diff --git a/backend/src/routes/invitations.ts b/backend/src/routes/invitations.ts
index 5ff5862..93fe942 100644
--- a/backend/src/routes/invitations.ts
+++ b/backend/src/routes/invitations.ts
@@ -3,6 +3,7 @@ import { PrismaClient, Prisma, UserRole } from '@prisma/client';
 import bcrypt from 'bcryptjs';
 import crypto from 'crypto';
 import { signAuthToken, requireAuth, requireRole, AuthedRequest } from '../middleware/auth';
+import { findOverlappingContract } from './contracts';
 
 const prisma = new PrismaClient();
 
@@ -20,13 +21,20 @@ function inviteLinkFor(req: Request, token: string): string {
   return `${baseUrl.replace(/\/$/, '')}/?invite=${token}`;
 }
 
-// POST /v1/invitations  (nur Vermieter/Admin) — { email, fullName?, roomId?, role? } -> { invitation, inviteLink }
+// POST /v1/invitations  (nur Vermieter/Admin)
+// { email, fullName?, roomId?, role?, contractStartDate?, contractEndDate? } -> { invitation, inviteLink }
+//
+// contractStartDate/contractEndDate sind nur relevant, wenn ein Zimmer
+// zugeordnet wird: sie legen fest, ab wann (und ggf. bis wann, sonst
+// unbefristet) der bei Einladungsannahme automatisch erzeugte Mietvertrag
+// laufen soll — Basis für die Plausibilitätsprüfung gegen bereits vergebene
+// Zimmer (siehe findOverlappingContract in contracts.ts).
 invitationsRouter.post(
   '/invitations',
   requireAuth,
   requireRole('LANDLORD', 'ADMIN'),
   async (req: AuthedRequest, res: Response) => {
-    const { email, fullName, roomId, role } = req.body || {};
+    const { email, fullName, roomId, role, contractStartDate, contractEndDate } = req.body || {};
     if (!email || !String(email).trim()) {
       return res.status(400).json({ error: 'E-Mail-Adresse erforderlich' });
     }
@@ -53,9 +61,29 @@ invitationsRouter.post(
       });
     }
 
+    let parsedContractStart: Date | null = null;
+    let parsedContractEnd: Date | null = null;
+
     if (roomId) {
       const room = await prisma.room.findUnique({ where: { id: roomId } });
       if (!room) return res.status(400).json({ error: 'Zimmer nicht gefunden' });
+
+      if (contractEndDate !== undefined && contractEndDate !== null && Number.isNaN(Date.parse(contractEndDate))) {
+        return res.status(400).json({ error: 'Mietende ist ungültig' });
+      }
+      if (contractStartDate !== undefined && contractStartDate !== null && Number.isNaN(Date.parse(contractStartDate))) {
+        return res.status(400).json({ error: 'Mietbeginn ist ungültig' });
+      }
+      parsedContractStart = contractStartDate ? new Date(contractStartDate) : new Date();
+      parsedContractEnd = contractEndDate ? new Date(contractEndDate) : null;
+
+      const overlap = await findOverlappingContract(prisma, roomId, parsedContractStart, parsedContractEnd);
+      if (overlap) {
+        const overlapUntil = overlap.endDate ? overlap.endDate.toLocaleDateString('de-DE') : 'unbefristet';
+        return res.status(409).json({
+          error: `Das Zimmer ist im gewählten Zeitraum bereits vergeben (belegt bis ${overlapUntil}).`,
+        });
+      }
     }
 
