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Basement conversions · London & Hertfordshire
Basement conversion London: cost by type, planning rules and build time — from £45,000
By Anderson & the LCCL Construction Team
Finchley N3 · Design & build, LABC award winners 2024 and 2026
A basement is the way to add a floor when the loft is done and the garden is short — a cinema room, a gym, a guest suite or a whole extra living floor under a Victorian or Georgian house. There are three very different jobs under one name: refitting an existing cellar, lowering its floor to full height, and digging a new basement where none exists. This page gives you the 2026 numbers for each, why basements need planning where extensions do not, what the Party Wall Act means for a dig, and how the job runs with one team.
Quick answer
Cellar refit £45,000–£95,000; floor lowering £95,000–£180,000; new basement £180,000–£300,000+. Design and build inclusive. Postcode dependent — most basements are in Inner and Central London.
A cellar refit usually needs no planning. Lowering and new digs usually need a planning application; several boroughs have basement-specific policies and Article 4 directions.
8–30 weeks on site depending on the type; 12–20 weeks of design, engineering and approvals before that.
A free 30-minute phone call with our team, no obligation.
LABC Building Excellence Award 2024
Best Residential Extension in London — Bohun Grove, East Barnet EN4
LABC Building Excellence Awards 2026
London Regional Winner, Best Conversion or Alteration — Weech Road NW6
One contract
Architecture, structural engineering, planning, construction and project management — all under one contract
Guaranteed & insured
12-month workmanship guarantee, structural guarantee to building-control sign-off, 10-year LABC Warranty available
What a basement conversion from LCCL includes
Design and build inclusive — one contract from the first sketch to the completion certificate.
Survey of the existing cellar or ground floor, the foundations, drainage and water table before anything is priced
Architectural design and drawings, with revisions until you're happy
Planning application with the basement impact assessment the borough requires; Lawful Development Certificate where a refit qualifies
Building-regulations drawings and structural calculations by our chartered engineer — underpinning sequence, retaining walls, new ground floor
Party wall notices and, on a dig, the party wall awards and schedules of condition the neighbours are entitled to
Excavation, underpinning in sequence, retaining walls and new slab; or, on a refit, the new floor and tanking
Waterproofing to BS 8102 — cavity drainage membrane with sump and pump, or tanking, as the design requires
Lightwells, escape windows and the new stair to building regulations
Insulation, plaster finish, first-fix electrics, plumbing and ventilation; bathroom where specified
Second fix, decoration and the items you supply fitted
LABC inspections and completion certificate; waterproofing guarantee
A dedicated project manager and a weekly progress update; 12-month workmanship and structural guarantees, 10-year LABC Warranty available
How much does a basement conversion cost in London in 2026?
Three different jobs, three ranges. Design and build inclusive; every project is priced from a survey and the ground conditions.
Basement type
What it involves
Guide range
Cellar refit (existing cellar, head height already there)
Waterproofing, insulation, new floor, stair, services and finishes — a usable room where there was storage
£45,000 – £95,000
Lowering (existing cellar, floor dug down for full height)
Underpinning the walls in sequence, new slab, waterproofing, stair, lightwell, full fit-out
£95,000 – £180,000
New basement (no cellar — dug under the house or garden)
Full excavation and underpinning, retaining walls, slab, waterproofing, lightwells, stair, fit-out
£180,000 – £300,000+
Also consider
Guide range
On site
Loft conversion
A bedroom floor above instead of below — cheaper and usually no planning
£27,000 – £124,000
4–16 wks
Rear extension
Ground-floor space where the garden allows
£35,000 – £75,000
10–14 wks
Basement + rear extension together
The Chipstead Street SW6 combination — one programme, one party wall process
Priced per project
Phased
What the price includes: survey, drawings, planning and basement impact assessment, building-regulations drawings, structural calculations, all excavation and underpinning, waterproofing, the stair and lightwells, insulation, first and second fix, decoration, LABC sign-off and guarantees. On top: sanitaryware and kitchen items above the allowance, cinema or gym fit-out, and party wall surveyor fees where neighbours appoint one (usual on a dig, from £750 per neighbour). The figure on your itemised estimate is the figure you pay — nothing is added afterwards. Postcode dependent — the instant estimator adjusts the range for your area in thirty seconds, and a detailed itemised estimate follows one survey.
