A house extension is one of the biggest investments you can make in your property. In 2026, a single-storey rear extension in the UK costs between £30,000 and £60,000, while a two-storey extension runs £40,000 to £90,000.

Single-Storey Extension Costs

The average cost per square metre for a single-storey extension is £1,500 to £2,500. A typical 4m x 5m extension (20m²) costs £30,000 to £50,000 including foundations, walls, roof, windows, electrics, plumbing, plastering, and basic finishing.

Two-Storey Extension Costs

A two-storey extension costs roughly 50-60% more than single-storey, not double. You are reusing the same foundations and roof structure.

What Pushes Costs Up

Structural steelwork for open-plan layouts adds £2,000-5,000. Underfloor heating adds £50-80 per m². Bi-fold doors cost £3,000-7,000. Building in London adds 20-40% to labour costs.

🚿 Adding a bathroom to your extension? Our bathroom cost guide helps you budget the fit-out separately from the build.

🚿 Adding a bathroom to your extension? Our bathroom cost guide covers what to budget for the fit-out.