Average Cost of Basement Tanking per m² (2026 UK)
Basement tanking costs roughly £50–£90 per m² for cementitious slurry tanking and £80–£150 per m² for a cavity drain membrane system in 2026. External tanking that needs digging out runs far higher at £150–£400 per m². For a typical 30m² basement that puts most jobs at £3,000–£9,000 before a sump pump and re-plastering. The method, the amount of water coming in and access are what move the figure. Here is the full breakdown, and where waterproofing sits alongside a full basement conversion.
Basement tanking cost per m² by method — 2026
Basement tanking cost per m² by method (2026)
| Method | Cost per m² fitted | Best for |
|---|---|---|
| Waterproof render / additive | £45–£80 | Sound walls, light damp, budget barrier |
| Cementitious slurry tanking (Type A) | £50–£90 | Stable masonry staying dry, cellars |
| Cavity drain membrane (Type C) | £80–£150 | Habitable basements, active water ingress |
| External tanking (Type A, dig-out) | £150–£400 | Full waterproofing where access allows excavation |
Ranges are for labour and materials on the waterproofing itself. A whole 30m² basement typically lands at £3,000–£9,000 depending on method. London and the South-East usually sit 15–25% above these figures. For the cost of the room itself, see the basement conversion cost guide.
The three ways to tank a basement
Type A — barrier (tanking)
A cementitious slurry or render coat forms a physical barrier against water. Cheapest per m² and reliable on sound, stable walls, but unforgiving if the structure moves or water pressure is high.
Type C — cavity drain membrane
A dimpled membrane lines the walls and floor, channelling any water to a perimeter drain and a sump. The go-to for habitable basements because it manages water rather than fighting it, and it can be maintained.
External tanking
Waterproofing applied to the outside of the wall after excavation. The most thorough, but digging around the property makes it the most expensive route and it is not always possible on a terrace.
What is in the price and what is not
| Usually included | Usually extra |
|---|---|
| Waterproofing system and labour | Damp and structural survey (£150–£400) |
| Preparing and cleaning the substrate | Sump pump plus battery backup (£800–£1,800) |
| Membrane, mesh and fixings | Re-plastering and decoration after tanking |
| Perimeter drainage channel (Type C) | Excavation and reinstatement (external tanking) |
The single biggest swing is whether you need a pumped drainage system. On a cavity drain job a sump and pump are almost always required, and a battery backup is worth having because the system only works while it can move water. Re-plastering is a separate trade and is rarely in the waterproofing quote.
How long basement tanking takes
- Single room, slurry tanking: 3–5 working days, plus drying time before plaster.
- Single room, cavity drain membrane: 4–7 working days including the sump and pump.
- Whole basement: 1–3 weeks depending on size, access and how much prep the walls need.
- External tanking: add time for excavation, which is weather-dependent.
Keeping the cost down and hiring well
- Get a diagnosis first. A damp survey (£150–£400) tells you whether you need full Type C tanking or a cheaper barrier — do not pay for more system than the water justifies.
- Ask what guarantee is offered and who stands behind it. Reputable specialists give a 10–30 year product guarantee; check whether it is transferable when you sell.
- Get 3 like-for-like quotes on the same scope so you compare the same method, not slurry against membrane.
- Check membership of a waterproofing body. Ask to see Property Care Association (PCA) membership and confirm the surveyor holds a CSSW qualification.
BestBuilders vets the specialists we pass your job to, but you should still check each firm’s insurance, guarantees and accreditations yourself before you hire.
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