House Extension Cost Estimator
Rough-cost a single or double storey home extension.
What is the House Extension Cost Estimator?
A house extension cost estimator gives a rough total cost for a home extension based on the floor area, number of storeys, your estimated build cost per square metre, and any additional costs like fit-out or professional fees. It works by multiplying the floor area by your cost-per-m² rate and the number of storeys, then adding on your extra costs to reach a total. Because real build costs per m² vary hugely by UK region, specification and builder, this tool is deliberately input-driven rather than using a single national average — to get an accurate figure, get quotes from local builders for your specific project and budget a contingency of 10-15% on top of any total.
How it works
This estimator multiplies your extension's floor area by a cost-per-square-metre rate (which you enter, since build costs vary hugely by region and specification) and by the number of storeys, then adds any additional costs you specify separately, like fit-out or professional fees, to give a total estimate.
UK context
Extension costs in the UK vary enormously by region, specification and current material/labour market conditions — a basic single-storey extension in one part of the country can cost meaningfully less per square metre than a high-specification build in London or the South East, which is exactly why this tool asks you to enter your own local rate rather than assuming a single UK-wide figure.
Tips
- Get quotes from at least two or three local builders before committing — this gives you a far more accurate local rate than any generic online figure.
- Budget a contingency of 10-15% on top of the quoted build cost for unexpected issues, which are common in extension projects once work begins.
- Remember to check whether you need planning permission or can proceed under permitted development rights, and factor in any building control and design fees separately.
Frequently asked questions
Why do I need to enter my own cost per square metre?
Because build costs vary hugely by region, specification and current market conditions — there's no single accurate UK-wide figure, so entering your own local estimate (from quotes or research) gives a far more realistic result than a generic average.
Does this estimate include planning and design fees?
Only if you include them in the "additional costs" field — the core calculation covers build cost only, so fees, fit-out and furnishing should be added separately for a complete picture.
A quick note
This is a rough estimate using the rate you provide. Actual costs depend heavily on specification, ground conditions, local labour rates and market conditions at the time of building — get quotes from local builders for an accurate figure.