Basement conversions

Basement conversions in East Sheen & South West London

Going down is often the only way to add space where the garden is small and the roof has already been converted. Whether you are turning an existing cellar into a usable room or excavating a new basement, the work is specialist: it depends on careful structural design, waterproofing and sequencing, and on the basement policies of councils across East Sheen and South West London.

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Basement

Types of basement conversions

Cellar conversions

Making an existing cellar dry, insulated and habitable, often by lowering the floor.
Basement excavations

Digging out beneath the house to create a new floor of living space.
Lightwells

Openings at the front or rear that bring daylight and ventilation into the basement.

What a basement conversion involves

A cellar conversion usually means lowering and relaying the floor, underpinning the walls where needed, installing a waterproofing system, and then insulating, wiring and finishing the room. A full excavation is a bigger undertaking: the house is supported while the ground beneath it is removed in carefully sequenced sections, new retaining walls and floor slab are cast, and the space is waterproofed before any fit-out begins.

Waterproofing is the heart of any basement. It is designed to the relevant British Standard for below-ground structures and usually combines a cavity drainage membrane with a sump and pump. Timescales vary more than for any other project: a straightforward cellar conversion may take a few months, while a full excavation can take the better part of a year.

Planning, basement policies and Building Regulations

Converting an existing cellar without changing the outside of the house often doesn't need planning permission, but new excavations, lightwells and changes to the front of a property usually do. Many London boroughs have their own basement policies, which can set limits on size and require construction management plans, so check with your local council early. The Planning Portal is a good starting point.

All basement work needs Building Regulations approval, covering structure, waterproofing, fire escape and ventilation.

Living with the work

Excavation means spoil has to leave the site, often by conveyor to a skip or grab lorry, so access, parking and any skip or scaffold licences from the council are planned before digging starts. Families often stay in the house during a cellar conversion; for a full excavation beneath the whole ground floor, it can be easier to move out for the noisiest stages. We talk this through with you at the feasibility stage.

Neighbours and the Party Wall Act

Because basements involve digging close to neighbouring foundations, they almost always fall under the Party Wall etc. Act 1996, and notices must be served before work starts. Neighbours often appoint their own surveyor, so allow time for this. The government's Party Wall guidance explains your obligations. We keep neighbours informed throughout, because good relations make the whole project run more smoothly.

How your project runs

1
Feasibility

A visit to look at the existing cellar or ground floor, access, and what the council is likely to allow.
2
Design and approvals

Structural and waterproofing design by specialists, planning where needed, building control and party wall awards.
3
Excavation and structure

Carefully sequenced digging, underpinning and retaining walls, with the house supported throughout.
4
Waterproofing and fit-out

The waterproofing system, then insulation, services and finishes, followed by sign-off.

Frequently asked questions

Does a basement conversion need planning permission?

Converting an existing cellar without external changes often doesn't, but new excavations and lightwells usually do, and many London boroughs have specific basement policies. Check with your council and the Planning Portal early.

How is a basement kept dry?

With a waterproofing system designed for below-ground structures, typically a cavity drainage membrane that channels water to a sump and pump, sometimes combined with other protection.

How long does a basement conversion take?

A straightforward cellar conversion may take a few months; a full excavation can take the better part of a year. You receive a programme specific to your project before work starts.

Will my neighbours be affected?

Basement work almost always falls under the Party Wall etc. Act 1996, so notices are served before work starts. We keep neighbours informed throughout the build.