Extension Cost Calculator
Single or double storey. A rough, honest UK build-cost estimate in under a minute, including the contingency most people forget.
- Build (25m² × £2,700)
- £67,500
- Kitchen
- £15,000
- Contingency (15%)
- £12,375
- Estimated build cost
- £94,875
+ our design fee
We don't charge a percentage of the build — it's a fixed fee, agreed with you up front. Book a free call and we'll confirm the exact figure for your project, no guessing.
A rough guide to build cost only. Costs vary by home, ground conditions and region — your design fee is quoted separately and is always fixed.
Thinking about the loft instead? Try the loft conversion cost calculator.
How the estimate is worked out
The calculator multiplies your extension's floor area by a cost per square metre for the finish you choose: £2,200 for a standard finish, £2,700 for a higher specification, and £3,200 for premium. Those bands reflect what UK builders are quoting for single-storey work, and they line up with the ranges on our home extensions page.
For a double-storey extension, the footprint is multiplied by 1.7. The upper floor shares the foundations, groundworks and roof with the ground floor, so it adds around 70% of the ground-floor cost, not another 100%.
A new kitchen is added as a flat £15,000 allowance if you tick it, and the contingency slider adds 10–20% on top. Ten percent is the minimum we'd carry on a straightforward build; fifteen is sensible for an older house.
What the calculator deliberately leaves out is our design fee. It's fixed, agreed in writing before any work starts, and never a percentage of the build. If you want the full picture before we talk, the extension cost guide covers every line that ends up on the bill.
Extension cost questions
How much does a house extension cost in the UK?
As a guide, a single-storey extension costs around £2,200 to £3,200 per square metre to build depending on finish, so a typical 25m² rear extension is £55,000 to £80,000 before a kitchen. A double-storey extension on the same footprint is roughly 1.7 times that, because the upper floor reuses the foundations and roof budget. Add 10–20% contingency.
How accurate is this extension cost calculator?
It's a realistic starting range, not a quote. Ground conditions, access, the roof type, how much glazing you want and your region all move the number. Use it to decide whether a project is in reach, then get a proper estimate from a builder working from real drawings.
Why doesn't the calculator estimate the design fee?
Because ours is fixed, not a percentage of the build. Many calculators add 10–15% for 'architect fees', which is the model we deliberately don't use. We agree a fixed design fee in writing after a free call, once we've seen your home.
Does a double-storey extension cost twice as much as single-storey?
No. The expensive parts, foundations, groundworks and roof, are shared, so the second floor typically adds around 70% of the ground-floor cost rather than 100%. That's why double-storey is often the better value per square metre, if planning allows it.
What isn't included in the estimate?
Council planning and building control fees, the structural engineer's calculations, specialist surveys, VAT where your builder charges it, and furnishing. We list every one of these before you commit.
Not sure whether to build, wait or move?
That's exactly what the call is for. We'll be straight with you — no pressure, no jargon, no nasty surprises.