Basement Conversion in London: Costs, Planning and Waterproofing

Basements · Costs

What a basement conversion in London costs, how converting a cellar differs from digging a new basement, and what planning, party wall and waterproofing involve, from an architect’s view.

Last updated: 30 October 2026 · Reviewed by Graham Sproul RIBA, Design & Operations Director

The figures here are approximate and meant to give you an idea, not a quote. Every house and brief is different, so your costs and programme will depend on your project.

How much does a basement conversion cost in London?

It depends on which of three routes your house needs. Converting a dry cellar that already has the height is the least work. Lowering a cellar floor means underpinning. Digging a new basement under the house or garden is the most involved. Our guide rates for new basement space are approx. £3,000 to £4,500 per m² under the house and £2,500 to £3,500 per m² under the garden.

Route	What’s involved	What pushes the cost up
Convert an existing dry cellar	Waterproofing, insulation, services, finishes, a proper stair; the floor stays where it is	Damp, low headroom, getting light in
Lower an existing cellar floor	Digging down and underpinning the walls, then everything above; lowering by about 400 mm can mean rebuilding the existing stair	Depth, ground conditions, party walls, the stair
New basement under the house	Excavation and underpinning under the existing house, structure, waterproofing, fit-out	Site access and spoil removal, ground and water table, party walls
New basement under the garden	Open excavation in the garden, new structure, waterproofing, fit-out	Access, walk-on rooflights and lightwells, drains and trees

Our guide rates cover London and the South East. Site access and logistics play a large part, which is why the ranges are wide.

The rate is the build cost. Your budget needs the items on top of it too, and on a basement some of them are larger than on an extension, because there’s more to survey and more neighbours to notify.

On top of the rate	Figure	What to know
Professional and statutory fees	About 12% of the build cost on average	From our analysis of hundreds of projects; see the renovation cost guide for how it’s worked out
Party wall surveyors	As published on our fees page: about £200 per notice, about £900 to £2,000+ per award	An award is needed only if a neighbour doesn’t consent
Thames Water build-over agreement	Class 1 £130, Class 2 £490, Class 3 £585 plus legal costs from £1,000	Only within 3m of a public sewer or 1m of a public lateral drain; Thames Water’s area only; fees change every 1 April
Planning application	£548 (householder, from 1 April 2026)	A new basement commonly needs permission
Contingency	A healthy contingency, set for your house	Digging down finds things no survey can see

A worked example: 40 m² under a London terraced house

Take a new 40 m² basement under a mid-terrace house, with underpinning to both party walls. This is illustrative, built from the guide rates above.

Line	How it’s worked out	40 m² basement
Build cost	40 m² x £3,000 to £4,500	£120,000 to £180,000
Professional and statutory fees	About 12% of the build cost	£14,400 to £21,600
Party wall notices	2 neighbours x about £200	About £400
Party wall awards, if both neighbours dissent	2 x about £900 to £2,000+	About £1,800 to £4,000+
Subtotal	Build + fees + party wall	About £136,600 to £206,000+
Surveys, planning fee, contingency	As above	On top

Converting an existing cellar: what’s involved?

Many Victorian and Edwardian London houses have a coal cellar or larder basement under the front of the house. If it’s dry, reachable by a stair and has enough headroom, turning it into a room is closer to a fit-out than a construction project. The work is waterproofing, insulation, heating, wiring, light and a safe way in and out.

Most cellars need more than that. Few were built to be lived in: they’re often damp, low and dark, so a cellar conversion usually starts with waterproofing and sometimes with lowering the floor, which brings in underpinning and the cost of the next route up. Your architect should tell you early which one your cellar is.

Headroom is the other early question. Building Regulations set no minimum room height, but a stair needs 2m of headroom over it. Lowering a floor by about 400 mm can mean rebuilding the existing stair to get that.

Converting an existing cellar is unlikely to need planning permission, according to the Planning Portal. It does if you add a lightwell, change the outside of the house, or create a separate home. Building regulations still apply: fire escape routes, ventilation, ceiling height, damp proofing and wiring are all checked.

Converted cellars make good rooms for things that don’t need much daylight:

• a wine cellar
• a sauna
• a gym
• a utility room
• a home office or reception room, where a lightwell can bring in daylight

On Melbury Gardens in SW20, for example, we’ve designed a basement with a sauna and a wine cellar for a private client.

If the cellar is part of a wider renovation, our guide to house renovation cost per m² in London covers the rest of the house.

Basement extension: under the house or under the garden?

Digging under the house means underpinning the existing walls, including the party walls, while the house stands above you. Digging under the garden is an open excavation with more room to work, and walk-on rooflights can bring daylight straight down. That’s why our guide rate is lower under the garden: approx. £2,500 to £3,500 per m², against £3,000 to £4,500 under the house.

Question	Under the house	Under the garden
Structure	Underpinning the existing walls and party walls	A new retaining structure in open ground
Daylight	Lightwells, front or rear	Walk-on rooflights and lightwells
Access during works	Through the house or a front lightwell; spoil carried out	Usually easier, but only if there’s a way into the garden
Planning	Commonly needs permission; front lightwells face stricter rules	Commonly needs permission; garden coverage limits in some boroughs
AURA guide rate	Approx. £3,000 to £4,500 per m²	Approx. £2,500 to £3,500 per m²

Do you need planning permission for a basement in London?

For a new basement, usually yes. The Planning Portal says excavating a new basement is likely to need planning permission because of the major works involved, and it commonly does because of the effect on the outside of the house. Converting an existing cellar is unlikely to need it, unless you add a lightwell, change the outside or create a separate home.

