In 2026, a basic single-storey rear extension in the UK runs from £2,200 to £3,200 per square metre for the build alone. A 25m² extension is therefore £55,000 to £80,000 — before VAT, before fees, and before fit-out.
That headline figure assumes a standard cavity-wall construction, a flat roof with rooflights, French doors to the garden and basic plasterboard finish. Costs scale quickly once you add bi-fold doors (£4,000–£10,000), underfloor heating (£60–£90 per m²), or a vaulted ceiling.
The costs that catch most homeowners by surprise are the soft costs: architect (3–8% of build), structural engineer (£800–£1,500), Building Control (£600–£1,200), party-wall surveyor if relevant (£800–£2,000 per neighbour), planning fees and CIL.
Add 10% contingency on top of everything. Drains nobody knew about, ground worse than expected, a steel beam that turned out to need a different padstone — there's always something.
Our advice: get three written quotes from local firms with checkable references, ignore the cheapest, and pay for a paid consultation rather than rely on free quotes.
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Our team is happy to talk through any of this on a free 15-minute call.