The short answer
Converting an existing cellar in the UK typically costs around £1,000–£3,000 per square metre, because the structure is already there and you avoid the dig. A cellar that is dry with reasonable headroom mostly needs lining, insulation and finishes, so it sits at roughly £1,000–£2,000/m². Once you add a designed waterproofing system, ventilation and a compliant escape, the rate rises to about £2,000–£3,000/m². If you also have to lower the floor for headroom, underpinning brings the rate up toward new-basement levels. The per-square-metre figure is a guide only, since water, headroom and access matter more than floor area. A cellar conversion is one of the lower-cost ways to add usable space, well below the rate of a new dig.
Because a cellar already has walls and a floor, converting it is far cheaper per square metre than digging new. The sections below set out the typical rates and what moves them.
Typical UK rates
- Dry cellar refit~£1,000–£2,000/m²
- With waterproofing~£2,000–£3,000/m²
- Floor lowering addedRises toward £3,500/m²
- Main driversWater, headroom, access
- EscapeRequired if habitable
What sets the rate
A cellar conversion rate is driven mainly by how much water the space has and whether the headroom is already adequate. If the cellar is dry and tall enough, you are paying for a waterproofing or damp-proofing layer, insulation, lining, electrics, heating and finishes, which keeps the per-square-metre figure modest. If it is damp or below the water table, a cavity drainage membrane with a sump and pump adds to the rate, as does the ventilation and means of escape a habitable room needs. The biggest single jump comes if the floor has to be lowered, since underpinning and a new slab are structural work that lifts the rate sharply.
| Cellar condition | Typical rate | Main work |
|---|---|---|
| Dry, good headroom | £1,000–£1,800/m² | Line, insulate, finish |
| Some damp | £1,800–£2,500/m² | Waterproofing, ventilation |
| Wet / habitable | £2,500–£3,000/m² | Full CDM, escape provision |
| Floor lowering needed | £3,000–£3,500/m²+ | Underpinning, new slab |
Indicative UK figures for guidance only. Costs vary with water, headroom and access.
Why area is only part of the picture
Floor area gives you a rough budget, but the per-square-metre rate for a cellar conversion depends heavily on factors that have nothing to do with size. A small cellar that needs a full drained waterproofing system and a new escape window can cost more per m² than a larger, drier one that only needs lining. Headroom is the key swing: a cellar that is already tall enough stays at the low end, while one needing the floor lowered moves up by a large margin because of the underpinning involved. Access for any excavation, and the quality of finish you want, also move the figure.
What the rate usually excludes
As with any basement work, a per-square-metre rate for a cellar conversion typically covers the build and finishes but often leaves out the fees and statutory costs. A structural engineer is needed if you lower the floor, a party wall surveyor may be required where you share walls, and building control inspects the waterproofing, escape and ventilation regardless. Planning permission is usually not needed for internal cellar work, but a new lightwell or external door can trigger it. These add to the all-in cost beyond the headline rate.
The other exclusion is high-end finish. A plain dry room is at the bottom of the range, while a cellar kitchen, bathroom or media room adds plumbing, pumped waste and fitted units. When comparing cellar conversion quotes, check that each rate states the waterproofing system, whether floor lowering is included, and which fees and finishes are inside the figure, so you are comparing the same scope rather than two different jobs that happen to share a per-square-metre headline.
Frequently asked questions
Is a cellar conversion cheaper per m² than a new basement?
Considerably. A cellar already has its structure, so you avoid the excavation, underpinning and concrete box that dominate a new dig. That typically halves the per-square-metre rate or better, depending on how much waterproofing and headroom work the cellar needs.
What pushes a cellar conversion rate to the top of the range?
Mainly lowering the floor for headroom, which needs underpinning and a new slab, and managing active water with a full drained system. A wet, low cellar that has to be both deepened and waterproofed sits at the top of the per-square-metre range.
Does a small cellar cost more per square metre?
Often yes, because fixed costs such as waterproofing design, escape provision and access are spread over a smaller area. A compact cellar needing a full system can have a higher per-m² rate than a larger, drier space.
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.