Many London boroughs limit how deep and how far a basement can go, so we check your council’s basement policy before design starts. Many also ask for a Basement Impact Assessment, which looks at ground conditions, drainage and the effect on neighbours. Where the council requires one, we commission it through the structural engineer.

A front lightwell visible from the street can face stricter rules on appearance. Whether any work counts as permitted development, and the routes in general, is covered in our planning permission vs permitted development guide.

Which surveys does a basement need?

Surveys before design limit the risks and give the structural engineer what they need to design the basement safely. They also make a planning application stronger, because many boroughs ask for the evidence. These are the surveys we usually recommend.

Survey	Why it matters
Basement Impact Assessment (where the council requires it)	Commissioned through the structural engineer; shows the council the effect on ground, groundwater and neighbouring buildings
Geotechnical (ground investigation)	The soil and groundwater decide the structure, the dig and the waterproofing
Flood risk assessment	Some areas and boroughs require it; it shapes the waterproofing
Drainage survey	Finds drains and sewers under or near the dig, including any that need a build-over agreement
Topographical survey	Levels and boundaries for the design and the planning drawings
Arboricultural survey	Trees near the dig affect foundations and may be protected

Party wall and Thames Water: what to sort before digging

Almost every London basement needs party wall notices. Digging within 3m of a neighbour’s building and deeper than its foundations, or within 6m in some deeper cases, needs an adjacent excavation notice at least one month before work starts. Underpinning a shared wall needs a party structure notice, at least two months ahead. A schedule of condition records the neighbour’s house before work begins.

Our guide to party wall agreements explains the notices and the 3m and 6m rules with diagrams. If you’re in Thames Water’s area and the dig is within 3m of a public sewer or 1m of a public lateral drain, you’ll usually need a Thames Water build-over agreement too.

How do you keep a basement dry? Types A, B and C

The British Standard for below-ground waterproofing, BS 8102, sets out three types of protection: a barrier (Type A), a watertight structure (Type B), and drained protection (Type C). They’re often combined. Choose the system with a waterproofing specialist at design stage, before any digging, because a leaking basement is hard to trace and expensive to put right.

Type	How it works	What to know
Type A: barrier protection	A tanking membrane or coating applied to the structure keeps water out	Relies on the barrier staying unbroken; repairs mean reaching it
Type B: structurally integral protection	The concrete structure itself is designed to be watertight	Relies on the quality of the concrete and its joints; not suitable for every case
Type C: drained protection	Lets water reach the structure, then collects it in a cavity and drains it away from the rooms, usually to a pump	The pump and channels need maintenance; it manages water rather than stopping it

On our basement projects we specify Type C (drained cavity) systems, often combined with another type. Redirecting water is more manageable and predictable than trying to stop it completely, especially next to neighbours where there’s no room to work on the outside of the walls. Ask for the paperwork and guarantees at the end; you’ll need them if you sell, and your insurer may ask for them.

How do you get daylight into a basement?

Plan the light before the layout. Lightwells at the front or back, and walk-on rooflights under a garden, are the main sources, so put the rooms that need daylight next to them. Rooms that don’t, like a utility room, wine cellar or sauna, can go in the darker parts of the plan.

Fewer internal walls let light travel further. If you need separate rooms, glazed doors and screens keep the light moving while still closing a space off. A lightwell is also an outside change, so it’s part of the planning question above.

When we’d recommend a basement, and when we wouldn’t

A basement makes most sense when the house can’t grow anywhere else: the garden is small, the loft is already done, or the street won’t accept more upwards. It makes least sense when a dry cellar can’t be found, the ground is difficult, or a loft or rear extension would give the same rooms for less.

We’d look hard at a basement when there’s an existing cellar to build on, the rooms you want don’t need much daylight, and the borough’s policy allows what you need.

We’d suggest another route first when the garden is the only place for light and access, the water table is high, or the same floor area is available in the loft or at the back of the house. Our comparison of a basement, a loft or an extension weighs the three side by side.

Private-client basement and cellar projects we’ve worked on include Nevern Square (SW5), Fitzwilliam Road and Clapham Manor Street (SW4) and Melbury Gardens (SW20). Outside London, we’ve designed a basement for Theobalds Manor, a Grade II listed, mid-18th-century house in Hertfordshire.

If you’d like help with the design, our basement architects in London page explains how we work on basement projects.

What can go wrong, and how to avoid it

Most basement problems start as decisions left to site. They’re cheaper to solve on paper, so we settle them in the design and the tender documents. These are the ones we see come up.

The drained cavity system is left to the builder. A Type C system needs a sump, pumps, an alarm and an agreed route for the water to leave the site. Thames Water may refuse discharge to the sewer, so the route has to be designed and agreed early, with the pumps and alarm shown on the drawings.

A floor or courtyard ends up below sewer level. Lowering a basement floor or a front courtyard can put it below the sewer. Drainage then needs a pump or an anti-flood valve, and both belong in the design.

The stair stops working. Lowering a floor by about 400 mm changes the stair’s rise and the headroom over it. The stair may need rebuilding to keep 2m of headroom, so price it from the start.

Water gets in. A leak below ground is hard to find and can cost far more than the waterproofing did. Choose the system with a specialist at design stage, use an experienced installer and keep the certificates.

The ground isn’t what you expected. Digging down can uncover buried services, old foundations or a higher water table. A ground investigation and drainage survey before design narrow the surprises, and a contingency covers the rest.

Access and spoil. Every tonne of soil has to leave the site, and on a terrace that often means through the house or a front lightwell, with skips and parking suspensions on the road. Ease of access affects the cost, so plan it at design stage.

Neighbours. Late party wall notices delay the start, and a missing schedule of condition makes damage claims hard to settle. Serve the notices early and record the neighbours’ houses before work begins.

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