diff --git a/Website/main.py b/Website/main.py
index 5a9543b..4e17fc2 100644
--- a/Website/main.py
+++ b/Website/main.py
@@ -3569,22 +3569,42 @@ def avv():
     return render_template("avv.html")
 
 @app.route('/instance_request/check_subdomain', methods=['GET'])
-@login_required
 def check_subdomain():
-    name = (request.values.get('school_name') or '').strip()
-    slug = _slugify_subdomain(name)
-    if not slug:
-        return jsonify({"available": False, "slug": "", "message": "Ungültiger Name für Subdomain."})
+    slug = request.args.get('slug', '').lower().strip()
+    
+    if not slug or len(slug) < 3:
+        return jsonify({"available": False, "message": "Ungültiger oder zu kurzer Name"}), 400
 
+    # 1. Prüfen, ob die Subdomain bereits in den aktiven Instanzen existiert
+    inst_client, inst_col = _get_collection("instances")
     try:
-        existing = steering.instace.list() or []
-    except Exception:
-        existing = []
+        existing_instance = inst_col.find_one({"subdomain": slug})
+    finally:
+        inst_client.close()
 
-    if slug in existing:
-        return jsonify({"available": False, "slug": slug, "message": "Subdomain bereits vergeben."})
-    return jsonify({"available": True, "slug": slug, "message": "Subdomain verfügbar."})
+    if existing_instance:
+        return jsonify({"available": False}), 200
 
+    # 2. Prüfen, ob bereits eine Anfrage (Instance Request) dafür läuft
+    req_client, req_col = _get_collection("instance_requests")
+    try:
+        existing_request = req_col.find_one({
+            "tenant_slug": slug,
+            "provision_status": {"$in": ["pending", "in_progress"]}
+        })
+    finally:
+        req_client.close()
+
+    if existing_request:
+        return jsonify({"available": False}), 200
+
+    # 3. Optional: Gegen reservierte System-Subdomains prüfen (z.B. www, admin, api, mail)
+    reserved_slugs = {"www", "admin", "api", "mail", "auth", "dashboard", "support"}
+    if slug in reserved_slugs:
+        return jsonify({"available": False}), 200
+
+    # Wenn alles frei ist:
+    return jsonify({"available": True}), 200
 
 @app.route('/impressum')
 def impressum():