When is a basement permitted development?
A pure internal cellar conversion — converting an existing void under the house into habitable space without any external alteration, excavation or structural change — is generally not development at all under the Town and Country Planning Act 1990. It requires no planning permission, only building regulations approval. This applies where: no new openings are created in the external walls or floor; no additional excavation is carried out; no external light wells or access stairs are added; the conversion is entirely internal. If the cellar already has adequate headroom (1.7m minimum for habitable use, 2.0m preferred) and no physical enlargement of the void is needed, you can proceed to building regulations without any planning involvement.
When does basement work require planning permission?
Planning permission is almost always required when: (1) Excavating below the existing floor level — any new excavation, however small, constitutes an operational development requiring planning permission in most London boroughs. (2) Extending under the rear garden — sub-garden basement extensions always need planning, as they enlarge the footprint of the dwelling. (3) Creating new external openings — adding light wells, external access stairs or area ways constitutes external alteration requiring planning. (4) The property is a flat — flats have no PD rights; any basement work requires planning and listed building consent if applicable. (5) Listed buildings — any basement work, internal or external, requires planning permission and Listed Building Consent. Most London boroughs also have adopted Supplementary Planning Documents (SPDs) on basements — check Camden, Kensington & Chelsea, Westminster, Hammersmith & Fulham, and Islington specifically, as these have the most restrictive policies.
Basement Supplementary Planning Documents in London
Several London boroughs have adopted specific basement SPDs that impose restrictions beyond the basic planning rules. Kensington and Chelsea: arguably the most restrictive in the UK — bans double-storey basements, limits sub-garden extensions to 50% of rear garden depth, bans front-garden basements entirely, and requires comprehensive BIAs. Camden: requires BIA, limits basement to one storey, restricts sub-garden extension to 50% of garden depth. Westminster: strong presumption against basements under listed buildings and in conservation areas; BIA required for all applications. Hammersmith and Fulham: BIA required, groundwater assessment essential given proximity to Thames terraces. Islington: BIA required, strong policy protection for communal gardens; basement beneath front garden generally refused. For all other boroughs, the NPPF applies and applications are assessed on their individual merits — but a BIA is still expected.
What is a Basement Impact Assessment?
A Basement Impact Assessment (BIA) is a technical report, prepared by a specialist structural engineer, that assesses the impact of a proposed basement on the surrounding ground, structures, drainage and the water environment. Most London LPAs require a BIA with every basement planning application — even where the principle of development is acceptable. A BIA typically includes: a ground investigation (trial pits or boreholes) to establish soil conditions and groundwater levels; a structural assessment of neighbouring buildings and their foundations; a hydrogeological assessment of groundwater flow and the risk of groundwater level change; an assessment of the impact on trees (particularly where Tree Preservation Orders exist); a construction methodology statement. BIAs typically cost £4,000–£12,000 and take 4–8 weeks to prepare. Builderr works with specialist basement engineers who produce BIAs accepted by all 33 London boroughs.