     const token = crypto.randomBytes(24).toString('hex');
@@ -68,6 +96,8 @@ invitationsRouter.post(
         token,
         role: invitedRole,
         roomId: roomId || null,
+        contractStartDate: parsedContractStart,
+        contractEndDate: parsedContractEnd,
         invitedById: req.user!.id,
         expiresAt,
       },
@@ -128,6 +158,8 @@ invitationsRouter.get('/invitations/:token/preview', async (req: Request, res: R
     fullName: invitation.fullName,
     role: invitation.role,
     room: invitation.room,
+    contractStartDate: invitation.contractStartDate,
+    contractEndDate: invitation.contractEndDate,
     expiresAt: invitation.expiresAt,
   });
 });
@@ -240,7 +272,8 @@ invitationsRouter.post('/invitations/:token/accept', async (req: Request, res: R
           data: {
             userId: created.id,
             roomId: room.id,
-            startDate: new Date(),
+            startDate: invitation.contractStartDate || new Date(),
+            endDate: invitation.contractEndDate,
             totalWarmRent: Number(room.baseRent) + Number(room.utilityPauschal),
             depositAmount: Number(room.baseRent) * 3,
             isActive: true,
diff --git a/web-dashboard/index.html b/web-dashboard/index.html
index e052823..3305100 100644
--- a/web-dashboard/index.html
+++ b/web-dashboard/index.html
@@ -670,6 +670,17 @@
               <option value="">– kein Zimmer zuordnen –</option>
             </select>
           </div>
+          <div class="field" id="inviteStartField" style="display:none">
+            <label for="inviteStartDate">Mietbeginn</label>
+            <input type="date" id="inviteStartDate" />
+          </div>
+          <div class="field" id="inviteEndField" style="display:none">
+            <label for="inviteEndDate">Mietende</label>
+            <input type="date" id="inviteEndDate" />
+            <label style="font-weight:normal; margin-top:4px;">
+              <input type="checkbox" id="inviteUnbefristet" checked /> unbefristet
+            </label>
+          </div>
           <button type="submit" class="btn-primary" style="width:auto" id="inviteSubmitBtn">Einladen</button>
         </form>
         <div id="inviteLinkResult" style="display:none" class="invite-link-box">
@@ -1492,22 +1503,71 @@
   let lastRooms = [];
   let lastCockpitData = null;
 
-  function populateRoomSelect() {
+  // Zimmerauswahl bei der Einladung ist datumsbewusst: statt nur nach
+  // Room.status zu filtern (das kennt nur "aktuell frei/belegt"), fragt sie
+  // /contracts/room-availability für den gewählten Mietbeginn/-ende ab. So
+  // lässt sich z.B. ein zum 1.9. gekündigtes Zimmer schon vorab ab 1.9. neu
+  // vergeben, während ein unbefristet/darüber hinaus vergebenes Zimmer für
+  // diesen Zeitraum ausgegraut bleibt (Plausibilitätsprüfung, siehe
+  // findOverlappingContract in contracts.ts).
+  async function refreshInviteRoomOptions() {
     const select = document.getElementById('inviteRoom');
+    const previousValue = select.value;
+    const startInput = document.getElementById('inviteStartDate');
+    const endInput = document.getElementById('inviteEndDate');
+    const unbefristet = document.getElementById('inviteUnbefristet').checked;
+    const startDate = startInput.value || new Date().toISOString().slice(0, 10);
+    const endDate = unbefristet ? '' : endInput.value;
+
     select.innerHTML = '<option value="">– kein Zimmer zuordnen –</option>';
-    // Nur freie Zimmer zur Auswahl anbieten — ein bereits belegtes Zimmer
-    // kann nicht zusätzlich vergeben werden.
-    lastRooms.filter(r => r.status === 'VACANT').forEach(r => {
-      const opt = document.createElement('option');
-      opt.value = r.id;
-      opt.textContent = r.roomNumber;
-      select.appendChild(opt);
-    });
+    try {
+      const params = new URLSearchParams({ startDate });
+      if (endDate) params.set('endDate', endDate);
+      const data = await apiFetch(`/contracts/room-availability?${params.toString()}`);
+      (data.rooms || []).forEach(r => {
+        const opt = document.createElement('option');
+        opt.value = r.id;
+        opt.textContent = r.available ? r.roomNumber : `${r.roomNumber} (im Zeitraum belegt)`;
+        opt.disabled = !r.available;
+        if (!r.available && r.conflictReason) opt.title = r.conflictReason;
+        select.appendChild(opt);
+      });
+      if (previousValue && [...select.options].some(o => o.value === previousValue && !o.disabled)) {
+        select.value = previousValue;
+      }
+    } catch (err) {
+      console.error('[invite] Zimmerverfügbarkeit laden fehlgeschlagen:', err.message);
+    }
+  }
+
+  function populateRoomSelect() {
+    refreshInviteRoomOptions();
   }
 