What moves the price: the ground — London clay, sand, gravel and the water table decide the underpinning method and the pump; whether the dig extends under the garden; the number of lightwells; the depth (a 2.7 m finished ceiling costs more than 2.4 m); a bathroom or kitchen below ground; and access — spoil on a Central London street goes out through the house or a conveyor. Inner London adds 10–15% and Central London 20–25% to base ranges.
Basement conversions by house type
Victorian or Georgian terrace with a cellar
The best starting point — the cellar walls are already below ground, so a refit or a lowering gives a full floor without digging under the garden. Both neighbours get party wall notices; on a lowering they will usually appoint a surveyor.
Terrace or semi with no cellar
A new dig under the house, sometimes extended under the garden for a lightwell and a larger floor. This is the £180,000+ job: full underpinning in sequence, a planning application with a basement impact assessment, and party wall awards on both sides.
Detached house
Digs under a detached house are simpler — no party walls, easier access, and often a garden basement with a full lightwell. The ground and the water table still decide the cost.
Planning
Do you need planning permission for a basement?
A cellar refit usually not. Lowering and new digs usually yes — and several boroughs have their own basement rules.
Basement type
Usual route
Time
What we do
Cellar refit, no external change
Permitted development / no planning; building regulations only
—
Lawful Development Certificate where a lightwell is added within PD limits
Lowering or new dig with lightwells
Full planning with a basement impact assessment (ground, water, structure, neighbours)
8–13 weeks
Drawings, BIA by our engineer and a specialist where the borough requires it, application and negotiation
Camden, Kensington & Chelsea, Westminster, Islington and others
Basement-specific policies: one storey only, limits on extent under the garden, construction management plan
8–13 weeks
We design to the policy from the first sketch and prepare the CMP
Conservation area or listed house
Planning and, where listed, listed building consent
8–13 weeks
Heritage statement alongside the application
Building regulations, party walls & waterproofing
The approvals that matter most on a basement
Every basement needs building-regulations approval — structure, waterproofing, fire escape (a basement bedroom needs an escape window or a protected stair), ventilation, insulation and drainage, including a pumped system where the sewer is above the new floor. We do this through LABC with full-plans drawings and our own structural calculations.
On a terrace or semi the Party Wall Act applies in full: underpinning the shared wall and excavating within 3 m of the neighbour's foundations both require notice, and on a dig the neighbours nearly always appoint a surveyor. We serve notices at design stage and manage the awards and schedules of condition. Waterproofing is designed to BS 8102 and carries its own guarantee.
Basements are the one domestic project where the planning application is mostly about engineering: the basement impact assessment has to show how the dig affects the ground, the water and the neighbours' foundations. That is why the structural design is done before the application, not after — and why we start the party wall process at the same time.
Recent basement projects
Bohun Grove, East Barnet EN4 — rear extension, hip-to-gable loft and whole-house renovation
LABC 2024 Best Residential Extension in London
Semi-detached family home One programme loft + extension + renovation
A total renovation of a family semi: a rear extension opening into a full-width kitchen and dining space with floor-to-ceiling glazing and skylights, a hip-to-gable loft conversion with a rear dormer, two Lusso Stone bathrooms, enlarged bedrooms, new windows throughout, a regulated outbuilding and landscaping.
Because the loft and extension ran under one contract, the steelwork for the open-plan ground floor and the loft floor was designed together and lifted in on the same day — one crane hire, one scaffold, one building-control file.
See the Bohun Grove project · All projects
ANDERSON · FOUNDER, LCCL CONSTRUCTION
Anderson's advice: start the party wall process the week you sign
Most clients underestimate the party wall timeline — on a semi-detached or terraced house it can add three months to the project if it is left until the drawings are finished. We serve the notices at design stage, not build stage, so the two-month notice period runs while the planning application is in.
