Basement Conversion
Specialists in London

You already have the space. The job is making it worth being in.

A conversion starts from a structure that already exists: a Victorian coal cellar, a vaulted undercroft, a lower ground floor used for storage, a shell left behind by a previous owner. Nobody is digging a new box under your house. The work is to make what is already there dry, light, ventilated, safe to sleep or sit in, and finished to the standard of the floors above.

That makes it a different discipline from a new basement, and a considerably less disruptive one. The engineering risk moves from temporary works and neighbouring ground to the condition of the structure you have inherited: how much head height it gives you, how wet it is and why, and whether the services can be made to work below the level of the sewer. We survey all three before anyone quotes a number.

Head height decides the project

Most London cellars give between 1.8m and 2.1m from slab to joist. That is enough for plant, storage or a wine room and not enough for a room you want to spend an evening in. There is no minimum ceiling height for a habitable room in the England Building Regulations, but Part K requires 2m of headroom over a stair, and in practice that stair, not the room, is what sets the floor level.

Three routes out of it. Leave the level alone and use the space for what it suits. Lower the slab within the existing foundations, which wins a few hundred millimetres without touching the structure that holds the house up. Or underpin to go deeper, at which point you have crossed into basement construction: temporary works, party wall awards, movement monitoring and a different budget entirely. The honest answer is often the first or second. We will tell you which before you have spent anything.

Retrofit waterproofing: why Type C is usually the only honest answer

On a new basement you can tank the outside face of the concrete before you backfill against it. On a conversion you cannot: the outside face is buried under your garden, your neighbour's garden or the pavement, and reaching it means excavating around a standing building for the sake of a membrane. So retrofit waterproofing is done from the inside, and BS 8102:2022 Type C, a drained cavity, is normally the specification.

Type C accepts that water will reach the structure rather than pretending it will not. A studded membrane is fixed to the wall and floor, water that arrives behind it runs into a perimeter drainage channel, and a sump pumps it away. Done properly that means dual pumps rather than one, an automatic alarm, a battery backup so a power cut is not a flooded cinema, and access hatches so the channels can be rodded in ten years' time instead of being sealed behind fitted joinery. The design is produced by a CSSW certificated waterproofing designer we appoint, and our approach is set out in full on waterproofing design. Crown Building is an accredited DELTA, Triton and Mapei contractor.

Damp, salts, and what the old walls are telling you

An existing cellar has usually been damp for a century, and the wall has adapted to it. Hygroscopic salts have migrated into the masonry and will keep drawing moisture out of the air long after the source is dealt with. Cement render applied by a previous owner may be trapping moisture rather than excluding it. None of this is fatal and all of it changes the specification, which is why the survey comes before the quotation rather than after it. Where the structure is breathable we keep it breathable; where it is going behind a cavity membrane, the substrate has to be sound enough to hold the fixings without breaching the system.

Light, air, and a way out

Three things turn a dry cellar into a room. Daylight, usually through a lightwell or an enlarged existing opening, which is also the element most likely to need consent because it alters the outside of the building. Ventilation, which below ground means a mechanical system rather than an openable window, sized and commissioned rather than assumed. And escape: a basement with habitable rooms needs either a window or door suitable for escape, or a protected stairway to a final exit, and that requirement shapes the layout from the first sketch rather than being solved at the end.

Get those three right and the space reads as part of the house. Get them wrong and it reads as a basement, whatever has been spent on the finishes.

Drainage below the sewer

A shower, a WC or a utility room at basement level sits below the invert of the public sewer, so gravity will not do the work. Waste is collected in a sealed pumped station and lifted to the drain above. As with the waterproofing, the detail is the part that matters: duty and standby pumps, a high level alarm, and a position that can be serviced without dismantling the room around it. It is a small part of the budget and a large part of whether you regret the project in five years.

Planning, listed buildings and party walls: different rules from a new dig

This is where a conversion diverges most sharply from a new basement, and where a lot of bad advice circulates. Converting existing space internally, without changing the external appearance and without enlarging the volume, frequently does not require planning permission at all. The Article 4 Directions that Camden, Kensington & Chelsea and Westminster have used to close off permitted development bite on excavation and on the creation of new basement volume, not on making an existing cellar habitable.

