Cost guide
How much does a basement conversion cost?
Converting an existing cellar and excavating a new basement are different jobs at very different rates - published UK ranges for both.
Converting an existing cellar typically costs £1,000-£1,800 per m², according to Checkatrade. A new basement excavation runs considerably more - houseUP puts the broader range at £1,200-£4,650 per m², and MyJobQuote quotes an average total cost of around £100,000 for a full basement conversion project.
Last updated 20 August 2026
| Range | ||
|---|---|---|
| Existing cellar/basement conversion, per m² - converting a space that already exists | £1,000-£1,800 | Per m² |
| New basement excavation, per m² - broader range, likely includes new-build excavation | £1,200-£4,650 | Per m² |
| Average total, full conversion project - single published average, not a range | £100,000 | Total project |
Sources: Checkatrade (existing cellar conversion, per m², checked August 2026), houseUP (broader per m² range, checked August 2026), MyJobQuote (average total cost, checked August 2026)
Worth knowing
Converting an existing cellar and excavating a brand new basement are very different jobs at very different rates, and our named sources don't always separate them clearly. Checkatrade's per-m² figure specifically describes remodelling an existing cellar; houseUP's much wider per-m² range and MyJobQuote's total-cost average most likely include new-excavation projects, which involve underpinning and structural work an existing-cellar conversion doesn't. Treat these as two different scenarios, not one range to average together.
Key points
- Whether you're converting a cellar that already exists, or excavating new - this is the single biggest driver by far.
- Waterproofing specification (tanking) to BS 8102.
- Ceiling height - a new excavation can be dug to the height you need; an existing cellar is constrained by what's there.
- Means of escape in a fire and ventilation, both Building Regulations requirements regardless of the planning route.
These are general published ranges, not a quote. Get an estimate based on your own project.
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