Talk to the neighbours before the letter lands. A five-minute conversation over the fence, with the drawings in your hand, is the difference between a consent form back in a week and a surveyor's bill.
How a basement conversion runs with LCCL
From the first call to the completion certificate. On a lowering or a new dig, engineering, planning and party wall run together for 12–20 weeks before the first spade — there is no shortcut.
WEEK 0
Free consultation call
A 30-minute phone call about the house, whether there is an existing cellar, what you want the space for and a realistic budget.
WEEKS 1–6
Survey, ground investigation & design
Survey of the cellar or ground floor, trial pits or boreholes where the design needs them, drawings, structural concept and your detailed itemised estimate — design, engineering, waterproofing, construction and warranty priced line by line.
WEEKS 6–20
Approvals & party wall
Planning application with basement impact assessment where needed, building-regulations drawings, full structural design and underpinning sequence, party wall notices, awards and schedules of condition. Nothing starts until every one is in place.
BUILD
On site
8–12 weeks for a cellar refit, 16–24 for a lowering, 20–30 for a new dig: excavation and underpinning in sequence, slab and retaining walls, waterproofing, stair and lightwells, first fix, plaster, second fix. Weekly updates from your project manager.
HANDOVER
Sign-off
Snagging, LABC final inspection and completion certificate, waterproofing guarantee, party wall final schedules, guarantees in writing. 10-year LABC Warranty available.
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Basement conversion questions we get asked most
In 2026, design and build inclusive: refitting an existing cellar £45,000–£95,000, lowering a cellar floor to full height £95,000–£180,000, digging a new basement £180,000–£300,000 or more. Central London adds 20–25%. The three are different jobs, which is why the range is so wide — the survey tells us which one your house is.
A cellar refit with no external change usually does not. Lowering the floor or digging a new basement usually does, with a basement impact assessment, and Camden, Kensington & Chelsea, Westminster, Islington and several other boroughs have basement-specific policies that limit depth and extent under the garden. We design to the policy from the first sketch.
Eight to twelve weeks on site for a cellar refit, sixteen to twenty-four for a lowering, twenty to thirty for a new dig — plus 12–20 weeks of engineering, planning and party wall before the first spade. The pre-construction stage on a dig cannot be compressed.
Not if it is designed to BS 8102. Most London basements use a cavity drainage membrane with a sump and pump behind the finishes, sometimes combined with tanking — a designed system with its own guarantee, not a coat of paint. The survey and the water table decide which.
On a terrace or semi, everything: underpinning the shared wall and digging within 3 m of the neighbour's foundations both need notice, the neighbours will usually appoint a surveyor, and a party wall award with a schedule of condition is agreed before work starts. We serve notices at design stage; left late, this is the item that adds three months.
Yes, if it has a means of escape — an escape window into a lightwell or a protected stair to the ground floor — plus the ventilation and daylight the regulations require. A lightwell is usually the answer and it is designed into the planning application.
A loft is cheaper, faster and usually needs no planning, so it comes first on almost every house. A basement makes sense when the loft is done, the garden is short, and the house is in an area where the value per square metre justifies £2,000–£4,000 per m² of new floor. We are not valuers — talk to a local agent before you commit.
For a cellar refit, yes. For a lowering or a new dig, most clients move out for the excavation and underpinning weeks — the ground floor is opened, the house is propped and the noise is constant — and return once the slab is in. We tell you honestly which weeks those are.
Where we build basements
From Finchley N3 we cover North London, North-West London, Central London, South-West and West London, and Hertfordshire — and we also work across the rest of London, Buckinghamshire, Surrey, Kent and Essex. Recent basements: St John's Wood NW8 and Fulham SW6 — and the period terraces of Central, North-West and South-West London where a basement is the last floor left to add.
Start with a figure, then a conversation.
Get an instant estimate for your postcode, or book a free 30-minute call with our team. Call us on 020 8037 7332, Monday to Saturday 08:00–18:00.
One team, one contract, one point of contact — from first sketch to handover.