommand({
+          schemaVersion: 2,
+          type: "project-floor.insert",
+          payload: { floor: legacyFloor },
+        })
+      );
+    } finally {
+      context.close();
+    }
+  });
+
+  it("validates building and floor requests", () => {
+    assert.equal(
+      createBuildingSchema.safeParse({ expectedRevision: 0, name: "Haus A" }).success,
+      true
+    );
+    assert.equal(
+      createBuildingSchema.safeParse({ expectedRevision: 0, name: " " }).success,
+      false
+    );
+    assert.equal(
+      createBuildingSchema.safeParse({
+        expectedRevision: 0,
+        name: "Haus A",
+        sortOrder: 1,
+      }).success,
+      false
+    );
+    for (const buildingId of [undefined, null, "building-1"]) {
+      assert.equal(
+        createFloorSchema.safeParse({ expectedRevision: 0, name: "EG", buildingId })
+          .success,
+        true
+      );
+    }
+    assert.equal(
+      updateFloorSchema.safeParse({ expectedRevision: 0, name: "EG", buildingId: "" })
+        .success,
+      false
+    );
+  });
+});
--- a/tests/project-command.service.test.ts
+++ b/tests/project-command.service.test.ts
@ -41,6 +41,7 @@ import { projectChangeSets } from "../src/db/schema/project-change-sets.js";
 import { projectDevices } from "../src/db/schema/project-devices.js";
 import { projectRevisions } from "../src/db/schema/project-revisions.js";
 import { projects } from "../src/db/schema/projects.js";
+import { buildings } from "../src/db/schema/buildings.js";
 import { floors } from "../src/db/schema/floors.js";
 import { rooms } from "../src/db/schema/rooms.js";
 import { externalCsvConfigurations } from "../src/db/schema/external-csv-configurations.js";
@ -78,6 +79,8 @@ import {
  createDistributionBoardComponentUpdateProjectCommand,
 } from "../src/domain/models/distribution-board-component-structure-project-command.model.js";
 import {
+  createProjectBuildingInsertProjectCommand,
+  createProjectBuildingSnapshot,
  createProjectFloorInsertProjectCommand,
  createProjectFloorSnapshot,
  createProjectRoomInsertProjectCommand,
@ -1610,6 +1613,43 @@ describe("project command service", () => {
    }
  });

+  it("dispatches building commands and floor assignments with persisted inverses", () => {
+    const context = createTestDatabase();
+    try {
+      const service = createService(context);
+      const building = createProjectBuildingSnapshot("project-1", "Haus A", 0);
+      service.executeUser({
+        projectId: "project-1",
+        expectedRevision: 0,
+        command: createProjectBuildingInsertProjectCommand(building),
+      });
+      const floor = createProjectFloorSnapshot("project-1", "EG", 0, building.id);
+      const created = service.executeUser({
+        projectId: "project-1",
+        expectedRevision: 1,
+        command: createProjectFloorInsertProjectCommand(floor),
+      });
+      assert.equal(created.history.currentRevision, 2);
+      assert.equal(
+        context.db.select().from(floors).where(eq(floors.id, floor.id)).get()
+          ?.buildingId,
+        building.id
+      );
+
+      createService(context).undo({ projectId: "project-1", expectedRevision: 2 });
+      createService(context).undo({ projectId: "project-1", expectedRevision: 3 });
+      assert.equal(context.db.select().from(buildings).all().length, 0);
+      createService(context).redo({ projectId: "project-1", expectedRevision: 4 });
+      createService(context).redo({ projectId: "project-1", expectedRevision: 5 });
+      assert.deepEqual(
+        context.db.select().from(floors).where(eq(floors.id, floor.id)).get(),
+        floor
+      );
+    } finally {
+      context.close();
+    }
+  });
+
  it("dispatches floor and room setup with their persisted inverses", () => {
    const context = createTestDatabase();
    try {
--- a/tests/project-snapshot.repository.test.ts
+++ b/tests/project-snapshot.repository.test.ts
@ -20,6 +20,7 @@ import { circuits } from "../src/db/schema/circuits.js";
 import { distributionBoardComponentProtectionDevices } from "../src/db/schema/distribution-board-component-protection-devices.js";
