House Extension Cost Calculator — UK Prices 2025
Extension Cost Calculator
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How much does extension cost in the UK?
Use our calculator above to get an accurate estimate based on your specific requirements. Prices are based on real quotes from UK tradespeople and updated quarterly.
What affects the cost of extension?
- Location: London and the South East are 15–40% more expensive than the national average.
- Size and complexity: Larger or more complex jobs cost proportionally more.
- Material quality: Budget, mid-range and premium specifications can vary by 2–3x.
- Time of year: Demand peaks in spring and summer — book in winter for better availability and pricing.
- Access: Difficult access (scaffolding, confined spaces) adds to labour costs.
How to save money on extension
- Get at least 3 quotes from different tradespeople.
- Book during quieter periods (January–March for most trades).
- Be flexible on start dates — tradespeople often discount for convenient scheduling.
- Supply your own materials where possible (but confirm with your tradesperson first).
- Combine jobs — having multiple trades on site at once can reduce overall costs.
How to find a good tradesperson
Always check reviews on Checkatrade, Rated People or MyBuilder. Ask for references from recent similar jobs. Ensure they are appropriately qualified and insured. Never pay the full amount upfront — a deposit of 10–25% is standard.
Frequently asked questions
How much does a house extension cost in the UK?
A single storey extension costs £1,500–£2,500 per m². A 20m² extension typically costs £30,000–£50,000. A double storey extension costs £1,800–£3,000 per m². Prices vary by region, specification, and complexity.
Do I need planning permission for a house extension?
Single storey rear extensions up to 4m (detached) or 3m (semi/terraced) deep fall under permitted development. Larger extensions, side extensions, and two-storey extensions usually require planning permission.
How long does a house extension take to build?
A single storey extension takes 8–16 weeks. A double storey extension takes 12–24 weeks. This includes groundwork, structure, roofing, windows, first fix, plastering, and second fix.
What is the cheapest type of house extension?
A single storey rear extension with a flat or lean-to roof is the most cost-effective. Using standard brick to match the existing house and avoiding full-height glazing keeps costs down.
Do I need an architect for a house extension?
You don't legally need an architect, but it is strongly recommended. An architect will produce drawings for planning and Building Regulations, and can save money through good design. Expect to pay £2,000–£5,000 for architectural services.