Calculate Your Basement Conversion Cost
2026 Basement Conversion Cost Guide
Basement conversions are among the most expensive home improvement projects but can unlock substantial additional living space, especially in cities where extending outward or upward is not possible.
| Item | Low | High |
|---|---|---|
| Existing Cellar Conversion | £1,000/m² | £1,500/m² |
| New Excavation Basement | £2,500/m² | £4,000/m² |
| Waterproofing / Tanking | £50/m² | £100/m² |
| Party Wall Agreement (per neighbour) | £700 | £1,000 |
Frequently Asked Questions
How much does a basement conversion cost in the UK in 2026?
Converting an existing cellar typically costs £1,000–£1,500 per m². A full new excavation basement costs £2,500–£4,000 per m², making a 40 m² basement £100,000–£160,000 before finishing. London and the South East add a 25–30% premium to all costs.
Do I need planning permission for a basement conversion in the UK?
Converting an existing basement often falls under Permitted Development in England, but a new dig-out typically requires full planning permission. Rules vary by local authority — London boroughs in particular have strict policies on basement excavations. Always consult your local planning authority.
What is a party wall agreement and do I need one for a basement?
A Party Wall Agreement (or Award) is a legal agreement under the Party Wall Act 1996. Most basement excavations that affect a shared boundary or wall require one. Budget £700–£1,000 per neighbour for a party wall surveyor to prepare and agree the award.
How much does waterproofing a basement cost in the UK?
Basement tanking (waterproofing) costs £50–£100 per m² in 2026. For a 40 m² basement, this adds £2,000–£4,000 to the project. A cavity drain membrane system is the most widely used solution for reliable long-term waterproofing in both new and converted basements.
How much value does a basement conversion add to a London property?
In London, a quality basement conversion can add 10–15% to property value. For a £1 million home this means a potential uplift of £100,000–£150,000. The value uplift tends to be greatest in prime London postcodes where space is at a premium and above-ground extensions are not possible.