Planning Portal distinguishes adapting existing space from basement works that significantly change the building, its appearance or its use. [1]
First describe all the work
Start with the existing lawful use and a drawing of everything proposed. Include floor lowering, extensions beyond the existing cellar, external doors, lightwells, railings and rooflights. Assessing only the internal floor plan can miss the part of the project that needs permission. [1] [2]
Then make the proposed use explicit. Additional rooms belonging to the existing house and a self-contained basement flat are not interchangeable descriptions. A separate home also introduces housing-design and flood-vulnerability considerations. [3] [4]
Check local restrictions
An Article 4 direction can remove specified permitted development rights in its mapped area. In Camden, the basement direction has applied since 1 June 2017 and covers the specified basement and lightwell excavation works. It is not a statement that ordinary internal decoration itself requires planning permission. [2]
Richmond's direction took effect on 1 April 2018 in the areas identified by its direction and map. Check the actual address and the works covered; do not assume either that it covers the whole borough or that a neighbouring borough uses the same approach. [5]
A basement policy and an Article 4 direction do different jobs. The policy helps assess an application, while the direction can remove a route that would otherwise avoid an express application. Read both where relevant. [2]
Planning permission is not the only approval
Making a cellar habitable normally involves building regulations matters, including structure, fire safety, damp protection, ventilation and drainage. This remains a separate assessment even where an express planning application is unnecessary. [6]
Works affecting the special character of a listed building can need listed building consent, including internal alterations. Excavation close to a neighbour or work to a party structure may also engage the Party Wall Act. Neither process is replaced by planning approval. [7] [8]
Where a public sewer is affected, Thames Water's requirements need to be checked separately. A drawing accepted by the planning authority does not itself authorise works over or close to the sewer. [9]
Establish the route before detailed pricing
Ask the design team to record the planning conclusion against the complete proposal, with any restrictions identified. Where there is doubt, obtain a formal route to establish lawfulness or permission rather than relying on a builder's assurance that the work is hidden underground. Planning Portal identifies the need to check the council's position before basement work. [1]
If permission is needed, establish the local technical submissions early. Camden, for example, requires a site-specific Basement Impact Assessment for new or extended basements, including engineering input. Other councils may use different document names and validation requirements. [10]
As checked on 2 October 2026, Camden's replacement basement policy remains under examination. A draft change should not be treated as an adopted permission or a relaxation of the current route. [11] [12]