Basement conversion Cost Calculator
A basement conversion is one of the most underrated UK home-improvement projects — if you already have a cellar, it's typically the cheapest way to add a habitable bedroom or living space. In 2026 a habitable cellar conversion costs £1,800–£4,000/m²; a brand-new excavated basement runs…
Open the calculator →Price basement conversion in 30 seconds
2 steps, using the figures from our cost guide.
How this calculator works
Every figure in this calculator is the one published in the cost table on our basement conversion guide. Pick the option that matches your job and the calculator returns that published range, adjusted for your region - it does not model anything the guide does not already say.
That is deliberate. A calculator whose rates drift from the page it sits under is worse than no calculator, because two numbers on the same site disagree.
What it cannot know: what's under and around your house, which is what makes basements the least predictable job on this site. The water table, the existing foundation depth, and whether you're underpinning an old cellar or digging a new one can swing the figure enormously, and the waterproofing design is a specialist cost in its own right. It rarely happens in isolation either: a building survey and a structural engineer set the scope, and because you're working right next to neighbours a party wall surveyor is almost always needed.
Published basement conversion costs (2026)
| Option | Typical 2026 cost | Notes |
|---|---|---|
| Cellar tidy-up (storage) | £700–£1,200 | |
| Habitable cellar conversion | £1,800–£2,800 | |
| Cellar + lightwell | £2,500–£4,000 | |
| Lower floor (1–1.5m dig) | £3,500–£5,500 | |
| New basement (under footprint) | £4,500–£7,500 | |
| Garden basement | £5,500–£8,500 |
These are the figures from our basement conversion cost guide.
Basement conversion calculator — FAQs
It uses the same published figures as the cost guide it sits under, adjusted for your region. It is a budgeting estimate, not a quote - only a site visit produces one of those.
Related cost guides
- Basement conversion cost guide — The full price guide
- All builders services — Browse the hub
- All BestBuilders services — Every trade we cover
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