 import { distributionBoardComponents } from "../src/db/schema/distribution-board-components.js";
 import { distributionBoards } from "../src/db/schema/distribution-boards.js";
+import { buildings } from "../src/db/schema/buildings.js";
 import { floors } from "../src/db/schema/floors.js";
 import { projectDevices } from "../src/db/schema/project-devices.js";
 import { projectSnapshots } from "../src/db/schema/project-snapshots.js";
@ -59,6 +60,15 @@ function createTestDatabase(): DatabaseContext {
    .where(eq(circuitSections.circuitListId, list.id))
    .get();
  assert.ok(section);
+  context.db
+    .insert(buildings)
+    .values({
+      id: "building-1",
+      projectId: "project-1",
+      name: "Haus A",
+      sortOrder: 0,
+    })
+    .run();
  context.db
    .insert(floors)
    .values({
@ -66,6 +76,7 @@ function createTestDatabase(): DatabaseContext {
      projectId: "project-1",
      name: "EG",
      sortOrder: 10,
+      buildingId: "building-1",
    })
    .run();
  context.db
@ -248,7 +259,9 @@ describe("project snapshot repository", () => {
        "device-1"
      );
      assert.equal(state.projectDevices.length, 1);
+      assert.equal(state.buildings.length, 1);
      assert.equal(state.floors.length, 1);
+      assert.equal(state.floors[0].buildingId, "building-1");
      assert.equal(state.rooms.length, 1);
      assert.deepEqual(repository.listByProject("project-1"), [
        created,
@ -400,8 +413,14 @@ describe("project snapshot repository", () => {
        schemaVersion: number;
        project: Record<string, unknown>;
        distributionBoards: Array<Record<string, unknown>>;
+        buildings?: unknown;
+        floors: Array<Record<string, unknown>>;
      };
      payload.schemaVersion = 5;
+      delete payload.buildings;
+      for (const floor of payload.floors) {
+        delete floor.buildingId;
+      }
      for (const field of ["siteAddress", "workStage", "projectLead", "projectEditor"]) {
        delete payload.project[field];
      }
@ -434,6 +453,9 @@ describe("project snapshot repository", () => {
          (board) => board.boardNumber === null && board.fedFrom === null
        )
      );
+      // Version 5 predates buildings: the upgraded state has none.
+      assert.deepEqual(targetState.buildings, []);
+      assert.ok(targetState.floors.every((floor) => floor.buildingId === null));
    } finally {
      context.close();
    }
@ -497,6 +519,11 @@ describe("portable project transfer", () => {
        copied.state.distributionBoards[0].floorId,
        copied.state.floors[0].id
      );
+      assert.notEqual(copied.state.buildings[0].id, "building-1");
+      assert.equal(
+        copied.state.floors[0].buildingId,
+        copied.state.buildings[0].id
+      );
      assert.equal(
        copied.state.distributionBoards[0].supplyType,
        "SV"
--- a/tests/project-state-snapshot.test.ts
+++ b/tests/project-state-snapshot.test.ts
@ -8,10 +8,22 @@ import {
 } from "../src/domain/models/project-state-snapshot.model.js";
 import { createNamedProjectSnapshotSchema } from "../src/shared/validation/project-snapshot.schemas.js";

-/** Removes the fields introduced by snapshot schema version 6. */
-function withoutVersionSixFields<
-  T extends { project: object; distributionBoards: object[] },
+/** Removes the fields introduced by snapshot schema version 7. */
+function withoutVersionSevenFields<
+  T extends { floors: object[] },
 >(snapshot: T): T {
+  delete (snapshot as Record<string, unknown>).buildings;
+  for (const floor of snapshot.floors) {
+    delete (floor as Record<string, unknown>).buildingId;
+  }
+  return snapshot;
+}
+
+/** Removes the fields introduced by snapshot schema versions 6 and 7. */
+function withoutVersionSixFields<
+  T extends { project: object; distributionBoards: object[]; floors: object[] },
+>(snapshot: T): T {
+  withoutVersionSevenFields(snapshot);
  const project = snapshot.project as Record<string, unknown>;
  for (const field of ["siteAddress", "workStage", "projectLead", "projectEditor"]) {
