Basement Conversion London 2026: Cost, Planning Permission and What the Council Asks For
Updated: 20 minutes ago
By Anderson & the LCCL Construction Team | Finchley N3
London houses run out of roof before they run out of demand for space, and on a terrace or a semi that has already had its loft and its rear extension, the basement is what is left. It is also the most engineered project a house can take on: a new floor built below the foundations of the house and, on a terrace, below the foundations of the neighbours. The questions are always the same three. What does it cost, does it need planning permission, and what will the council and the neighbours ask for before a spade goes in. This guide answers all three for 2026, with the figures we price from and the rules borough by borough.
As LABC Building Excellence Award Winners 2024 for Best Residential Extension in London — and London Regional Winners again in 2026 for Best Conversion or Alteration to an Existing Home — we design and build basements as one contract, from the structural method statement to the tanking certificate, and the St John's Wood NW8 basement in the picture above is one of ours. Thinking about a basement alongside a loft or extension? Check which parts of the project are permitted development first.
The three kinds of basement, and what each costs in 2026
Cellar refit. The house already has a cellar or a lower-ground floor with usable headroom; the work is waterproofing, insulating, a new floor, a staircase, services and finishes, sometimes with a lightwell cut to the front or rear. From £54,000 to £114,000 inc. VAT (£45,000 – £95,000 + VAT) for 20 to 35 square metres, eight to twelve weeks of design and eight to twelve weeks on site. Lowering. The existing cellar floor is dug down to give full headroom, which means underpinning the walls in sections and usually the party walls too. From £114,000 to £216,000 inc. VAT (£95,000 – £180,000 + VAT) for 25 to 40 square metres, ten to fourteen weeks of design and fourteen to twenty on site. New dig. There is no cellar; a new storey is excavated under the footprint of the house, or under the garden, with the whole structure underpinned and a new slab, walls and lightwells formed. From £216,000 to £360,000 inc. VAT (£180,000 – £300,000 + VAT) for 30 to 50 square metres, twelve to sixteen weeks of design and twenty to thirty on site, and more where the dig extends under the garden or below the water table. All figures are design and build at economy specification, shell-and-plaster finish, postcode dependent; the instant estimate gives a range for your own house in about two minutes.
Does a basement need planning permission?
It depends on the borough more than on the basement. In principle a basement under a house can be permitted development where it does not change the external appearance, and a cellar refit with no new lightwell is internal work that needs no application at all. In practice the inner London boroughs removed that right years ago. Camden has had a borough-wide Article 4 direction since 1 June 2017: every basement needs planning permission, limited to one storey, no more than half of each garden, less than one and a half times the footprint of the house and no further than half the depth of the house beyond the rear wall, with a Basement Impact Assessment by a chartered engineer that the council has independently audited. Westminster did the same city-wide from 31 July 2016, with one storey below the lowest original floor, a metre of soil over any garden basement and an engineer-certified method statement. Islington decides under its Basement Development SPD: single storey below the lowest original habitable floor, no more than 3 metres floor to ceiling, less than half the original garden, a metre of soil and 200 mm of drainage beyond 3 metres of the rear wall, and a structural method statement with the application. Hackney limits the footprint to one and a half times the house and half the garden depth, with lightwells to no more than half the front garden. Further out, Enfield asks for a Basement Impact Assessment on its validation checklist and Haringey asks for structural stability to be demonstrated, while Barnet has no basement-specific policy and judges on amenity and appearance. The council has the final say; the house extension planning permission guide covers the householder application itself.
The party wall notice: the one you cannot skip
On a terrace or a semi, a basement is dug within 3 metres of the neighbours' foundations and below them, and usually within the 6 metre, 45-degree line as well. That is an adjacent excavation notice under section 6 of the Party Wall etc. Act 1996, served at least one month before work starts, with drawings showing the depth and the method; where the party wall itself is underpinned there is a party structure notice too, at two months. The neighbours almost always dissent on a basement, which is not hostility but prudence, and the award that follows records the condition of their house room by room, fixes the sequence of underpinning, the monitoring and the insurance, and is the document that protects both sides. Surveyors' fees on a basement are higher than on a loft, often several thousand pounds per neighbour, and we list them separately in every estimate. The party wall notice guide follows the timeline step by step.
