What Does a Basement Conversion Cost?
A basement conversion costs £1,500–£3,000 per m² in the UK — roughly £45,000–£90,000 for a typical 30m² basement. A light-touch existing cellar conversion starts at £1,000–£1,500/m², while a new basement dig under the property runs to £2,500–£3,000+/m². Figures are indicative — the final price depends on waterproofing method and level of finish, so always get several quotes.
Quick Answer
£1,500–3,000/m². Typical 30m² basement: £45,000–90,000
Depends on: existing cellar vs new dig, waterproofing method, and level of finish
Cost by Conversion Type
- Existing cellar conversion (light touch): £1,000–1,500/m² — waterproofing, damp-proofing, basic fit-out
- Cellar conversion (full): £1,500–2,000/m² — underpinning, lowering floor, full fit-out
- New basement dig: £2,500–3,000+/m² — full excavation under existing property
What’s Included
- Structural underpinning or bench foundations
- Waterproofing (cavity drain membrane or tanking)
- New concrete floor slab
- Insulation (walls and floor)
- Electrics, plumbing, and HVAC
- Staircase access
- Plastering and decoration
- Light well or window installation
Additional Costs
- Planning permission: £548 (if required; many conversions are PD)
- Building Regulations: £400–800
- Structural engineer: £1,000–3,000
- Party wall agreements: £700–1,500 per neighbour
- Sump pump system: £1,500–3,000
Is It Worth It?
A well-executed basement conversion can add 20–30% to a property’s value in high-value areas. In London and the South East, the value added often exceeds the cost. In lower-value areas, the return may be marginal.
Last updated: April 2026