    delete project[field];
@ -66,7 +78,19 @@ function minimalSnapshot() {
    distributionBoardComponentProtectionDevices: [],
    circuits: [],
    projectDevices: [],
-    floors: [],
+    buildings: [] as Array<{
+      id: string;
+      projectId: string;
+      name: string;
+      sortOrder: number;
+    }>,
+    floors: [] as Array<{
+      id: string;
+      projectId: string;
+      name: string;
+      sortOrder: number;
+      buildingId: string | null;
+    }>,
    rooms: [],
  };
 }
@ -306,7 +330,10 @@ describe("project state snapshot model", () => {
    delete (outdated.project as Record<string, unknown>).description;
    assert.throws(() => parseProjectStateSnapshot(outdated));
    assert.throws(() =>
-      parseProjectStateSnapshot({ ...minimalSnapshot(), schemaVersion: 7 })
+      parseProjectStateSnapshot({
+        ...minimalSnapshot(),
+        schemaVersion: projectStateSnapshotSchemaVersion + 1,
+      })
    );
  });

@ -355,12 +382,99 @@ describe("project state snapshot model", () => {
      projectId: "project-2",
      name: "EG",
      sortOrder: 10,
+      buildingId: null,
    });
    assert.throws(
      () => parseProjectStateSnapshot(wrongOwner),
      /floor belongs to a different project/
    );
  });
+
+  it("keeps buildings and validates floor building references", () => {
+    const snapshot = minimalSnapshot();
+    snapshot.buildings.push({
+      id: "building-1",
+      projectId: "project-1",
+      name: "Haus A",
+      sortOrder: 0,
+    });
+    snapshot.floors.push(
+      {
+        id: "floor-1",
+        projectId: "project-1",
+        name: "EG",
+        sortOrder: 0,
+        buildingId: "building-1",
+      },
+      {
+        id: "floor-2",
+        projectId: "project-1",
+        name: "Keller",
+        sortOrder: 1,
+        buildingId: null,
+      }
+    );
+    const parsed = deserializeProjectStateSnapshot(
+      serializeProjectStateSnapshot(parseProjectStateSnapshot(snapshot))
+    );
+    assert.deepEqual(parsed.buildings, snapshot.buildings);
+    assert.deepEqual(
+      parsed.floors.map((floor) => floor.buildingId),
+      ["building-1", null]
+    );
+
+    const unknownBuilding = structuredClone(snapshot);
+    unknownBuilding.floors[0]!.buildingId = "building-x";
+    assert.throws(
+      () => parseProjectStateSnapshot(unknownBuilding),
+      /floor building/
+    );
+
+    const foreignBuilding = structuredClone(snapshot);
+    foreignBuilding.buildings[0]!.projectId = "project-2";
+    assert.throws(
+      () => parseProjectStateSnapshot(foreignBuilding),
+      /building belongs to a different project/
+    );
+
+    const floorWithoutBuildingField = structuredClone(snapshot);
+    delete (floorWithoutBuildingField.floors[1] as Record<string, unknown>)
+      .buildingId;
+    assert.throws(() => parseProjectStateSnapshot(floorWithoutBuildingField));
+  });
+
+  it("upgrades snapshot version 6 without buildings", () => {
+    const snapshot = minimalSnapshot();
+    snapshot.floors.push({
+      id: "floor-1",
+      projectId: "project-1",
+      name: "EG",
+      sortOrder: 0,
+      buildingId: null,
+    });
+    const previous = withoutVersionSevenFields(structuredClone(snapshot));
+    previous.schemaVersion = 6 as typeof previous.schemaVersion;
+
+    const upgraded = parseProjectStateSnapshot(previous);
+
+    assert.equal(upgraded.schemaVersion, projectStateSnapshotSchemaVersion);
+    assert.deepEqual(upgraded.buildings, []);
+    assert.deepEqual(upgraded.floors, [
+      {
+        id: "floor-1",
+        projectId: "project-1",
+        name: "EG",
+        sortOrder: 0,
+        buildingId: null,
+      },
+    ]);
+    // Version 6 cover values survive the upgrade.
+    assert.equal(upgraded.project.workStage, "LPH 5");
+
+    const mislabeled = structuredClone(snapshot);
+    mislabeled.schemaVersion = 6 as typeof mislabeled.schemaVersion;
+    assert.throws(() => parseProjectStateSnapshot(mislabeled));
+  });
 });

 describe("named project snapshot request", () => {