Basement extensions in London: cost, policy and consent

Do you need planning permission for a basement in London?

Almost always, yes. The permitted development rights in Schedule 2, Part 1 of the Town and Country Planning (General Permitted Development) (England) Order 2015 are written around enlargement at and above ground level. Excavating a new storey beneath a house is not the kind of enlargement those classes were drafted for, and the moment the work involves a lightwell, a new external staircase, a raised pavement vault or any alteration to the front elevation, it engages parts of the Order that householder rights do not cover.

There is a narrower question that catches people out: converting an existing cellar into habitable space. Where the space already exists, is not being enlarged, and the work involves no external alteration, that can be a change of use rather than an extension — and may not need planning permission at all. It still needs Building Regulations approval, which is a separate consent under a separate statute. The distinction is between digging and fitting out, and it is worth establishing before anything else, because it decides which of the two routes below you are on.

The safe answer for any scheme that removes soil is to assume a full planning application. In the boroughs listed below, that assumption is close to a certainty.

Which London boroughs have their own basement policy?

Basement development is the clearest example on the whole planning system of local policy doing the real work. National policy says very little about it. A cluster of inner-London boroughs, responding to a decade of large subterranean schemes, adopted detailed policies that constrain size, depth, garden coverage and construction method far more tightly than anything in national guidance. Two are worth knowing by name.

The Royal Borough of Kensington and Chelsea — Local Plan Policy CL7

The Royal Borough's basement policy is the most restrictive in the country and the one most often quoted back at applicants. Its core limits: no more than a single storey below ground; no more than 50% of the garden or open part of the site may be built under; and a minimum depth of natural soil above the structure so that planting can survive. It also requires a construction traffic management plan, because in practice the objections that sink a basement application in RBKC are as often about lorry movements on a narrow street as about the structure itself.

The London Borough of Camden — Local Plan Policy A5 and the Basement Impact Assessment

Camden's approach is different in kind. Rather than only setting dimensional limits, Policy A5 requires the applicant to prove the scheme will not harm neighbouring structures, the local water environment or ground stability — through a Basement Impact Assessment, which the council has independently reviewed at the applicant's expense. This is the requirement that most often determines the programme, because a BIA is produced by a chartered engineer and hydrogeologist and cannot be assembled in a fortnight.

Everywhere else

Westminster, Hammersmith and Fulham, Islington, Wandsworth, Richmond and Merton all carry basement policies in their adopted Local Plans, and several publish separate supplementary planning documents on top. The numbering and detail differ enough that quoting one borough's rule at another's officer is worse than useless. Find the adopted Local Plan basement policy for the borough deciding your application and read it before the design is fixed — not after, when changing it means redrawing.

What is a Basement Impact Assessment, and who has to produce one?

A Basement Impact Assessment is a technical report demonstrating that a proposed excavation will not cause harm in three areas: land stability (will the ground move, and will neighbouring buildings move with it), hydrogeology (will the structure dam or divert groundwater), and surface water and flooding (will the loss of permeable ground push water somewhere it did not go before).

It is not a planning statement written by a planning consultant. The stability and groundwater sections are signed by a chartered civil or geotechnical engineer, and in the boroughs that require one, the council commissions its own independent audit of the report before the application is determined. Where a BIA is required and absent, the application is normally not validated at all — it does not get refused, it simply never starts, which is a slower and more expensive outcome.

Even where no formal BIA is demanded, any London basement needs a ground investigation before the structural design can be completed. London's geology varies across a few hundred metres: London Clay behaves differently from the river terrace gravels along the Thames, and a scheme designed on an assumption about which one is under the house is a scheme designed on a guess.

The Building Regulations a basement has to satisfy

Planning permission decides whether the basement may exist. The Building Regulations, made under the Building Act 1984, decide whether what you have built is lawful to occupy. They are assessed by building control — either the local authority's team or an approved inspector — and a basement engages more parts of them than almost any other domestic project.

Part A — structure

The existing house has to stay up while the ground beneath it is removed. In a London terrace that normally means underpinning in short bays, excavated and filled in a staggered sequence so that only a small proportion of the wall is unsupported at any moment. The sequence is part of the engineer's design, not a decision for the site, and it is the single most common point at which an under-specified basement goes wrong.

Part C and BS 8102 — keeping water out

Below ground, waterproofing is a designed system rather than a product. BS 8102:2022 sets out the approach: Type A (barrier — a tanking membrane), Type B (structurally integral — waterproof concrete), and Type C (drained protection — a cavity drain membrane routed to a sump and pump). Habitable basement space in London is normally designed to combine two of the three, because any single system has a failure mode, and a Type C system introduces something no other part of a house has: a pump that must keep working, with a maintenance obligation and usually a battery backup.

