diff --git a/synapse/handlers/room_summary.py b/synapse/handlers/room_summary.py
index 720459f1e7..da4b4ca75c 100644
--- a/synapse/handlers/room_summary.py
+++ b/synapse/handlers/room_summary.py
@@ -183,8 +183,10 @@ class RoomSummaryHandler:
     ) -> JsonDict:
         """See docstring for SpaceSummaryHandler.get_room_hierarchy."""
 
-        # First of all, check that the room is accessible.
-        if not await self._is_local_room_accessible(requested_room_id, requester):
+        # First of all, check that the room is accessible locally.
+        # OR accessible through federation.
+        local_room = await self._store.is_host_joined(room_id, self._server_name)
+        if local_room and not await self._is_local_room_accessible(requested_room_id, requester):
             raise UnstableSpecAuthError(
                 403,
                 "User %s not in room %s, and room previews are disabled"
@@ -192,6 +194,23 @@ class RoomSummaryHandler:
                 errcode=Codes.NOT_JOINED,
             )
 
+        if not local_room:
+            (
+                room_entry,
+                children_room_entries,
+                inaccessible_children,
+            ) = await self._summarize_remote_room_hierarchy(
+                _RoomQueueEntry(requested_room_id, ()),
+                False,
+            )
+            if not room_entry or not await self._is_remote_room_accessible(requester, requested_room_id, room_entry.room):
+                raise UnstableSpecAuthError(
+                    403,
+                    "User %s not in room %s, and room previews are disabled"
+                    % (requester, requested_room_id),
+                    errcode=Codes.NOT_JOINED,
+                )
+
         # If this is continuing a previous session, pull the persisted data.
         if from_token:
             try: