Basement Planning Permission in London

Basements are one of the few ways left to add real space in London, and they are also the most heavily regulated. This guide sets out what you apply for, what it costs, how long the borough has to decide, and what happens if it says no.

Converting an existing cellar

Converting an existing cellar into habitable space, with no change to the outside of the house, normally needs no planning permission at all, only building regulations approval. Planning control bites on external appearance and on operations such as excavation, and lining out a dry cellar is neither.

Two things break that rule quickly. Lowering the existing floor slab to gain head height is an excavation, and cutting a lightwell or a new opening in the front elevation is an external alteration. Because the line is fine, it is worth applying for a proposed Lawful Development Certificate first. It costs half a householder fee and gives you a signed council document proving the work never needed permission, which is what a buyer's solicitor will ask for later.

Digging a new basement

Excavating a new basement, lowering the floor level, or forming lightwells and other external alterations needs full planning permission, applied for as a householder application if the property is a single house. You apply to your borough, normally through the Planning Portal, and the borough has 8 weeks from validation to decide a householder or minor application. If it does not decide in time, you have a right of appeal on the ground of non-determination rather than an obligation to keep waiting. Flats are different: works to a flat are not householder development, so a basement under a lower ground floor flat is a full application at a higher fee, and the freeholder's consent is a separate matter that planning permission does not give you.

What you apply for, and what it costs

Almost every London basement needs more than one consent, and they run in parallel rather than in sequence. These are the statutory fees in force from 1 April 2026.

Those are the council and utility fees only. The professional costs are the larger number: structural engineer, party wall surveyors and the Basement Impact Assessment itself. For construction, our basement conversion cost guide puts London work at £1,200 to £5,000 per m² depending on whether you are lining an existing cellar or underpinning and digging out.

London borough basement policies

Several inner London boroughs adopted dedicated basement policies after a wave of large digs, and they are stricter than national policy. Kensington and Chelsea (Policy CD11), Camden (Policy A5) and Westminster all operate detailed local plan policies that typically require:

• Basements limited to a single storey below ground, with exceptions rarely granted.
• A cap on how much of the garden or open part of the site can be excavated, commonly no more than half.
• A Basement Impact Assessment and a structural method statement, submitted with the application rather than conditioned.
• A construction traffic management plan covering lorry routes, hours and spoil removal.
• Retention of a minimum depth of soil over the basement for planting and drainage.

Read the policy before the architect draws anything. A scheme designed to a generic permitted development mindset and then tested against a basement policy usually has to be redrawn, and that is where months are lost.

What is in a Basement Impact Assessment?

A Basement Impact Assessment is a technical report, prepared by suitably qualified engineers, that models what your excavation will do to the ground and to the buildings around it: surface water and flood risk, groundwater flow, ground stability and the predicted movement of neighbouring structures. In boroughs with an adopted basement policy it is a validation requirement, so the council will refuse to register the application without one.

It usually drives the design rather than describing it. Predicted ground movement dictates the underpinning sequence, the temporary propping and often the depth you can realistically reach, so commission it early enough that its findings can still change the drawings.

The other approvals

Planning permission is one of four separate consents, and they are not sequential. A basement dig will normally need a party wall award, because excavating within 3 metres of a neighbouring structure below the level of its foundations triggers a notice of adjacent excavation under section 6 of the Party Wall etc. Act 1996, with a minimum of one month's notice. You serve that notice as the building owner, or your party wall surveyor does for you; we supply the drawings and programme it needs and build to the award (see party wall work in London). It will need building regulations approval covering structure, waterproofing, ventilation, means of escape and ceiling height. And where a public sewer runs within 3 metres, it will need a Thames Water build over agreement, which can change your foundation design.

What if it is refused?

You have two routes, and the refusal notice tells you which one to take. If the reasons are about detail, such as the design of a lightwell or the wording of a method statement, amending and resubmitting is faster. Note that the old free repeat application was abolished in December 2023, so a resubmission now costs a further £548. If the reasons go to the principle, for example that the borough considers a basement of that size unacceptable in that location, that is a policy disagreement and the route is an appeal to the Planning Inspectorate. A householder appeal must reach the Inspectorate within 12 weeks of the date on the decision notice, and there is no fee and no discretion on the deadline.

GS Renovation manages basement projects across London, from the pre-application meeting through to sign-off. For a free assessment, contact us or call 07472 424 226, or read our guide to getting planning permission in London.

The service side of this is covered by our extension service in London, and the next step is usually cellar conversion against a new dig.