Do you need planning permission for a basement?

... works which (i) affect only the interior of the building, or (ii) do not materially affect the external appearance of the building, and are not works for making good war damage or works begun after 5th December 1968 for the alteration of a building by providing additional space in it underground

What the courts have said

The Order has no Class for basements, and the government's technical guidance does not mention them. Two High Court judgments set out the position.

Kensington and Chelsea, 2015. Two single-storey basements, 3.2 metres deep and entirely under the footprint of mews houses, were held to fall within Class A. The "enlarged part" was the basement alone, so it had a single storey, and the court drew "no distinction ... between storeys above or below ground" (Royal Borough of Kensington and Chelsea v SSCLG [2015] EWHC 2458 (Admin), paragraph 40).

Camden, 2016. The court quashed a lawful development certificate for a basement under a terraced house. Whether excavating the space and supporting the house and its neighbours is a separate "activity of substance" is a question of fact and degree, and where it is, "The Class A right grants planning permission for one of the two activities of the development but not for the engineering aspect" (Eatherley v London Borough of Camden [2016] EWHC 3108 (Admin), paragraph 59).

On that reasoning, the other Class A limits apply to a basement as they would to an extension above ground. On Article 2(3) land, which includes conservation areas, Class A permits no enlargement beyond a side wall (A.2(b)).

Lightwells

Nothing in Part 1 mentions lightwells. A lightwell in front of the house is either an engineering operation outside Class A or part of an enlargement beyond the principal elevation, which A.1(e) excludes, so it needs planning permission. A report the government published in November 2008, quoted in the Camden judgment, said that basement lightwells, being classed as engineering operations, did not benefit from permitted development rights (paragraph 40), and the Planning Portal lists adding a lightwell among the works that need planning permission. A small lightwell at the rear could only come within Class A as part of the enlargement and within every Class A limit, which is for the council to judge on the facts.

Councils that require permission

An Article 4 direction can remove Class A rights for basements. Camden confirmed one covering the whole borough on 3 October 2016, requiring planning permission for basements from 1 June 2017 (Eatherley, paragraph 25). Richmond upon Thames states that planning permission is required for basement developments. Check your own council's local plan and any Article 4 direction before you design.

Building Regulations and party walls

The Planning Portal says Building Regulations apply to basement works, including fire escape, ventilation, ceiling height and damp proofing. Underpinning is building work in its own right (regulation 3(1)(f) of the Building Regulations 2010). Excavating within 3 metres of a neighbour's building to below its foundations, or within 6 metres where the dig cuts a 45 degree line down from them, needs notice at least one month before you start (section 6 of the Party Wall etc. Act 1996). See the 3 metre and 6 metre rules and do you need a party wall agreement.

What this means for you

Start with the council: its local plan, any Article 4 direction and its validation requirements for basements. Then run the rest of the project through the permitted development checker, and bring in a structural engineer early.

Getting certainty

Because the answer turns on fact and degree, a lawful development certificate is how to have the council decide it before you dig. Describe the excavation, temporary works and underpinning, not only the finished room: in the Camden case the council had no description of the engineering works in front of it (paragraph 67). From 1 April 2026 the fee is £274, and the council has 8 weeks to decide.