Extension Cost Estimator

A single-storey kitchen extension in the UK typically costs £2,200 – £2,500 per m² for the builder's work, before you add the kitchen itself. That figure is the standard cost per m² benchmark: it covers structural work, specialist trades, windows and doors, flooring, and professional fees, but not the kitchen units, appliances, or worktops.

This estimator turns that rate into a realistic ballpark for your extension. Enter your floor area, choose single or double storey, pick your region, and decide whether to include a new kitchen. The result splits across the main build phases, so you can check a builder's quote line by line instead of staring at one number.

How the cost per m² scales

Start with the build rate of £2,200 – £2,500 per m² and scale it by size and region. A small extension costs more per m² than a large one, because fixed costs like steelwork and the kitchen do not shrink with floor area. London and the South East sit at the top of the range; the North and Scotland run lower.

A worked example

Take a 30m² single-storey rear extension in the Midlands. At the standard £2,200 – £2,500 per m² build rate, the builder's work lands in the region the calculator shows above, before the kitchen. Add a kitchen and the total climbs; then add 15-20% contingency on top of everything.

A documented 30m² Oxfordshire extension ran well above its first estimate once premium glazing, mid-build steelwork changes, and a long programme were counted, which is exactly why the contingency line matters. See budgeting and contingency for how to hold that buffer through the build.