Part B — getting out

Fire escape is what most often forces a basement layout to change late. A habitable basement generally needs either a protected escape route to the outside or an external door or window suitable for egress. An open-plan staircase connecting a basement to the ground floor — the arrangement almost every homeowner sketches first — can compromise the protected route for the whole house, and the fix is usually a fire-rated enclosure that changes how the space feels.

Part H — drainage and building over sewers

Basement drainage cannot rely on gravity, so waste normally discharges to a pumped station. Separately, if a public sewer runs within three metres of the excavation, the sewerage undertaker's consent is required — a build-over or build-near agreement under the Water Industry Act 1991, entirely separate from both planning and building control, and with its own timescale.

How does the Party Wall etc. Act 1996 apply to a basement dig?

More heavily than for any other domestic project, and on a statutory timetable that cannot be compressed. Two sections matter.

Section 6 is triggered by excavation depth and proximity. Excavating within 3 metres of a neighbouring structure and to a depth below the bottom of its foundations requires notice. So does excavating within 6 metres, where a line drawn down at 45 degrees from the bottom of the neighbour's foundation would meet the excavation. Practically every London terrace basement engages one or both.

Section 2 covers the work to the party wall itself — underpinning it, cutting into it, or inserting beams — which a basement invariably involves.

A section 6 notice requires one month's notice before work starts, and a section 2 notice requires two months. A neighbour who does not consent triggers the appointment of surveyors and a party wall award, which takes longer again. This runs alongside the planning application rather than after it, and treating it as paperwork to sort out once permission arrives is how a basement loses a season.

What does a basement application actually have to contain?

A householder application for a basement is a substantially larger submission than one for a rear extension. Beyond the standard existing and proposed plans, sections and elevations, expect to provide a structural method statement describing the excavation and temporary works sequence; a ground investigation report; a Basement Impact Assessment where the borough requires one; a construction traffic management plan; an arboricultural report where trees are within influencing distance; and a drainage strategy showing how the basement discharges and how surface water run-off is managed.

Sections carry more weight here than plans. A basement is judged on what happens vertically — how deep, how close to the boundary, how much soil sits above the roof of the structure, where daylight reaches. A set of drawings that shows a convincing plan and a vague section will attract exactly the questions that add months.

What statutory charges apply?

These are set by regulation and are the same for every applicant, whoever prepares the drawings.

• £548 — the householder planning application fee, where the basement serves an existing single dwelling.
• £610 — the fee for a full application, which is the route where the scheme creates a new dwelling rather than extending one.
• £274 — a lawful development certificate, the sensible route where you believe an existing cellar conversion needs no permission and want that established on the record before you sell.
• Community Infrastructure Levy — charged on additional floor area, and a basement is floor area. CIL is calculated on gross internal area, rates are set by the borough and the Mayor, and householder extensions may qualify for relief that must be claimed before work begins. Missing that sequence is expensive and cannot be retrofitted.

Building control charges and the sewerage undertaker's build-over fee are separate again. What drives the professional design cost is the engineering: the ground investigation, the structural design of the underpinning sequence, and the impact assessment — see what your project involves.

How long does a basement approval take in London?

The statutory determination period for a householder application is eight weeks from validation, and thirteen weeks for a major application. Those are the headline figures, and for a basement they are the least useful part of the programme, because the work that governs the timeline happens before validation and alongside determination.

The realistic sequence is: ground investigation and survey first; structural and impact assessment work next, which is where a Basement Impact Assessment sits; then validation, then the eight-week clock, during which the council's independent audit of the BIA runs. Party wall notice periods — one month under section 6, two under section 2 — run in parallel and must be served early enough that the award is in place when the excavation is ready to start. Where a scheme is refused and appealed, add substantially more.

The compressible parts of that are the ones done in the wrong order. The statutory periods are not compressible at all.

What gets basement applications refused?

Refusals cluster into a small number of recurring reasons, and none of them are surprises once the local policy has been read.

• Exceeding the garden or footprint limit. Where policy caps the proportion of the open site that may be built under, a scheme that exceeds it is refused on that ground alone, regardless of how well it is designed.
• More than one storey below ground, in the boroughs whose policy permits only one.
• An inadequate or absent impact assessment. Usually a validation failure rather than a refusal — the application never starts.
• Harm to trees. Excavation inside the root protection area of a protected tree, or of one in a conservation area, without an arboricultural method statement.
• Construction impact. Lorry movements, spoil removal and duration on a constrained street, particularly where the council has an adopted construction management requirement.
• Heritage harm. On a listed building, or in a conservation area, a lightwell or external stair altering the front elevation is judged against the character of the whole terrace, not the individual house.

The pattern is that basements are rarely refused on the merits of the room. They are refused on depth, on garden coverage, on trees, on traffic, and on the absence of evidence. All five are decided at the drawing stage.

Choose your borough