There is no single London-wide basement size allowance. Borough design limits, Article 4 directions, flood risk and construction controls address different questions and must be checked separately. The examples below compare published approaches in Camden, Richmond, Kensington and Chelsea, and Westminster; they are not a permission certificate for any particular property. [1][2][3][4]
Camden: size, impact assessment and independent audit
Camden identifies Local Plan Policy A5 and its basement guidance as the starting point. Its published process addresses effects on groundwater, flooding and neighbouring structures through a Basement Impact Assessment, with independent audit undertaken for the council by CampbellReith. The assessment is therefore not simply a drawing showing the proposed room layout. [1]
The January 2021 guidance explains the policy's ordinary limits, including a single basement storey and restrictions on how much garden can be developed below ground. The garden calculation applies separately to each garden: it is not permission to combine front and rear areas and excavate whichever is more convenient. The general limit is 50% of each garden, alongside the policy's other spatial limits and qualifications. [5]
These figures are Camden's published policy framework, not national basement permitted-development dimensions. A proposal must address all applicable criteria rather than selecting the garden percentage and disregarding building footprint, depth or environmental effects. [5]
Richmond: the permission trigger
Richmond's two basement Article 4 directions cover the borough's flood-risk and non-flood-risk areas. They took effect on 1 April 2018 and remove the specified permitted-development rights for basement development. This is a rule about the need for an application, not a statement that all basements are prohibited. [2]
The signed direction and its map should be checked against the work proposed. An Article 4 direction answers a different question from a design policy: it removes a particular automatic permission route, leaving the planning merits to be assessed through the application process. [2]
Kensington and Chelsea: use the current plan version
Kensington and Chelsea adopted its replacement Local Plan on 24 July 2024. The council also publishes a basement SPD dated April 2016. These dates matter: an older supplementary guide should not be treated as proof of every numerical limit in the current adopted plan. [3][6]
The borough also publishes a Code of Construction Practice. Its construction requirements need checking alongside the planning design assessment. A basement that has reached an acceptable planning design still needs a suitable construction-control response; these are not competing alternatives. [7]
Use Kensington and Chelsea's adopted basement policy and current supporting requirements for the particular site before deciding an excavation percentage or storey limit. [3]
Westminster: construction controls are not just for large basements
Westminster's 2026 Code of Construction Practice applies to new sites from 1 June 2026 and existing sites from 1 September 2026 under the arrangements explained by the council. Its Level 3 category includes new or extended basements of any size where the higher categories do not apply. Small floor area alone is therefore not a reason to disregard the code. [4]
The council describes construction and site environmental management documentation and the use of planning conditions. Treat this as a separate workstream from establishing the acceptable basement envelope. It can affect what must be prepared before construction starts. [4]
How to compare two properties
Prepare separate checks for the permission route, the adopted design limits, the required impact evidence and the construction-management controls. Camden's BIA and audit process, Richmond's Article 4 directions and Westminster's construction code demonstrate why one borough's checklist cannot simply be copied to another. [1][2][4]
Finally, check emerging plans without describing them as adopted. Camden's examination news records its replacement plan's examination activity during 2026. That is a reason to monitor policy changes, not to substitute an unadopted policy for the current one. [8]