Basement Conversion Calculator

Costs vary significantly by type. A simple tanking conversion (where existing headroom is adequate) costs roughly £1,200-£2,250/m². Lowering the floor to gain headroom (which involves underpinning) costs around £2,000-£3,150/m². A brand-new basement excavation costs about £3,000-£4,650/m². These are indicative UK averages for 2026 (sources: houseup, MyJobQuote, Checkatrade) and include waterproofing, insulation, electrics and basic finishes. London and the South East typically run 20-25% higher, so always confirm against local quotes.

Converting an existing cellar or basement usually falls under permitted development rights, provided the work does not extend the footprint of the building. However, Building Regulations approval is always required for structural work, waterproofing, fire safety, ventilation, and drainage. In Conservation Areas or with Listed Buildings, planning permission and/or Listed Building Consent will be needed.

England's Building Regulations set no statutory minimum ceiling height for habitable rooms — the old by-law minimum was repealed. Approved Document K's 2.0m headroom figure applies to stairs and landings (measured along the pitch line), not to rooms. In practice 2.1m is the widely-used minimum for a comfortable habitable basement, while 2.3m over 75% of the floor area is the optional Nationally Described Space Standard (London's London Plan uses 2.5m). Most conversion specialists aim for 2.3-2.4m to allow for floor build-up (insulation, screed, floor finish) and ceiling finishes. If existing headroom is insufficient, underpinning to lower the floor is needed.

In high-value areas (London, South East, major cities), basement conversions typically add £2,000-£4,000 per square metre to property value. Where the conversion cost is £1,500-£2,500/m², the return on investment can be significant. In lower-value areas, the cost may approach or exceed the value added. A local estate agent can advise on likely value uplift.

A simple tanking conversion takes 4-8 weeks. Lowering the floor with underpinning takes 12-20 weeks. A new-build basement takes 16-30 weeks depending on size and complexity. Underpinning work is sequential (you cannot do all sections at once for structural stability), which extends the programme significantly.