Basement
conversions
in Hampstead, NW3

Every basement or lightwell dug in Camden needs planning permission: an Article 4 Direction in force since 1 June 2017 removed the permitted development route across the whole borough. In Hampstead the ground adds its own difficulty. The Neighbourhood Plan describes the hill as very permeable, erodible Bagshot Sand at the top, the Claygate Member beneath it, and then Band D of the London Clay Formation.

That is why we treat a Hampstead basement as an engineering project first and a room second. Our structural engineers, working with chartered geotechnical and hydrogeological specialists, lead the investigation, and the same appointment carries the scheme through planning, excavation, waterproofing and fit-out.

Sources: Camden Planning Guidance: Basements (January 2021), Hampstead Neighbourhood Plan 2025-2040 (adopted by Camden)

Policy A5 of the Camden Local Plan only allows a basement where the Council is satisfied it will not harm neighbouring buildings, the ground and water conditions, the character and amenity of the area, the architecture of the house or the significance of heritage assets.

It then sets the scale. A basement should be:

• no more than one storey, which Camden reads as roughly 3 to 4 metres of total excavation depth measured externally;
• not built beneath an existing basement;
• no larger than half of each garden;
• less than one and a half times the footprint of the house;
• no further into the garden than half the depth of the house, measured from the main rear elevation, and no further than half the depth of the garden;
• set back from neighbouring boundaries wherever it goes beyond the house footprint, and arranged to avoid losing valued garden space or trees.

A deeper section is allowed only for a swimming pool, and only where the extra depth harms neither neighbours nor ground and water conditions and is used for nothing but the pool.

Converting an existing cellar without lowering it is a different proposition from digging a new storey. Where a floor is to be lowered, we confirm with Camden at pre-application stage how the proposal will be treated before the engineering is commissioned.

Sources: Camden Local Plan 2017, Camden Planning Guidance: Basements (January 2021)

The Hampstead Neighbourhood Plan 2025-2040, adopted by Camden on 21 July 2025, covers Hampstead Town ward, a significant part of Frognal ward, the greater part of Hampstead Heath and a small part of Gospel Oak ward; Church Row and Perrin's Walk sit outside it. It has the same legal status as the Local Plan.

• Predicted damage to every neighbouring building inside the zone of influence, typically about twice the basement's depth from the excavation, should be no worse than Burland Scale 1.
• The plan recommends a basement search radius of 50m around the site, widened to 100m for large detached villas.
• A basement connected to the sewer should be fitted with a positive pumped device, or similar, against sewer flooding.
• Support depends on a landscaping scheme showing how existing trees are kept and protected during construction; losing an ancient or veteran tree is not supported without wholly exceptional reasons and compensation.
• Applicants are encouraged to consult neighbours before applying and to submit a statement showing what was said and what changed.

Sources: Camden Council: Hampstead Neighbourhood Forum, Hampstead Neighbourhood Plan 2025-2040 (adopted by Camden)

In a listed house, a new basement, or an extension of existing basement accommodation, needs listed building consent even where planning permission is not required. Hampstead's conservation area contains nearly 500 listed buildings, according to Camden's design guide.

In a conservation area, or next to a listed building, Camden expects a management plan for the demolition or construction stage of basement works.

For listed houses we record the existing cellar walls, vaults and floors before design starts, and the engineers set out how the historic structure will be supported while the new structure is built beneath or beside it.

Sources: Camden Planning Guidance: Basements (January 2021), Camden: Hampstead Conservation Area Design Guide (updated June 2026)

A basement falls in the second level of Camden's homeowner pre-application advice, alongside roof extensions and whole-house remodelling.

• Desk study: planning history, neighbouring basements, utilities and the geological maps, so the ground investigation is targeted rather than generic.
• Ground investigation and monitoring standpipes arranged by our engineers, with the chartered specialists who will sign the assessment.
• Structural scheme, temporary works and waterproofing designed together, so the assessment describes the basement that will actually be built.
• Construction Management Plan and party wall notices issued with the same drawings, so neighbours and their surveyors see one consistent proposal.
• Fit-out planned for daylight, ventilation and drainage pumps from the start, with the finishes coordinated with our house refurbishment team where the rest of the house is being renovated.

Source: Camden Council: Pre-planning application advice for homeowners