UK cost calculator
House Extension Cost Calculator
Estimate a planning budget for a single or double-storey extension. Adjust the region, size and finish level to see the range change instantly.
Enter a postcode to set the cost region automatically.
Estimated project range
£50,760 – £84,600Typical estimate £67,680 · Suggested 10% contingency £6,768 · Budget incl. contingency £74,448
Postcode matching uses the postcode area to select an approximate cost region; some postcode areas cross regional boundaries, so the region can always be overridden manually. Core extension benchmark: £1,800–£3,000 per m².
How the estimate works
This first build uses a central cost model with regional multipliers and finish-level adjustments. The production version will attach source dates and methodology to each price input.