The short answer
Per square metre, the UK rate depends entirely on the type of work. Converting an existing cellar is typically around £1,200–£2,250 per square metre. Lowering the floor to gain head height runs to about £2,000–£3,500 per square metre. Digging out a new basement is roughly £3,000–£5,000 per square metre, and in central London projects can start well above that once access and structure are factored in. The per-square-metre rate is a useful sense-check, but the real total also depends on fixed items — staircase, waterproofing system, party-wall fees and professional fees — that do not scale neatly with floor area.
A per-square-metre rate is the quickest way to sense-check whether a quote is in the right ballpark. The rates below are typical UK figures for guidance, not quotations.
Per square metre (UK)
- Convert existing cellar£1,200–£2,250 / m²
- Lower the floor£2,000–£3,500 / m²
- Dig out new basement£3,000–£5,000 / m²
- Central London new digoften £6,000+ / m²
- Plus fixed costsstairs, fees, party wall
How the per-square-metre rate breaks down
On a typical project, the cost splits roughly into excavation and underpinning (around 30–40% on a dig-out), waterproofing (around 15–20%), professional fees (around 10–15%) and fit-out and finishes (around 20–25%). On a straight cellar conversion the excavation share falls away, which is why the per-square-metre figure drops sharply. Use the rate to multiply by your usable floor area for a rough total, then add the fixed costs that do not scale with area.
| Type of work | Typical cost / m² |
|---|---|
| Convert existing cellar | £1,200–£2,250 |
| Lower the floor for head height | £2,000–£3,500 |
| Dig out a new basement | £3,000–£5,000 |
| Central London new basement | £6,000+ |
Indicative UK per-square-metre figures for guidance. Sources: HouseUp and Architecture for London cost guides.
Why the rate alone won't give your total
Some costs barely change with floor area: a staircase, the waterproofing system, party-wall agreements and professional fees are broadly fixed whether the room is small or large. That means a small basement can look expensive per square metre, while a larger one spreads those fixed costs and looks better value per square metre. Use the rate as a guide, but always get a measured quote for the real figure.
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Frequently asked questions
How much does a basement conversion cost per square metre?
Converting an existing cellar is typically around £1,200–£2,250 per square metre, lowering the floor is about £2,000–£3,500 per square metre, and digging a new basement is roughly £3,000–£5,000 per square metre. Central London new digs often start above £6,000 per square metre.
Why does the cost per square metre vary so much?
Because the type of work differs. A straight cellar conversion has no excavation, while a new dig-out adds excavation, underpinning and structure — which roughly doubles the rate or more.
Can I just multiply the rate by my floor area?
It gives a rough total, but several costs — the staircase, waterproofing system, party-wall and professional fees — are broadly fixed and don't scale with area, so a measured quote gives the accurate figure.
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.