   function initInvitePanel() {
     const form = document.getElementById('inviteForm');
     const errorBox = document.getElementById('inviteFormError');
+    const roomSelect = document.getElementById('inviteRoom');
+    const startField = document.getElementById('inviteStartField');
+    const endField = document.getElementById('inviteEndField');
+    const startInput = document.getElementById('inviteStartDate');
+    const endInput = document.getElementById('inviteEndDate');
+    const unbefristetCheckbox = document.getElementById('inviteUnbefristet');
+
+    roomSelect.onchange = () => {
+      const hasRoom = !!roomSelect.value;
+      startField.style.display = hasRoom ? 'flex' : 'none';
+      endField.style.display = hasRoom ? 'flex' : 'none';
+      if (hasRoom && !startInput.value) startInput.value = new Date().toISOString().slice(0, 10);
+    };
+    unbefristetCheckbox.onchange = () => {
+      endInput.disabled = unbefristetCheckbox.checked;
+      if (unbefristetCheckbox.checked) endInput.value = '';
+      refreshInviteRoomOptions();
+    };
+    startInput.onchange = refreshInviteRoomOptions;
+    endInput.onchange = refreshInviteRoomOptions;
+    endInput.disabled = unbefristetCheckbox.checked;
 
     form.onsubmit = async (e) => {
       e.preventDefault();
@@ -1518,16 +1578,24 @@
       try {
         const fullName = document.getElementById('inviteName').value.trim() || undefined;
         const email = document.getElementById('inviteEmail').value.trim();
-        const roomId = document.getElementById('inviteRoom').value || undefined;
+        const roomId = roomSelect.value || undefined;
+        const contractStartDate = roomId ? (startInput.value || undefined) : undefined;
+        const contractEndDate = roomId && !unbefristetCheckbox.checked ? (endInput.value || undefined) : undefined;
         const data = await apiFetch('/invitations', {
           method: 'POST',
-          body: JSON.stringify({ email, fullName, roomId }),
+          body: JSON.stringify({ email, fullName, roomId, contractStartDate, contractEndDate }),
         });
         document.getElementById('inviteLinkText').textContent = data.inviteLink;
         document.getElementById('inviteLinkResult').style.display = 'flex';
         document.getElementById('inviteName').value = '';
         document.getElementById('inviteEmail').value = '';
+        startField.style.display = 'none';
+        endField.style.display = 'none';
+        startInput.value = '';
+        endInput.value = '';
+        unbefristetCheckbox.checked = true;
         loadInvitations();
+        refreshInviteRoomOptions();
       } catch (err) {
         errorBox.textContent = err.message;
         errorBox.style.display = 'block';
@@ -3582,17 +3650,14 @@
     if (!isLandlord) { section.style.display = 'none'; return; }
     section.style.display = 'block';
     try {
-      const [archiveData, roomsData] = await Promise.all([
-        apiFetch('/contracts/archive'),
-        apiFetch('/contracts/vacant-rooms'),
-      ]);
-      renderContractArchive(archiveData.contracts, roomsData.rooms);
+      const archiveData = await apiFetch('/contracts/archive');
+      renderContractArchive(archiveData.contracts);
     } catch (err) {
       console.error('[contracts] Archiv laden fehlgeschlagen:', err.message);
     }
   }
 
-  function renderContractArchive(contracts, vacantRooms) {
+  function renderContractArchive(contracts) {
     const list = document.getElementById('contractArchiveList');
     list.innerHTML = '';
     if (!contracts.length) {
@@ -3620,13 +3685,13 @@
         row.appendChild(docWrap);
       }
 
-      row.appendChild(buildReactivateSection(c, vacantRooms));
+      row.appendChild(buildReactivateSection(c));
 
       list.appendChild(row);
     });
   }
 
-  function buildReactivateSection(c, vacantRooms) {
+  function buildReactivateSection(c) {
     const wrap = el('div');
 
     const toggleBtn = document.createElement('button');
@@ -3638,75 +3703,122 @@
     formArea.style.display = 'none';
     formArea.style.marginTop = '10px';
 
