How much does it cost to tank a cellar?
Cellar tanking (waterproofing) is priced by the area of wall and floor treated. Estimate yours below.
How cellar waterproofing works
- 1Diagnose the water
A specialist assesses water ingress and specifies a system to BS 8102 — barrier tanking or a managed cavity-drain membrane.
- 2Tank or drain
Either seal the structure (cementitious tanking) or manage water to a sump and pump behind a membrane.
- 3Finish
Battens/board over a membrane, or render/plaster over tanking, then decorate — turning the space into usable rooms.
A dry cellar is bonus floor area, which is why basement space is such a value lever in higher-value areas. The whole thing rests on the waterproofing design — use a specialist, insist on BS 8102 and a guarantee.
Cellar tanking costs (indicative)
| System | Per m² | Notes |
|---|---|---|
| Cementitious tanking | £55 – £110 | Barrier system |
| Cavity-drain membrane | £70 – £140 | Managed water + sump/pump |
| Small cellar (~25 m²) | — | £1,500 – £3,500 |
| Sump & pump | each | £600 – £1,500 |
Waterproofing should be designed to BS 8102 by a specialist. Full basement conversions cost much more.
Frequently asked questions
How much does it cost to tank a cellar?+
Cellar tanking typically costs £55–£140 per m² of wall and floor treated, so a small cellar (~25 m²) is roughly £1,500–£3,500. A sump and pump for a cavity-drain system is extra.
Is tanking a cellar worth it?+
Converting a damp cellar into dry, usable space can add valuable floor area — but waterproofing must be designed properly (BS 8102) by a specialist, ideally with a guarantee, or it can fail expensively.
Related tools & guides
Want to know how these figures are calculated? See our cost methodology.
Cost figures shown are indicative estimates, not quotations. You are responsible for verifying all costs (obtain contractor quotes) and any figures submitted to a lender. ScopeWise is a documentation tool, not financial, tax, structural or planning advice. HMO compliance prompts are guidance only — confirm requirements with your local council, as standards and licensing vary by authority.