Converting a basement

Plenty of older houses have a cellar that nobody uses much.

Some owners turn it into a room. Others dig a new basement under the house. Each project has its own rules, and these notes set them out for homes in England.

A cellar you already have, or a new dig

The Planning Portal is the government's planning service for England. It says you are unlikely to need planning permission to turn an existing basement into living space.

That changes if you add a lightwell or change how the outside of the house looks. It also changes if the basement becomes a separate home or is used to run a business.

Digging a new basement is different. The Planning Portal says it is likely to need planning permission, because of the major works involved.

Local rules

Some councils have their own rules for basements. These can limit how deep and how large a new basement can be. The Planning Portal suggests talking to your local planning authority before you apply.

A lightwell lets daylight into a basement. On the front of a house, where people see it from the street, councils may be stricter about how it looks.

Building regulations

Building regulations apply to basement work. They cover fire escape routes, ventilation, ceiling height, damp proofing, electrical wiring and water supplies.

In England and Wales the regulations treat a storey as a basement when it is at least 1.2 metres below the ground next to it.

You can use your council's building control team or a private registered building control approver. GOV.UK says that without approval you may not have the certificates you need when you sell your home.

Getting out in a fire

A basement room people live in needs a way out if there is a fire. Guidance on the fire safety rules says it should have one of two things.

An escape window or an outside door that leads up to ground level.
A protected stairway that leads to a final exit.

A window that opens into a well may need a guard around the well, so that nobody falls in.

Keeping water out

Designers waterproof basements to a British Standard called BS 8102. It is a code of practice for keeping ground water out of buildings below ground. The current version was published in March 2022.

The standard describes three types of protection. Type A is a barrier, such as a membrane or a waterproof render. Type B is built into the structure, for example concrete with a waterproofing additive. Type C collects any water that gets through behind a cavity membrane and drains it away, often to a pump.

A pumped system has to discharge its water in line with the drainage rules in the building regulations. Its pumps and channels need to stay reachable for checks and repairs.

The Property Care Association keeps a register of specialists who design waterproofing. Their qualification is called Certificated Surveyor in Structural Waterproofing.

Your neighbours

A basement dig near a neighbour's home usually brings in the Party Wall etc. Act 1996. It applies in England and Wales.

The Act covers digging within 3 metres of a neighbour's building, where the dig goes deeper than their foundations. It can also cover digging within 6 metres, depending on the depth. You give your neighbour at least one month's notice before the excavation starts.

GOV.UK sets out the notices, the 14 days a neighbour has to reply and the surveyor's award in its explanatory booklet on the Act.