House Extension Cost Calculator UK
Get a ballpark cost estimate for your house extension. Select your extension type, size, region, and finish level to see an estimated cost range based on 2026 UK market rates.
Extension Cost Estimator
Ballpark build costs for UK home extensions (2024)
What Affects Extension Costs?
- ·Size — larger extensions are cheaper per m²
- ·Location — London costs 40% more than the North
- ·Specification — premium finishes cost more
- ·Ground conditions — clay soils need deeper foundations
- ·Roof type — flat roofs are cheaper than pitched
- ·Structural work — removing walls adds cost
Extension Cost FAQs
How much does a typical house extension cost in the UK?
A typical single-storey rear extension costs £1,500–£2,500 per square metre in most of England. A 25m² extension would therefore cost approximately £37,500–£62,500 in the Midlands, or £55,000–£90,000 in London and the South East. These figures are for the structure only — fixtures, fittings, and a kitchen or bathroom would be additional.
Does the cost include VAT?
No — the estimates shown in our calculator exclude VAT. Most building contractors charge 20% VAT on top of their quoted price. Budget for VAT on the total build cost.
How long does a house extension take?
A typical single-storey extension takes 12–16 weeks to build once the planning permission (if required) and building regulations are in place. A double-storey extension may take 16–20 weeks. You should also allow 2–3 months for planning (if required), architect drawings, and tendering.
Do I need planning permission for a house extension?
Many extensions are permitted development (PD) and do not need planning permission. The key PD rules for a rear extension are: no more than 3m depth (attached houses) or 4m depth (detached), under the height of the eaves, and not covering more than 50% of the garden. Larger extensions may be possible with prior approval (up to 6m attached, 8m detached).
How do I get an accurate quote?
The only way to get an accurate quote is to have detailed architectural drawings prepared and then invite at least three builders to tender. Our calculator provides a useful ballpark figure, but actual costs will depend on your specific site, ground conditions, existing structure, and chosen specification.