-    if (!vacantRooms.length) {
-      formArea.appendChild(el('span', 'meta-text', 'Kein freies Zimmer verfügbar.'));
-    } else {
-      const roomField = el('div', 'field');
-      roomField.style.minWidth = '150px';
-      roomField.appendChild(el('label', null, 'Zimmer'));
-      const roomSelect = document.createElement('select');
-      vacantRooms.forEach(r => {
-        const opt = document.createElement('option');
-        opt.value = r.id;
-        opt.textContent = r.roomNumber;
-        opt.dataset.baseRent = r.baseRent;
-        opt.dataset.utilityPauschal = r.utilityPauschal;
-        roomSelect.appendChild(opt);
-      });
-      roomField.appendChild(roomSelect);
-      formArea.appendChild(roomField);
+    const roomField = el('div', 'field');
+    roomField.style.minWidth = '190px';
+    roomField.appendChild(el('label', null, 'Zimmer'));
+    const roomSelect = document.createElement('select');
+    roomField.appendChild(roomSelect);
+    formArea.appendChild(roomField);
 
-      const startField = el('div', 'field');
-      startField.style.minWidth = '150px';
-      startField.appendChild(el('label', null, 'Mietbeginn'));
-      const startInput = document.createElement('input');
-      startInput.type = 'date';
-      startField.appendChild(startInput);
-      formArea.appendChild(startField);
+    const startField = el('div', 'field');
+    startField.style.minWidth = '150px';
+    startField.appendChild(el('label', null, 'Mietbeginn'));
+    const startInput = document.createElement('input');
+    startInput.type = 'date';
+    startInput.value = new Date().toISOString().slice(0, 10);
+    startField.appendChild(startInput);
+    formArea.appendChild(startField);
 
-      const rentField = el('div', 'field');
-      rentField.style.width = '130px';
-      rentField.appendChild(el('label', null, 'Warmmiete (€)'));
-      const rentInput = document.createElement('input');
-      rentInput.type = 'number';
-      rentInput.min = '0';
-      rentInput.step = '0.01';
-      const firstRoom = vacantRooms[0];
-      rentInput.value = (Number(firstRoom.baseRent) + Number(firstRoom.utilityPauschal)).toFixed(2);
-      rentField.appendChild(rentInput);
-      formArea.appendChild(rentField);
+    const endField = el('div', 'field');
+    endField.style.minWidth = '150px';
+    endField.appendChild(el('label', null, 'Mietende'));
+    const endInput = document.createElement('input');
+    endInput.type = 'date';
+    endInput.disabled = true;
+    endField.appendChild(endInput);
+    const unbefristetLabel = el('label', null, ' unbefristet');
+    unbefristetLabel.style.cssText = 'font-weight:normal; margin-top:4px; display:block;';
+    const unbefristetCheckbox = document.createElement('input');
+    unbefristetCheckbox.type = 'checkbox';
+    unbefristetCheckbox.checked = true;
+    unbefristetLabel.prepend(unbefristetCheckbox);
+    endField.appendChild(unbefristetLabel);
+    formArea.appendChild(endField);
 
-      const depositField = el('div', 'field');
-      depositField.style.width = '130px';
-      depositField.appendChild(el('label', null, 'Kaution (€)'));
-      const depositInput = document.createElement('input');
-      depositInput.type = 'number';
-      depositInput.min = '0';
-      depositInput.step = '0.01';
-      depositInput.value = (Number(firstRoom.baseRent) * 3).toFixed(2);
-      depositField.appendChild(depositInput);
-      formArea.appendChild(depositField);
+    const rentField = el('div', 'field');
+    rentField.style.width = '130px';
+    rentField.appendChild(el('label', null, 'Warmmiete (€)'));
+    const rentInput = document.createElement('input');
+    rentInput.type = 'number';
+    rentInput.min = '0';
+    rentInput.step = '0.01';
+    rentField.appendChild(rentInput);
+    formArea.appendChild(rentField);
 
-      roomSelect.onchange = () => {
-        const opt = roomSelect.selectedOptions[0];
-        rentInput.value = (Number(opt.dataset.baseRent) + Number(opt.dataset.utilityPauschal)).toFixed(2);
-        depositInput.value = (Number(opt.dataset.baseRent) * 3).toFixed(2);
-      };
+    const depositField = el('div', 'field');
+    depositField.style.width = '130px';
+    depositField.appendChild(el('label', null, 'Kaution (€)'));
+    const depositInput = document.createElement('input');
+    depositInput.type = 'number';
+    depositInput.min = '0';
+    depositInput.step = '0.01';
+    depositField.appendChild(depositInput);
+    formArea.appendChild(depositField);
 
