House Extension Cost Calculator UK
Estimate the cost of your home extension project instantly. Get accurate build quotes based on size, type, and specification, including VAT, professional fees, and regional rates.
Extension Cost Estimator
Calculate your build costs based on 2024/25 UK averages
Select the type of extension you are planning to build.
The total internal floor area of the new extension in square metres.
Basic uses off-the-shelf materials; Premium includes bespoke joinery, high-spec glazing, and smart home tech.
Building costs in London and the South East are typically 15-20% higher due to labour rates and access constraints.
Your Extension Estimate
2024/25 UK build costs
Enter your project details and click Calculate to see your estimated extension costs.
Average Extension Costs by Type
Typical total project costs for standard-sized extensions in the UK (excluding VAT and professional fees). Costs vary significantly based on specification and location.
| Extension Type | Average Size | Basic Cost | Standard Cost | Premium Cost |
|---|---|---|---|---|
| Single-storey rear | 15 – 20m² | £30k – £45k | £45k – £60k | £60k+ |
| Double-storey | 20 – 30m² | £50k – £70k | £70k – £90k | £90k+ |
| Wrap-around | 25 – 35m² | £60k – £80k | £80k – £110k | £110k+ |
| Loft conversion | 20 – 30m² | £35k – £50k | £50k – £70k | £70k+ |
| Garage conversion | 15m² | £15k – £25k | £25k – £35k | £35k+ |
House Extension Costs FAQ
Everything you need to know about budgeting, planning, and building a house extension in the UK.
In 2024/25, a standard single-storey extension in the UK typically costs between £40,000 and £65,000, while a double-storey extension ranges from £60,000 to £100,000. The final cost depends heavily on the size, specification (basic, standard, or premium), and your location, with London and the South East costing up to 20% more due to higher labour rates.
As a general rule, you should budget between £1,500 and £2,500 per square metre for a single-storey extension. Double-storey extensions are slightly more expensive per square metre (£1,700–£2,800/m²) due to the need for deeper foundations and structural support. Premium finishes and complex designs can push costs above £3,000 per square metre.
Yes, standard VAT at 20% applies to most house extension projects, including labour and materials. However, if you are incorporating certain energy-saving materials (like insulation, heat pumps, or solar panels), the VAT rate on those specific elements may be reduced to 5% or even 0%. New build properties are zero-rated, but extensions to existing homes are standard-rated.
Professional fees typically account for 10% to 15% of your total construction budget. This usually includes an architect (5-8% for design and planning), a structural engineer (2-3% for calculations and building regulations), and party wall surveyors or planning consultants. For a £50,000 extension, expect to pay around £5,000–£7,500 in professional fees.
Absolutely. It is highly recommended to set aside a contingency fund of 10% to 15% of your total budget. Extensions often uncover unexpected issues once the build starts, such as poor ground conditions, outdated drainage, or asbestos. A contingency ensures you can cover these surprises without halting the project or compromising on quality.
Building a double-storey extension is generally more cost-effective per square metre of added space than a single-storey extension. While the base costs for foundations and the roof are similar, you are essentially getting two floors of living space for the price of one roof and foundation. However, double-storey projects often require more complex structural engineering and may face stricter planning permission rules.
The timeline depends on the size and complexity of the project. A simple single-storey rear extension typically takes 8 to 12 weeks to build once construction starts. A double-storey or wrap-around extension will take 12 to 16 weeks. However, you must also factor in 8 to 12 weeks for design, planning permission, and party wall agreements before any physical work begins.
Yes, a well-designed extension can significantly increase your property’s value. According to industry estimates, a double-storey extension can add up to 20% to the value of your home, while a single-storey rear extension can add 10-15%. The key is to ensure the cost of the build does not exceed the value it adds relative to the ceiling price of properties in your specific street.