The short answer
Converting an existing cellar in the UK typically costs between £15,000 and £80,000, depending on its current condition. A cellar that is already dry, with reasonable headroom, mainly needs insulation, lining, electrics, heating and finishes, which often comes in around £15,000–£40,000. Once you need a proper waterproofing system, a cavity drainage membrane with a sump and pump or tanking to BS 8102, plus a compliant means of escape and ventilation, the figure usually rises to £40,000–£80,000. You avoid the biggest basement cost, the dig itself, because the structure already exists. The price climbs if you also lower the floor for headroom, which brings underpinning into play and pushes it toward new-basement territory.
A cellar conversion is far cheaper than a new dig because the hard structural work is already done. The sections below set out what the price covers and the factors, chiefly water and headroom, that move it up.
Typical UK costs
- Dry cellar refit£15,000–£40,000
- With waterproofing£40,000–£80,000
- Add floor loweringPushes toward £80,000+
- Per square metre~£1,000–£2,500/m²
- Means of escapeRequired if habitable
What the price covers
Most of the cost of a cellar conversion goes on turning a damp storage space into a dry, warm, habitable room. That means a waterproofing system sized to how wet the cellar is, then insulation, stud or membrane-backed walls, a finished floor, electrics, heating, lighting suited to a windowless space, and ventilation to meet building regulations. Because a habitable basement room needs a safe way out in a fire, you also pay for a compliant means of escape, which might be a protected stair, a proper egress window or a lightwell, depending on the layout. None of this involves digging, which is why even a fully finished cellar conversion sits well below the cost of a new basement.
| Cellar condition | Typical UK cost | Main work |
|---|---|---|
| Dry, good headroom | £15,000–£30,000 | Insulate, line, finish, electrics |
| Some damp, decent height | £30,000–£50,000 | Waterproofing, fit-out, ventilation |
| Wet, low headroom | £50,000–£80,000 | Full CDM, escape, possible floor work |
| Floor lowering needed | £80,000+ | Adds underpinning and new slab |
Indicative UK figures for guidance. Sources: Checkatrade and MyBuilder cellar conversion guides.
Water and headroom set the cost
Two things decide which end of the range a cellar conversion falls in: how much water it has to deal with, and how much headroom it has. Waterproofing is rarely optional in a habitable basement, but the choice of system, tanking applied to the walls versus a cavity drainage membrane that channels water to a sump and pump, affects both cost and maintenance. Headroom is the bigger swing factor. If the ceiling height is already enough for a habitable room, you keep the existing floor and stay at the lower end. If it is too low, gaining height means lowering the floor, which requires underpinning the walls and forming a new slab, the most expensive single item in any cellar job.
Fees, finishes and what is excluded
As with any basement work, the build figure usually sits below the all-in cost. A structural engineer is needed if you lower the floor or alter the structure, a party wall surveyor may be required where you share walls, and building control will inspect the waterproofing, escape and ventilation. Many cellar conversions do not need planning permission because they are internal works, but a new lightwell, external door or change of use can trigger it, so it is worth checking with your local authority early.
The other variable is finish. A plain dry room costs much less than a basement kitchen, bathroom or media room, where you are adding plumbing, drainage (often pumped, since the basement sits below the sewer), tiling and fitted units. When comparing quotes, make sure each one states the waterproofing system, whether floor lowering is included, and which finishes and fees are inside the price, because these are where headline figures diverge most.
Frequently asked questions
Do I need planning permission to convert a cellar?
Often not, because converting an existing cellar is usually classed as internal alteration. Permission is more likely if you change the use, add a new external entrance, or form a lightwell. Building regulations approval is required regardless, so check with your local authority before starting.
Why does waterproofing matter so much in a cellar?
Cellars sit below ground and are exposed to damp and water pressure, so a habitable room needs a designed waterproofing system to stay dry. Skimping here risks damage to finishes and the structure, which is why a system designed to BS 8102 is treated as essential rather than optional.
Can I add a bathroom or kitchen in a converted cellar?
Yes, but because the basement usually sits below the main drain, waste has to be lifted by a pumped system, which adds cost. Otherwise the room is finished like any other, with the waterproofing system protected behind the new walls and floor.
Sources & further reading
Figures on this page are typical UK ranges drawn from published sources and depend on your specific property. They are guidance, not a quotation.