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.
These are general UK ranges. We build across the North West, including Darwen, Blackburn and the wider Lancashire and Greater Manchester area - site access, local trade rates and travel distance can move your actual price within (or occasionally outside) the range above.
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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Common questions
How much does a basement conversion cost? FAQs
Is this a fixed quote for my basement conversion?
No. The figures above are published UK industry ranges from Checkatrade and houseUP and MyJobQuote, not a quote for your project. An accurate price depends on a site survey and your specific requirements - get in touch for an estimate based on your own project.
What actually drives the price up or down?
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.
What's a typical existing cellar/basement conversion, per m² in per m² terms?
Existing cellar/basement conversion, per m² typically runs £1,000-£1,800 (per m²), for a converting a space that already exists, based on the sources cited above.
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