Cost FAQ
How much does a basement extension cost?
Drawings: from £4,500. Build: £90,000-£150,000 single-storey, £220,000-£380,000 double-storey. Add £15-30k for structural surveys.
Drawings: from £4,500. Build: £90,000-£150,000 single-storey, £220,000-£380,000 double-storey. Add £15-30k for structural surveys.
A London basement typically costs £4,000 to £6,500 per square metre — substantially more than any above-ground extension — because of underpinning, waterproofing and temporary works.
Three costs that no above-ground extension carries. Underpinning or piling to take the existing foundations below the new floor level, done in a hit-and-miss sequence that is slow by design. Waterproofing to BS 8102 Grade 3, which for habitable space normally means two independent protection systems. And temporary works — propping, sequencing and spoil removal through a terraced house.
Add a ground investigation, a structural scheme, a ground-movement assessment and often a construction management plan required by planning condition.
Converting an existing cellar without lowering the floor or altering the external appearance is far cheaper, and is often permitted development. Costs rise sharply the moment the floor is lowered, a lightwell is cut, or the footprint extends beyond the original house.
Many London boroughs operate specific basement policies that bite before the national rules: limits on the proportion of garden that can be built under, bans on double-storey basements outside commercial areas, and mandatory basement impact assessments. Those policies, not the GPDO, usually decide what is achievable.
Excavation within 3 m of an adjoining structure and below its foundations is section 6 work with two months' notice. Underpinning a party wall is section 2 work with one month's notice. A schedule of condition is essential — basement projects generate more damage claims than any other domestic work.
A worked Havering example of how much does a basement extension cost?: with 15 conservation areas — the Romford Market Conservation Area and the Upminster Conservation Area among the best known — London Borough of Havering (Havering Council) sees how much does a basement extension cost? constantly, and there is no borough-wide Article 4 direction, so permitted development generally survives outside conservation areas. Where how much does a basement extension cost? pushes a Havering scheme out of permitted development, the route becomes a £548 application decided in 8 weeks rather than a £249 prior approval in 6 weeks. is governed by the Town and Country Planning Act 1990, the GPDO 2015 and the Building Regulations 2010.
| London rate | £4,000–£6,500 per m² |
|---|---|
| Our basement drawing package | |
| Waterproofing standard | BS 8102, Grade 3 |
| Excavation notice | Two months, section 6 |
| Underpinning notice | One month, section 2 |
| Planning fee | £548 |
Often yes, if the external appearance is unchanged and no lightwell is added.
In several London boroughs, yes — check the local basement policy.
Only where the ceiling price supports it. Per square metre it is the most expensive space you can add.
Pick a slot that suits you. No obligation and no sales pressure, just straight answers on your planning route.
