diff --git a/backend/app.py b/backend/app.py
index d4545d2..43ea8bd 100644
--- a/backend/app.py
+++ b/backend/app.py
@@ -74,6 +74,23 @@ async def find_guest(party: str, token: str) -> DBGuest:
     return DBGuest.parse_obj(guest)
 
 
+class Party(MongoModel):
+    name: str
+    created: datetime
+    allowed_extra: list[str]
+
+
+async def find_party(name: str) -> Party:
+    party = await meta["parties"].find_one({"name": name})
+    if not party:
+        raise HTTPException(status.HTTP_404_NOT_FOUND)
+    return Party.parse_obj(party)
+
+
+def validate_extra(extra: dict[str, str], party: Party):
+    return all(k in party.allowed_extra and len(v) <= 64 for (k, v) in extra.items())
+
+
 # Guest methods
 
 
@@ -88,7 +105,8 @@ async def get_self(guest: DBGuest = Depends(find_guest)):
 
 
 class GuestUpdate(BaseModel):
-    coming: Coming
+    coming: Coming | None
+    extra: dict[str, str] | None
 
 
 @app.patch(
@@ -100,8 +118,31 @@ class GuestUpdate(BaseModel):
 async def update_self(
     party: str, update: GuestUpdate, guest: DBGuest = Depends(find_guest)
 ):
+    try:
+        party_obj = await find_party(party)
+    except HTTPException:
+        # should not happen since find_guest in Depends already
+        # implies that the party/token combo is correct
+        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED)
+
     guest_dict = guest.dict(exclude={"id"})
-    guest_dict.update(update.dict())
+    update_dict = update.dict(exclude_unset=True)
+
+    if "extra" in update_dict:
+        if not validate_extra(update_dict["extra"], party_obj):
+            raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED)
+
+        # overwrite allowed extra, but keep those that are not allowed/user-modifiable
+        update_dict["extra"].update(
+            {
+                k: v
+                for (k, v) in guest_dict["extra"].items()
+                if k not in party_obj.allowed_extra
+            }
+        )
+
+    guest_dict.update(update_dict)
+
     await db[party].replace_one({"_id": guest.id}, guest_dict)
     return await db[party].find_one({"_id": guest.id})
 
@@ -135,11 +176,6 @@ async def auth_admin(admin_token: str):
         raise HTTPException(status.HTTP_401_UNAUTHORIZED)
 
 
-class Party(MongoModel):
-    name: str
-    created: datetime
-
-
 @app.get(
     "/{admin_token}",
     response_model=list[Party],
@@ -152,6 +188,7 @@ async def list_parties(_=Depends(auth_admin)):
 
 class PartyCreate(BaseModel):
     name: str
+    allowed_extra: list[str] = []
 
 
 @app.post(
@@ -168,12 +205,11 @@ async def create_party(party: PartyCreate, _=Depends(auth_admin)):
         raise HTTPException(
             status.HTTP_400_BAD_REQUEST, f"Party {party.name!r} already exists"
         )
-    inserted = await meta["parties"].insert_one(
-        {
-            "name": party.name,
-            "created": datetime.now(),
-        }
-    )
+
+    party_dict = party.dict()
+    party_dict.update({"created": datetime.now()})
+
+    inserted = await meta["parties"].insert_one(party_dict)
     return await meta["parties"].find_one({"_id": inserted.inserted_id})
 
 
@@ -218,8 +254,7 @@ class GuestCreate(BaseModel):
     tags=["admin"],
 )
 async def create_new_guest(party: str, new_guest: GuestCreate, _=Depends(auth_admin)):
-    if not await meta["parties"].find_one({"name": party}):
-        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND)
+    await find_party(party)
 
     existing = await db[party].find_one({"token": new_guest.token})
     if existing:
@@ -232,6 +267,31 @@ async def create_new_guest(party: str, new_guest: GuestCreate, _=Depends(auth_ad
     return inserted
 
 
+@app.get(
+    "/{admin_token}/{party}/userAllowedExtra",
+    response_model=list[str],
+    responses=error_responses(401, 404),
+    tags=["admin"],
+)
+async def get_allowed_extra_keys(party: str, _=Depends(auth_admin)):
+    party_obj = await find_party(party)
+    return party_obj.allowed_extra
+
+
+@app.patch(
+    "/{admin_token}/{party}/userAllowedExtra",
+    response_model=Party,
+    responses=error_responses(401, 404),
+    tags=["admin"],
+)
+async def modify_allowed_extra_keys(party: str, keys: list[str], _=Depends(auth_admin)):
+    party_obj = await find_party(party)
+    party_dict = party_obj.dict(exclude={"id"})
+    party_dict["allowed_extra"] = keys
+    await meta["parties"].replace_one({"_id": party_obj.id}, party_dict)
+    return await meta["parties"].find_one({"_id": party_obj.id})
+
+
 class GuestModify(BaseModel):
     token: str | None
     name: str | None
@@ -249,8 +309,7 @@ class GuestModify(BaseModel):
 async def modify_guest(
     party: str, id: PyObjectId, modified_guest: GuestModify, _=Depends(auth_admin)
 ):
-    if not await meta["parties"].find_one({"name": party}):
-        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND)
+    await find_party(party)
 
     existing = await db[party].find_one({"_id": id})
     if not existing: