Extension Cost Calculator

House extension costs in the UK are quoted per square metre of floor area. In 2026 a single-storey rear extension costs roughly £1,800 to £2,800 per m², while a double-storey works out at £1,500 to £2,400 per m², the upper floor being cheaper because it shares foundations and roof with the storey below. A wrap-around, the largest and most complicated shape, runs £2,500 to £4,000 per m². London and the South East are typically 20 to 40% more expensive than the Midlands or the North.

Building work is only part of the bill. Professional fees add an architect at 7 to 12%, a structural engineer at 3 to 5% and a Building Regulations application at £200 to £900. A householder planning application, where one is needed, costs £258. Then come the finishes, meaning kitchen, bathroom and flooring, and premium fittings can double what that stage costs. VAT at 20% applies to labour and materials throughout.

Enter the size of the extension, whether it is single or double storey, and your region, and the calculator returns a cost range for the building work alongside estimates for professional fees, planning and finishes. Many single-storey rear extensions fall under Permitted Development and need no full planning application, the depth limit being 6 m for a detached house and 3 m for an attached one.

Other Permitted Development conditions apply alongside that depth limit. A side extension may be no wider than half the width of the original house, height is capped at 3 m for single-storey work and 4 m for anything taller, nothing may project in front of the property, and materials must match what is already there. Conservation areas are tighter still, and an Article 4 direction can remove permitted rights altogether. Two-storey extensions need permission in most cases. Talk to the council before commissioning a design, because a refusal costs months.

Building Regulations approval is mandatory for every extension, applied for by Full Plans or Building Notice. Structural calculations are needed wherever a load-bearing wall comes out, a beam spans a new opening or foundations are formed. If you build against a shared boundary or excavate within 3 m of one, the Party Wall Act 1996 obliges you to notify the neighbour 2 months in advance. The usual surprises are underground: clay soils need foundations 1 m deep or more, an oak or willow within 1.5 times its own height pushes them deeper still, and drains sometimes have to be diverted with the water company involved.

The spend divides fairly predictably. The structural shell of foundations, walls, roof and windows takes 30 to 35%, or £15,000 to £25,000 on a typical job. Fit-out covering plaster, electrics and plumbing is 15 to 20%, or £8,000 to £15,000. A kitchen, where the extension is a kitchen, accounts for 15 to 25% at £8,000 to £20,000, flooring and decoration 10 to 15% at £5,000 to £10,000, and professional fees 5 to 10% at £4,000 to £8,000. Hold 5 to 10% back as contingency, and at least 10% is sensible. A finished kitchen extension lands at £40,000 to £80,000 mid-market and £80,000 to £150,000 at premium level.

On value, a kitchen extension typically adds 10 to 20% to a property and turns a net positive return after 3 to 5 years in most areas. The worst outcome is over-extending a small house, since a £100k extension on a £200k home rarely returns its cost. The best is usually turning a 3-bed into a 4-bed, where the value jump is real. Loft conversions often give a better return than extensions, being cheaper for a similar gain. Check local values on Rightmove or Zoopla first, because no extension pushes a house above the ceiling price for its street.