-      const submitBtn = document.createElement('button');
-      submitBtn.type = 'button';
-      submitBtn.className = 'btn-primary';
-      submitBtn.style.width = 'auto';
-      submitBtn.textContent = 'Neuen Vertrag anlegen';
-      submitBtn.onclick = () => reactivateTenant(c.user.id, {
-        roomId: roomSelect.value,
-        startDate: startInput.value,
-        totalWarmRent: rentInput.value,
-        depositAmount: depositInput.value,
-      }, submitBtn);
-      formArea.appendChild(submitBtn);
+    const roomInfoById = {};
+
+    // Datumsbewusste Zimmerauswahl: bei jeder Änderung von Mietbeginn/-ende
+    // wird /contracts/room-availability neu abgefragt, damit z.B. ein zum
+    // gewählten Datum bereits gekündigtes (aber noch aktives) Zimmer als
+    // verfügbar erscheint, ein unbefristet/darüber hinaus vergebenes aber
+    // nicht (siehe findOverlappingContract in contracts.ts).
+    async function refreshRoomOptions() {
+      const startDate = startInput.value || new Date().toISOString().slice(0, 10);
+      const endDate = unbefristetCheckbox.checked ? '' : endInput.value;
+      const previousValue = roomSelect.value;
+      try {
+        const params = new URLSearchParams({ startDate });
+        if (endDate) params.set('endDate', endDate);
+        const data = await apiFetch(`/contracts/room-availability?${params.toString()}`);
+        roomSelect.innerHTML = '';
+        (data.rooms || []).forEach(r => {
+          roomInfoById[r.id] = r;
+          const opt = document.createElement('option');
+          opt.value = r.id;
+          opt.textContent = r.available ? r.roomNumber : `${r.roomNumber} (im Zeitraum belegt)`;
+          opt.disabled = !r.available;
+          if (!r.available && r.conflictReason) opt.title = r.conflictReason;
+          roomSelect.appendChild(opt);
+        });
+        const stillAvailable = [...roomSelect.options].some(o => o.value === previousValue && !o.disabled);
+        roomSelect.value = stillAvailable ? previousValue : ([...roomSelect.options].find(o => !o.disabled)?.value || '');
+        applyRoomDefaults();
+      } catch (err) {
+        console.error('[contracts] Zimmerverfügbarkeit laden fehlgeschlagen:', err.message);
+      }
     }
 
+    function applyRoomDefaults() {
+      const info = roomInfoById[roomSelect.value];
+      if (!info) return;
+      rentInput.value = (Number(info.baseRent) + Number(info.utilityPauschal)).toFixed(2);
+      depositInput.value = (Number(info.baseRent) * 3).toFixed(2);
+    }
+
+    roomSelect.onchange = applyRoomDefaults;
+    startInput.onchange = refreshRoomOptions;
+    endInput.onchange = refreshRoomOptions;
+    unbefristetCheckbox.onchange = () => {
+      endInput.disabled = unbefristetCheckbox.checked;
+      if (unbefristetCheckbox.checked) endInput.value = '';
+      refreshRoomOptions();
+    };
+    refreshRoomOptions();
+
+    const submitBtn = document.createElement('button');
+    submitBtn.type = 'button';
+    submitBtn.className = 'btn-primary';
+    submitBtn.style.width = 'auto';
+    submitBtn.textContent = 'Neuen Vertrag anlegen';
+    submitBtn.onclick = () => reactivateTenant(c.user.id, {
+      roomId: roomSelect.value,
+      startDate: startInput.value,
+      endDate: unbefristetCheckbox.checked ? undefined : (endInput.value || undefined),
+      totalWarmRent: rentInput.value,
+      depositAmount: depositInput.value,
+    }, submitBtn);
+    formArea.appendChild(submitBtn);
+
     toggleBtn.onclick = () => {
       formArea.style.display = formArea.style.display === 'none' ? 'flex' : 'none';
     };
@@ -3718,6 +3830,7 @@
 
   async function reactivateTenant(userId, options, btn) {
     if (!options.startDate) { alert('Bitte Mietbeginn angeben.'); return; }
+    if (!options.roomId) { alert('Bitte ein verfügbares Zimmer wählen.'); return; }
     btn.disabled = true;
     btn.textContent = 'Legt an…';
     try {