What does it mean if a building is listed?
A listed building is one included on a statutory list of buildings of special architectural or historic interest — and listing protects the entire building, inside and out, not just the frontage people assume.
The grades in England and Wales:
Grade I — exceptional interest. A very small fraction of listings.
Grade II* — particularly important buildings of more than special interest.
Grade II — special interest. The vast majority of listed buildings.
Scotland uses categories A, B and C; Northern Ireland uses A, B+ and B.
What listing actually covers, and where people get caught:
The whole building, including the interior — staircases, fireplaces, panelling, plasterwork, original doors, floors and windows.
Attached structures, and frequently curtilage structures such as walls, outbuildings, gates and garden features predating a cutoff date.
It is not just the façade, and it is not only the old parts — later additions can be covered.
What you need consent for. Listed building consent is required for any works that would affect the building's special interest, whether internal or external. That can include replacing windows, removing a wall or chimney breast, changing internal doors, replacing a staircase, re-roofing in a different material, installing new services, painting over historic finishes, and sometimes simple repairs using the wrong materials.
Listed building consent is separate from planning permission, and both may be needed.
The enforcement position is unusually serious. Carrying out unauthorised works to a listed building is a criminal offence, not merely a planning breach. It can attract prosecution and an unlimited fine, and the courts may take account of any financial benefit gained. There is no time limit after which unauthorised work becomes lawful, which is a crucial difference from ordinary planning enforcement.
What it means practically: repairs cost more and take longer; specialist materials and craftspeople are frequently required; insurance and mortgages can be more complicated; VAT relief on alterations was abolished; and grants exist but are limited.
Check the national register before buying, and commission a surveyor experienced with historic buildings.
General information, not legal advice.