Permitted development rights may be available for works that go beyond repair, maintenance, and very minor works. However, you should always check with the local planning authority to confirm that rights have not been removed.
You should clarify ownership and responsibility before modifying or carrying out maintenance. Drains, sewers and manholes may be shared with neighbours or owned by the relevant sewerage undertaker.
Failure to confirm these details or to comply with relevant standards/legislation could lead to legal and remedial action at your own cost.
Internal or external works to listed buildings are likely to require listed building consent.