diff --git a/changelog.d/11312.misc b/changelog.d/11312.misc
new file mode 100644
index 000000000000..86594a332db9
--- /dev/null
+++ b/changelog.d/11312.misc
@@ -0,0 +1 @@
+Add type hints to storage classes.
diff --git a/mypy.ini b/mypy.ini
index a06c7fc66b68..6bf09f43fd4e 100644
--- a/mypy.ini
+++ b/mypy.ini
@@ -49,7 +49,6 @@ exclude = (?x)
    |synapse/storage/databases/main/room_batch.py
    |synapse/storage/databases/main/roommember.py
    |synapse/storage/databases/main/search.py
-   |synapse/storage/databases/main/signatures.py
    |synapse/storage/databases/main/state.py
    |synapse/storage/databases/main/state_deltas.py
    |synapse/storage/databases/main/stats.py
diff --git a/synapse/events/builder.py b/synapse/events/builder.py
index 4f409f31e1cc..eb39e0ae3291 100644
--- a/synapse/events/builder.py
+++ b/synapse/events/builder.py
@@ -128,14 +128,12 @@ async def build(
             )
 
         format_version = self.room_version.event_format
+        # The types of auth/prev events changes between event versions.
+        prev_events: Union[List[str], List[Tuple[str, Dict[str, str]]]]
+        auth_events: Union[List[str], List[Tuple[str, Dict[str, str]]]]
         if format_version == EventFormatVersions.V1:
-            # The types of auth/prev events changes between event versions.
-            auth_events: Union[
-                List[str], List[Tuple[str, Dict[str, str]]]
-            ] = await self._store.add_event_hashes(auth_event_ids)
-            prev_events: Union[
-                List[str], List[Tuple[str, Dict[str, str]]]
-            ] = await self._store.add_event_hashes(prev_event_ids)
+            auth_events = await self._store.add_event_hashes(auth_event_ids)
+            prev_events = await self._store.add_event_hashes(prev_event_ids)
         else:
             auth_events = auth_event_ids
             prev_events = prev_event_ids
diff --git a/synapse/storage/databases/main/signatures.py b/synapse/storage/databases/main/signatures.py
index ab2159c2d378..3201623fe415 100644
--- a/synapse/storage/databases/main/signatures.py
+++ b/synapse/storage/databases/main/signatures.py
@@ -63,12 +63,12 @@ async def add_event_hashes(
             A list of tuples of event ID and a mapping of algorithm to base-64 encoded hash.
         """
         hashes = await self.get_event_reference_hashes(event_ids)
-        hashes = {
+        encoded_hashes = {
             e_id: {k: encode_base64(v) for k, v in h.items() if k == "sha256"}
             for e_id, h in hashes.items()
         }
 
-        return list(hashes.items())
+        return list(encoded_hashes.items())
 
     def _get_event_reference_hashes_txn(
         self, txn: Cursor, event_id: str