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A builder measuring the new blockwork of a single-storey rear extension on a 1930s semi
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Extension cost calculator

Enter the size and finish. We work the job out the way a builder prices it — foundations, walls, roof, glazing, services, site costs — and show you where the money goes.

Single-storey extension · 4m × 5m · about 16 m² · standard finish
£50,500 – £64,500

Including VAT at 20%. Central figure £56,300 · roughly £3,460 per m² · about 5 weeks on site.

Where the money goes

Preliminaries & site setup£840Excavation & foundations£2,800Floors£2,990Structural elements£2,250External walls£6,860Roof£7,990Windows & external doors£6,960Drainage£1,210Plumbing & heating£870Electrical£1,300Insulation & internal linings£2,010Internal finishes£1,930Scaffold & access£780Plant hire£930Waste removal£2,380Deliveries£330Testing & certification£120Fees & approvals£1,200Allowances (vary most between jobs)Steel beam and structural design for 1 × 3m opening£3,110Total before VAT£46,900VAT at 20%£9,400

A guide, not a quote. Worked out with typical UK labour rates and material prices dated 2026-09-24, for a straightforward site with good access and normal ground. Planning, architect and structural engineer fees, party wall costs and high-end kitchens are not included. Always get written quotes from local builders.

How much does an extension cost in the UK?

There is no honest single answer, which is why this calculator works the job out item by item instead of multiplying by one rate per square metre. The biggest swings come from the foundations (ground conditions and depth), the roof type, the amount and type of glazing, how much of the existing house is opened up (steels and making good), and above all how far it's finished — a watertight shell is a very different price from a finished kitchen-diner.

What makes extensions more expensive

  • Poor ground or nearby trees needing deeper foundations.
  • Large openings into the house needing steel beams.
  • Bifolds, sliders and roof lanterns.
  • Moving drains or adding a bathroom.
  • Two storeys: scaffold, a second floor and fire separation.
  • Tight access for materials and muck-away.

Single-storey vs two-storey

A two-storey extension costs more in total but usually less per square metre, because the foundations and roof are shared across two floors. Try both in the calculator at the same footprint to see the difference.

Extension Cost Calculator UK (2026) — Free, Itemised, No Sign-up