Home Extension Cost Calculator
Estimate the total budget for your home extension. Calculate build costs, professional fees, and contingency based on size, quality, and location.
Calculate Your Extension Budget
Enter your project details to get a realistic, itemised estimate of what your home extension will cost in today’s market.
Project Specifications
Enter your extension details
Budget Evaluation
Estimated total project cost breakdown
Average Build Costs per Sqm
Baseline construction costs (excluding professional fees, VAT, and contingency) for standard finish quality in the Rest of UK.
| Extension Type | Standard Finish | Good Finish | Premium Finish |
|---|---|---|---|
| Single-Storey | £1,800 / sqm | £2,250 / sqm | £2,700 / sqm |
| Double-Storey | £1,500 / sqm | £1,875 / sqm | £2,250 / sqm |
| Wrap-Around | £1,900 / sqm | £2,375 / sqm | £2,850 / sqm |
Extension Costs FAQ
Answers to the most frequently asked questions about budgeting for a home extension in the UK.
On average, a standard single-storey extension costs between £1,800 and £2,200 per square metre. Double-storey extensions are slightly cheaper per square metre of floor area (around £1,500 to £1,900 per sqm) because the roof and foundation costs are shared across two levels.
Besides the core construction build cost, you should budget approximately 10-15% for professional fees (architects, structural engineers, party wall surveyors) and a 10% contingency fund for unforeseen issues. VAT at 20% may also apply depending on the contractor.
Yes, significantly. Building in London typically costs 25-30% more than the national average due to higher labour rates, material delivery logistics, and stricter planning constraints. The South East is around 15% higher, while the rest of the UK is closer to the baseline average.
Many smaller extensions fall under ‘Permitted Development’ rights and do not require full planning permission, provided they meet specific size and height limits. However, building regulations approval is always required. It is highly recommended to consult your local planning authority or an architect.