diff --git a/changelog.d/9675.misc b/changelog.d/9675.misc
new file mode 100644
index 000000000000..35338cd33263
--- /dev/null
+++ b/changelog.d/9675.misc
@@ -0,0 +1 @@
+Add additional type hints to the Homeserver object.
diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index 7b723ead58ec..ee607e6e6576 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -80,7 +80,7 @@ class SyncConfig:
     filter_collection = attr.ib(type=FilterCollection)
     is_guest = attr.ib(type=bool)
     request_key = attr.ib(type=Tuple[Any, ...])
-    device_id = attr.ib(type=str)
+    device_id = attr.ib(type=Optional[str])
 
 
 @attr.s(slots=True, frozen=True)
@@ -723,7 +723,9 @@ async def compute_summary(
 
         return summary
 
-    def get_lazy_loaded_members_cache(self, cache_key: Tuple[str, str]) -> LruCache:
+    def get_lazy_loaded_members_cache(
+        self, cache_key: Tuple[str, Optional[str]]
+    ) -> LruCache:
         cache = self.lazy_loaded_members_cache.get(cache_key)
         if cache is None:
             logger.debug("creating LruCache for %r", cache_key)
diff --git a/synapse/rest/client/v2_alpha/sync.py b/synapse/rest/client/v2_alpha/sync.py
index 8e52e4cca4b2..a0db0a054bb3 100644
--- a/synapse/rest/client/v2_alpha/sync.py
+++ b/synapse/rest/client/v2_alpha/sync.py
@@ -15,6 +15,7 @@
 
 import itertools
 import logging
+from typing import TYPE_CHECKING, Tuple
 
 from synapse.api.constants import PresenceState
 from synapse.api.errors import Codes, StoreError, SynapseError
@@ -26,11 +27,15 @@
 from synapse.handlers.presence import format_user_presence_state
 from synapse.handlers.sync import SyncConfig
 from synapse.http.servlet import RestServlet, parse_boolean, parse_integer, parse_string
-from synapse.types import StreamToken
+from synapse.http.site import SynapseRequest
+from synapse.types import JsonDict, StreamToken
 from synapse.util import json_decoder
 
 from ._base import client_patterns, set_timeline_upper_limit
 
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
 logger = logging.getLogger(__name__)
 
 
@@ -73,7 +78,7 @@ class SyncRestServlet(RestServlet):
     PATTERNS = client_patterns("/sync$")
     ALLOWED_PRESENCE = {"online", "offline", "unavailable"}
 
-    def __init__(self, hs):
+    def __init__(self, hs: "HomeServer"):
         super().__init__()
         self.hs = hs
         self.auth = hs.get_auth()
@@ -85,7 +90,7 @@ def __init__(self, hs):
         self._server_notices_sender = hs.get_server_notices_sender()
         self._event_serializer = hs.get_event_client_serializer()
 
-    async def on_GET(self, request):
+    async def on_GET(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
         if b"from" in request.args:
             # /events used to use 'from', but /sync uses 'since'.
             # Lets be helpful and whine if we see a 'from'.
diff --git a/synapse/server.py b/synapse/server.py
index 98822d8e2f59..5e787e2281a8 100644
--- a/synapse/server.py
+++ b/synapse/server.py
@@ -650,13 +650,13 @@ def get_federation_registry(self) -> FederationHandlerRegistry:
         return FederationHandlerRegistry(self)
 
     @cache_in_self
-    def get_server_notices_manager(self):
+    def get_server_notices_manager(self) -> ServerNoticesManager:
         if self.config.worker_app:
             raise Exception("Workers cannot send server notices")
         return ServerNoticesManager(self)
 
     @cache_in_self
-    def get_server_notices_sender(self):
+    def get_server_notices_sender(self) -> WorkerServerNoticesSender:
         if self.config.worker_app:
             return WorkerServerNoticesSender(self)
         return ServerNoticesSender(self)
diff --git a/synapse/server_notices/consent_server_notices.py b/synapse/server_notices/consent_server_notices.py
index 9137c4edb122..a9349bf9a17a 100644
--- a/synapse/server_notices/consent_server_notices.py
+++ b/synapse/server_notices/consent_server_notices.py
@@ -13,13 +13,16 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import logging
-from typing import Any
+from typing import TYPE_CHECKING, Any, Set
 
 from synapse.api.errors import SynapseError
 from synapse.api.urls import ConsentURIBuilder
 from synapse.config import ConfigError
 from synapse.types import get_localpart_from_id
 
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
 logger = logging.getLogger(__name__)
 
 
@@ -28,16 +31,11 @@ class ConsentServerNotices:
     privacy policy consent, and sends one if we do.
     """
 
-    def __init__(self, hs):
-        """
-
-        Args:
-            hs (synapse.server.HomeServer):
-        """
+    def __init__(self, hs: "HomeServer"):
         self._server_notices_manager = hs.get_server_notices_manager()
         self._store = hs.get_datastore()
 
-        self._users_in_progress = set()
+        self._users_in_progress = set()  # type: Set[str]
 
         self._current_consent_version = hs.config.user_consent_version
         self._server_notice_content = hs.config.user_consent_server_notice_content
@@ -73,6 +71,10 @@ async def maybe_send_server_notice_to_user(self, user_id: str) -> None:
         try:
             u = await self._store.get_user_by_id(user_id)
 
