How much does a house extension cost in the UK?

Working out what a house extension will actually cost usually starts with a per-square-metre figure picked up from a website, and then gets complicated fast once VAT, professional fees and site-specific factors get added on top. Here's a realistic breakdown of what those numbers mean in practice.

As a rule of thumb, budgeting around £1,800-£3,000 per square metre for a standard-specification extension is a reasonable starting point nationally, though the range quoted varies by source and by exactly what "standard" includes. A single-storey rear or side extension commonly falls somewhere in the £1,500-£3,200 per m² range, with a mid-range figure around £1,800-£2,500 per m² typical for a straightforward kitchen or living-space extension without an especially high specification. Multiplying that by the floor area gives a rough build cost, but it's the finish level, not just the footprint, that pushes a project toward the top or bottom of that range.

Building up rather than out changes the maths in a way that catches some homeowners out. A double-storey extension typically works out cheaper per square metre, commonly around £1,200-£2,800/m², because the foundations and roof structure are shared across two floors rather than paid for once per storey. But because there's simply more floor area being built, a same-footprint double-storey extension commonly costs 50-75% more in total than an equivalent single-storey one, not less. Typical all-in totals are commonly cited around £45,000-£75,000 for a single-storey extension and £80,000-plus for a two-storey project, though a small single-storey extension or a large two-storey one can sit well outside those bands.

Where you live changes the numbers substantially. London and the South East commonly run 25-40% above the national range, which on a £60,000 national-average extension can mean a genuinely different quote once local labour and material costs, and often more constrained site access, are factored in. It's worth benchmarking any quote against costs for your specific region rather than a single national average, since the gap between London and, say, the North East or Wales is large enough to materially change what's affordable.

VAT adds a further 20% on top of most extension costs, since VAT on building work for an already-occupied single dwelling is standard-rated. A reduced 5% VAT rate exists, but it applies only in specific circumstances, chiefly renovating a residential property that has been empty for at least two years, or converting a non-residential building into a home, not to an ordinary extension of a house you're currently living in. It's worth checking whether a builder's quote is VAT-inclusive or needs 20% adding on top, since that distinction alone can meaningfully change a headline figure.

Professional fees sit alongside the build cost itself and are easy to under-budget for. A full architectural service on a medium-sized extension commonly costs 10-15% of the construction cost, so £10,000-£15,000 plus VAT on a £100,000 build; if you only need planning drawings rather than a full design-and-build service, that's commonly £1,000-£2,500, rising to around £3,000 if building-regulations drawings are included too. A structural engineer, needed for calculations on any loadbearing wall removal or new opening, commonly charges £550-£1,500 for a typical domestic extension. None of these figures are optional extras to skip; a lender, buyer or Building Control officer further down the line will expect to see proper structural calculations for anything load-bearing.

A kitchen extension is worth budgeting separately from the building work itself, since the two figures often get conflated in early planning. The building cost, walls, roof, floor, glazing, commonly falls in the same £1,500-£2,500/m² range as any other single-storey extension, but the kitchen units, worktops, appliances and fitting on top of that shell commonly add a further £5,000-£25,000 depending on specification, a range wide enough that it's worth deciding on a kitchen budget early rather than treating it as whatever's left over once the building work is paid for. A side-return or wrap-around extension, which extends around an existing rear addition rather than in a single straightforward direction, commonly costs more per square metre than a simple rear extension, reflecting the more complex roof junctions and structural tie-ins involved even when the total floor area added is similar.

Putting a realistic total together means adding the build cost, VAT where it applies, professional fees, and a contingency on top, rather than treating the headline per-square-metre figure as the whole answer. A £1,800/m² quote on a 25m² single-storey extension gives a £45,000 build cost before VAT, professional fees, and any site-specific extras such as underpinning, drainage diversion or difficult access are added — all of which are common enough on real projects that it's worth asking a builder directly whether they're included in a quote or likely to come up once work starts.

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