The exceptions matter. A new or enlarged lightwell alters the exterior and will usually need permission. A listed building requires Listed Building Consent for internal alterations as well as external ones, and that includes taking out historic fabric to gain head height. A flat will normally need freeholder consent and may be restricted by the lease. And if you are lowering the slab below the level of an adjoining owner's foundations, the Party Wall etc. Act 1996 applies exactly as it would on a new dig. We establish which of these apply at feasibility, in writing, before anything is committed.

What a basement conversion costs in London, and how it compares to digging

The reason a conversion is the cheaper route is arithmetic rather than salesmanship. On a new basement, excavation, temporary works, underpinning, the retaining structure and external tanking are largely fixed costs incurred before a single finish is chosen. As a published comparison, we delivered a new basement in a Knightsbridge mews of approximately 85m², around 915 sq ft, to luxury specification with a wine cellar and home gym, at a contract sum of approximately £493,000 net of VAT, roughly £5,800 per m². A conversion avoids nearly all of that structural spend, because the box already exists.

What remains is waterproofing, services, ventilation, drainage and finish, and those do scale with area and specification. Our Highgate project is the closest published example: approximately 230m² of raw shell turned into a private cinema, a bespoke bar and lounge, a gym and a WC, with full mechanical, electrical and AV infrastructure. For an indicative figure against your own floor area and specification, use our project estimator, and our basement conversion guide sets out the cost structure and the assumptions behind it.

Areas we serve

We convert lower ground floors across prime London, Hampstead, Kensington, Chelsea, Notting Hill, Fulham and Battersea among them, where the housing stock is largely period and almost every house has something under it. Our full coverage, London within the M25 and a fifteen mile radius of our Watford headquarters, is set out on our areas we cover page.

Talk to our team about your cellar

Send us the head height, the age of the house and a photograph of the wettest wall, and we will tell you honestly whether it is a conversion, a dig, or a space best left as storage. Call 020 3675 3999 or book a free consultation.

Frequently asked questions

What is the difference between a basement conversion and basement construction?

A conversion makes an existing cellar or lower ground floor habitable. Construction creates new volume by excavating beneath a standing building, with underpinning, temporary works and movement monitoring. A conversion is faster, cheaper and far less disruptive, because the structure is already there. If you need more head height than the existing foundations allow, the job becomes construction, and we will say so.

How much head height do I need?

There is no minimum ceiling height for a habitable room in the England Building Regulations, but Part K requires 2m of headroom over a stair, and that is usually the binding constraint. Most London cellars start at 1.8m to 2.1m. Lowering the slab within the existing foundations can win a few hundred millimetres without underpinning; going deeper than that means underpinning, and a different project.

Do I need planning permission to convert an existing cellar?

Often not. Converting existing space internally, without altering the external appearance or enlarging the volume, frequently falls outside the need for permission. A new or enlarged lightwell usually does require it, a listed building requires Listed Building Consent for internal alterations, and a flat will normally need freeholder consent. We confirm which apply at feasibility, in writing.

How do you waterproof an existing basement?

From the inside, because the outside face of the structure is buried. The usual specification is BS 8102:2022 Type C, a drained cavity: a studded membrane, a perimeter drainage channel and a sump that pumps water away, with dual pumps, an automatic alarm, battery backup and maintainable access to the channels. The design is produced by a CSSW certificated designer we appoint. Crown Building is an accredited DELTA, Triton and Mapei contractor.

Can I have a bathroom or utility room below the sewer?

Yes. Waste is collected in a sealed pumped station and lifted to the drain above, with duty and standby pumps, a high level alarm and a position that can be serviced without dismantling the room around it.

Can you convert a basement beneath a listed building?

Yes. Internal alterations to a listed building require Listed Building Consent in their own right, including the removal of historic fabric to gain head height, and the waterproofing approach has to respect how the existing structure handles moisture. We manage consent, structural engineering and conservation methodology as one package. Our current live project in Hampstead is a two level basement beneath a listed Victorian terrace, with cinema and gym, which you can read about on our basement construction page.