+            # The user doesn't exist.
+            if u is None:
+                return
+
             if u["is_guest"] and not self._send_to_guests:
                 # don't send to guests
                 return
diff --git a/synapse/server_notices/resource_limits_server_notices.py b/synapse/server_notices/resource_limits_server_notices.py
index 665245134614..a18a2e76c999 100644
--- a/synapse/server_notices/resource_limits_server_notices.py
+++ b/synapse/server_notices/resource_limits_server_notices.py
@@ -13,7 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import logging
-from typing import List, Tuple
+from typing import TYPE_CHECKING, List, Tuple
 
 from synapse.api.constants import (
     EventTypes,
@@ -24,6 +24,9 @@
 from synapse.api.errors import AuthError, ResourceLimitError, SynapseError
 from synapse.server_notices.server_notices_manager import SERVER_NOTICE_ROOM_TAG
 
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
 logger = logging.getLogger(__name__)
 
 
@@ -32,11 +35,7 @@ class ResourceLimitsServerNotices:
     ensures that the client is kept up to date.
     """
 
-    def __init__(self, hs):
-        """
-        Args:
-            hs (synapse.server.HomeServer):
-        """
+    def __init__(self, hs: "HomeServer"):
         self._server_notices_manager = hs.get_server_notices_manager()
         self._store = hs.get_datastore()
         self._auth = hs.get_auth()
diff --git a/synapse/server_notices/server_notices_manager.py b/synapse/server_notices/server_notices_manager.py
index c46b2f047d18..144e1da78e2e 100644
--- a/synapse/server_notices/server_notices_manager.py
+++ b/synapse/server_notices/server_notices_manager.py
@@ -58,7 +58,7 @@ async def send_notice(
         user_id: str,
         event_content: dict,
         type: str = EventTypes.Message,
-        state_key: Optional[bool] = None,
+        state_key: Optional[str] = None,
     ) -> EventBase:
         """Send a notice to the given user
 
diff --git a/synapse/server_notices/server_notices_sender.py b/synapse/server_notices/server_notices_sender.py
index 6870b67ca0ff..965c6458893d 100644
--- a/synapse/server_notices/server_notices_sender.py
+++ b/synapse/server_notices/server_notices_sender.py
@@ -12,25 +12,27 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-from typing import Iterable, Union
+from typing import TYPE_CHECKING, Iterable, Union
 
 from synapse.server_notices.consent_server_notices import ConsentServerNotices
 from synapse.server_notices.resource_limits_server_notices import (
     ResourceLimitsServerNotices,
 )
+from synapse.server_notices.worker_server_notices_sender import (
+    WorkerServerNoticesSender,
+)
+
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
 
 
-class ServerNoticesSender:
+class ServerNoticesSender(WorkerServerNoticesSender):
     """A centralised place which sends server notices automatically when
     Certain Events take place
     """
 
-    def __init__(self, hs):
-        """
-
-        Args:
-            hs (synapse.server.HomeServer):
-        """
+    def __init__(self, hs: "HomeServer"):
+        super().__init__(hs)
         self._server_notices = (
             ConsentServerNotices(hs),
             ResourceLimitsServerNotices(hs),
diff --git a/synapse/server_notices/worker_server_notices_sender.py b/synapse/server_notices/worker_server_notices_sender.py
index 9273e618953b..c76bd5746044 100644
--- a/synapse/server_notices/worker_server_notices_sender.py
+++ b/synapse/server_notices/worker_server_notices_sender.py
@@ -12,16 +12,17 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
 
 
 class WorkerServerNoticesSender:
     """Stub impl of ServerNoticesSender which does nothing"""
 
-    def __init__(self, hs):
-        """
-        Args:
-            hs (synapse.server.HomeServer):
-        """
+    def __init__(self, hs: "HomeServer"):
+        pass
 
     async def on_user_syncing(self, user_id: str) -> None:
         """Called when the user performs a sync operation.
diff --git a/synapse/storage/databases/main/deviceinbox.py b/synapse/storage/databases/main/deviceinbox.py
index 45ca6620a84e..691080ce742b 100644
--- a/synapse/storage/databases/main/deviceinbox.py
+++ b/synapse/storage/databases/main/deviceinbox.py
@@ -14,7 +14,7 @@
 # limitations under the License.
 
 import logging
-from typing import List, Tuple
+from typing import List, Optional, Tuple
 
 from synapse.logging.opentracing import log_kv, set_tag, trace
 from synapse.replication.tcp.streams import ToDeviceStream
@@ -115,7 +115,7 @@ def get_to_device_stream_token(self):
     async def get_new_messages_for_device(
         self,
         user_id: str,
-        device_id: str,
+        device_id: Optional[str],
         last_stream_id: int,
         current_stream_id: int,
         limit: int = 100,
@@ -163,7 +163,7 @@ def get_new_messages_for_device_txn(txn):
 
     @trace
     async def delete_messages_for_device(
-        self, user_id: str, device_id: str, up_to_stream_id: int
+        self, user_id: str, device_id: Optional[str], up_to_stream_id: int
     ) -> int:
         """
         Args:
diff --git a/synapse/storage/databases/main/monthly_active_users.py b/synapse/storage/databases/main/monthly_active_users.py
index d788dc0fc698..757da3d55dd1 100644
--- a/synapse/storage/databases/main/monthly_active_users.py
+++ b/synapse/storage/databases/main/monthly_active_users.py
@@ -13,7 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import logging
-from typing import Dict, List
+from typing import Dict, List, Optional
 
 from synapse.metrics.background_process_metrics import wrap_as_background_process
 from synapse.storage._base import SQLBaseStore
@@ -109,7 +109,7 @@ async def get_registered_reserved_users(self) -> List[str]:
         return users
 
     @cached(num_args=1)
-    async def user_last_seen_monthly_active(self, user_id: str) -> int:
+    async def user_last_seen_monthly_active(self, user_id: str) -> Optional[int]:
         """
         Checks if a given user is part of the monthly active user group