Is a basement conversion worth it?

Short answer

Converting an existing cellar is often good value at around £1,900–£3,200 per m² for converting an existing cellar including VAT, because the space already exists. Digging a new basement costs much more and is usually only viable in high-value areas. Both need careful waterproofing to BS 8102.

Cellar conversion

Uses existing space, sometimes lowering the floor. Waterproofing, ventilation, insulation and daylight are the main jobs.

New basement

Excavation under the house or garden with underpinning and a new concrete structure. Needs planning permission in most cases and party wall agreements.

Waterproofing

Designed to BS 8102, usually a cavity drain membrane with a sump pump, sometimes combined with tanking.

Escape and light

Habitable basement rooms need a safe escape route. Lightwells provide daylight and escape.

Sources and further reading

This guide is general information, not legal or tax advice. Rules change and depend on your property; we confirm what applies to your project at the survey.