The short answer
Converting an existing cellar into living space typically costs around £1,200–£2,250 per square metre, which usually means roughly £25,000–£75,000 for an average space. If the floor has to be lowered to gain head height, that rises to about £2,000–£3,500 per square metre. Digging out a brand-new basement is the most involved option at roughly £3,000–£5,000 per square metre, with whole-project costs commonly from £150,000 upwards on larger homes and considerably more in London. The main drivers are whether you convert or excavate, the head height you need, the waterproofing system, ground and water conditions, and the finish.
The single biggest factor is whether you already have a usable cellar to convert or are excavating new space from scratch. The figures below are typical UK costs for guidance, not quotations.
Typical UK costs
- Convert existing cellar£1,200–£2,250 / m²
- Average cellar project~£25,000–£75,000
- Lower the floor£2,000–£3,500 / m²
- Dig out new basement£3,000–£5,000 / m²
- Whole new basement£150,000+ (more in London)
What drives the price
- Convert vs excavate: reusing an existing cellar is far cheaper than digging out new space, where excavation and structural work can be 30–40% of the bill.
- Head height: if you need to lower the floor to stand up comfortably, that adds underpinning and structural cost.
- Waterproofing: a proper tanking or cavity-drain system is essential below ground and is typically 15–20% of the cost.
- Ground & water conditions: a high water table, difficult access or unstable ground all raise the figure.
- Fit-out & fees: finishes, plus professional and party-wall fees, add to the total.
| Type of work | Typical figure | Notes |
|---|---|---|
| Convert existing cellar | £1,200–£2,250 / m² | lowest-priced route where space exists |
| Lower the floor for head height | £2,000–£3,500 / m² | adds underpinning |
| Dig out new basement | £3,000–£5,000 / m² | excavation + structure |
| Whole new basement (project) | £150,000+ | far higher in London |
Indicative UK figures for guidance. Sources: HouseUp and MyJobQuote basement conversion cost guides.
Why converting is so much cheaper than digging
If you already have a cellar with reasonable head height, you are mostly paying for waterproofing, a damp-proof floor, insulation, fit-out and the staircase — a contained job. Digging a new basement adds excavation, underpinning, structural support and far more waterproofing, which is why the cost per square metre roughly doubles or more, and the whole-project figure runs into six figures. Knowing which route your property needs is the first thing to establish before reading any quote.
Want a measured basement conversion quote?
We'll match you with a vetted basement conversion specialist who assesses your space and quotes on a clear specification — waterproofing, head height, structure and fit-out all set out.
Frequently asked questions
How much does a basement conversion cost in the UK?
Converting an existing cellar typically costs around £1,200–£2,250 per square metre, or roughly £25,000–£75,000 for an average space. Digging out a new basement is far more, at around £3,000–£5,000 per square metre, with whole projects commonly from £150,000 upwards and higher in London.
Why is converting a cellar cheaper than digging a new basement?
Converting reuses space you already have, so you mainly pay for waterproofing, flooring, insulation and fit-out. Digging a new basement adds excavation, underpinning and structural work, which roughly doubles the cost per square metre or more.
Why is the price range so wide?
Because projects differ. Whether you convert or excavate, the head height you need, the waterproofing system, ground and water conditions and the finish all move the figure. A measured survey gives the accurate number for your property.
Sources & further reading
Figures on this page are typical UK ranges drawn from published sources and depend on your specific property. They are guidance, not a quotation.