The short answer
Tanking typically costs around £50–£90 per square metre, while a cavity drainage membrane system usually costs £75–£150 per square metre once the sump and pump are included. Tanking (Type A in BS 8102) is a barrier coating that holds water out, and suits drier walls with lower water pressure. A cavity drainage membrane (Type C) accepts that some water may enter, channels it behind a studded membrane to a sump and pump, and copes better with active water, which is why it is the common choice for habitable basements. The cavity system costs more upfront and needs a pump and periodic maintenance, but it is more forgiving of changing ground water. The right choice depends on the water risk, not the lowest price.
The two main basement waterproofing systems differ in both cost and how they handle water. The sections below compare them per square metre and explain which suits which situation.
Cost comparison
- Tanking (Type A)~£50–£90/m²
- Cavity drainage (Type C)~£75–£150/m²
- Sump & pump~£800–£2,500
- Backup pump~£300–£800
- StandardBoth designed to BS 8102
How the two systems differ
Tanking is a barrier approach: a waterproof slurry, render or membrane is applied to the walls and floor to keep water out, relying on the barrier staying intact under water pressure. It is well suited to basements with relatively dry walls and modest water pressure, and it leaves no pump to maintain. A cavity drainage membrane takes the opposite view: it accepts that water may pass through the structure, holds it behind a dimpled membrane fixed to the walls and floor, and channels it through perimeter conduits to a sump, from which a pump discharges it away. This drained approach copes with active and variable water far better, which is why it dominates habitable basement conversions.
| System | Cost per m² | Best for | Maintenance |
|---|---|---|---|
| Tanking (Type A) | £50–£90/m² | Drier walls, low pressure | Minimal, no pump |
| Cavity drainage (Type C) | £75–£150/m² | Active / variable water | Pump servicing needed |
| Combined A + C | Higher | Habitable, higher risk | Pump servicing needed |
Indicative UK figures for guidance. Sources: Property Care Association; Checkatrade waterproofing guides.
Why the cavity system costs more
A cavity drainage membrane costs more per square metre because it is more than a coating. It includes the membrane itself, perimeter drainage channels, a sump chamber, a pump with electrics, and usually an alarm and a backup or second pump for resilience against power failure. Those components and the labour to install them add to the rate, and there is an ongoing cost too, since the pump needs power and the channels need periodic clearing. Tanking has none of that mechanical equipment, which keeps its per-square-metre rate lower, but it depends entirely on the barrier remaining intact and is less tolerant of high or rising water.
Choosing between them
For a habitable basement, the deciding factor is how much and how variable the ground water is, not the upfront price. A space with consistently dry walls and low water pressure may be kept dry with tanking, saving on both installation and maintenance. A basement below the water table, or one where water can rise after heavy rain, is better served by a cavity drainage system, which manages water reliably and is more forgiving if conditions change. Many specialists combine both, tanking as a first defence and a drained membrane to manage anything that gets through, for the most resilient result in higher-risk situations.
Because remedial waterproofing in a finished basement is far more expensive than getting it right first time, the per-square-metre cost should be weighed against the consequences of failure. A drained system that costs more upfront but reliably keeps a habitable room dry is usually better value than a cheaper barrier that struggles with the water. The sound approach is a BS 8102 assessment by a CSSW-qualified specialist, who designs the system to the water risk and gives you a per-square-metre cost that reflects the protection your basement actually needs.
Frequently asked questions
Which is cheaper, tanking or a cavity drainage membrane?
Tanking is usually cheaper per square metre because it has no pump or drainage components. A cavity drainage system costs more upfront and has ongoing pump maintenance, but it copes better with active water, so the cheaper option is not always the right one for the ground.
Can I use both tanking and a cavity drainage membrane?
Yes, and in higher-risk basements specialists often combine them, tanking as a barrier and a drained membrane to manage any water that gets through. This belt-and-braces approach costs more but gives the most resilient result for a habitable room.
Does a cavity drainage system need a pump forever?
Yes. The system relies on draining water to a sump and pumping it away, so the pump must keep running for the life of the basement. A habitable space normally has a backup pump and an alarm, plus periodic servicing of the pump and channels.
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.