diff --git a/synapse/handlers/room.py b/synapse/handlers/room.py
index 2abd63ad0..ab72963d8 100644
--- a/synapse/handlers/room.py
+++ b/synapse/handlers/room.py
@@ -23,7 +23,7 @@ from synapse.types import UserID, RoomAlias, RoomID, RoomStreamToken
 from synapse.api.constants import (
     EventTypes, JoinRules, RoomCreationPreset
 )
-from synapse.api.errors import AuthError, StoreError, SynapseError
+from synapse.api.errors import AuthError, Codes, StoreError, SynapseError
 from synapse.util import stringutils
 from synapse.visibility import filter_events_for_client
 
@@ -115,7 +115,11 @@ class RoomCreationHandler(BaseHandler):
             )
 
             if mapping:
-                raise SynapseError(400, "Room alias already taken")
+                raise SynapseError(
+                    400,
+                    "Room alias already taken",
+                    Codes.ROOM_IN_USE
+                )
         else:
             room_alias = None