What building control checks
Four things above all. Structure: the underpinning sequence, the new retaining walls and slab, and the temporary works that hold the house up while the ground beneath it is removed, all from the engineer's calculations and method statement, inspected pin by pin. Waterproofing: designed to BS 8102, normally two forms of protection together, a cavity drainage membrane with a sump and pump behind the finishes plus either a tanked structure or a waterproof concrete box, with the pump on a battery back-up. Fire and escape: a habitable basement room needs a protected stair or an escape window or door to a lightwell big enough to climb out of, and that lightwell is why most new-dig basements have one. Ventilation and daylight: background and extract ventilation, and where a bedroom is planned, a lightwell or sunken terrace for daylight. Drainage below the level of the sewer runs through a pumped station, and the completion certificate is issued when all of it has been seen.
How long it takes
A basement is slower than a loft or an extension at every stage. Design takes eight to sixteen weeks because the structural engineer's method statement, the site investigation and, in Camden, Westminster and Islington, the impact assessment all have to be finished before the application goes in. The planning decision is eight weeks and basements go to committee more often than other householder applications. The party wall awards take their own six to ten weeks. On site, a cellar refit is eight to twelve weeks, a lowering fourteen to twenty, and a new dig twenty to thirty, most of it in the ground before anything looks like a room. Plan a year from the first conversation to the finished floor on a new dig.
Is it worth it?
In the parts of London where a basement makes sense, the inner boroughs where land values are highest and lofts and rear extensions are already done, a well-built basement adds a floor's worth of accommodation to a house that could not otherwise grow, and that is what the price reflects. The honest test is the one we apply before we quote: if the house still has an unconverted loft or a rear extension to build, do those first. They cost far less per square metre and need a fraction of the engineering. The basement is the project for the house that has used its other allowances, which is why the planning check asks about earlier extensions before it gives a verdict.
Anderson's advice
Use the council's pre-application service on a basement, and choose your borough with your eyes open. Our experience is that Wandsworth's pre-application advice is more useful than Islington's: in Wandsworth you get a written view on the scheme that the case officer then follows, which on a basement is worth the fee many times over. In Islington the SPD is the answer and the pre-app tends to repeat it. Either way, go in before the engineer has finished the design, not after, because the one thing you cannot change at low cost on a basement is the depth.
Why homeowners choose LCCL for a basement
One contract and one point of contact: our team surveys the house and the ground, designs the basement with a chartered structural engineer, prepares the impact assessment or method statement the borough asks for, makes the planning application, serves the party wall notices and builds it, with the waterproofing warranted and the council's building control inspecting every stage. Every project carries a 12-month workmanship guarantee and a 10-year LABC structural warranty.
What our clients say
“LCCL Constructions was professional, trustworthy and worked to the highest standard. The process was smooth and I'm very happy with the result - it has transformed the quality of my home.”
— KashImaging, Trustpilot
Frequently asked questions
How much does a basement conversion cost in London in 2026?
From £54,000 inc. VAT (£45,000 + VAT) to refit an existing cellar, from £114,000 inc. VAT (£95,000 + VAT) to lower one with underpinning, and from £216,000 inc. VAT (£180,000 + VAT) for a new dig under the house, all design and build at economy specification for 20 to 50 square metres. Postcode, ground conditions and the water table move the figure; surveyors' fees and council fees are separate.
Do I need planning permission for a basement in London?
In Camden and Westminster, always: both have Article 4 directions removing permitted development for basements. Islington, Hackney and most inner boroughs decide under basement policies that limit depth, footprint and garden coverage. A cellar refit with no external change usually needs no application; a new lightwell or a dig under the garden usually does. The council has the final say.
Do I need a party wall agreement for a basement?
Yes on a terrace or semi. Digging within 3 metres of the neighbours' foundations and below them needs an adjacent excavation notice at least one month before work starts, and underpinning the shared wall needs a party structure notice at two months. Neighbours usually dissent on a basement and the surveyors' award then sets the method, monitoring and schedule of condition.
Get a figure for your own project
Not sure whether your project needs planning permission? Enter your postcode and tick what you want to build — we look up the conservation areas, Article 4 directions and listed buildings for you, and you see which parts are permitted development and the from-price for each. Lawful Development Certificate included on every permitted development job.
To book a free 30-minute consultation call about your basement conversion, call LCCL Construction on 020 8037 7332 or get an instant estimate online — six questions, about two minutes, and you see a real range for your postcode on screen. Finchley N3 based, covering all of London and Hertfordshire.
One team, one contract, one point of contact — from first sketch to handover.
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