Typical cost in 2025
Most homeowners pay around £50,000 for a single storey rear extension, 25m², standard spec
What affects the price?
Size (m²)
Building costs are quoted per m². Standard single-storey extensions cost £1,500–£2,500/m². Double-storey is more efficient — roughly £1,200–£2,000/m² as the roof and foundations are shared.
Single vs double storey
A double-storey extension adds significantly more space for a relatively modest increase in cost — often 40–60% more than a single storey for double the space. Planning permission is usually required.
Specification
Standard spec (timber frame, UPVC windows, plasterboard) is cheapest. High spec (steel frame, aluminium glazing, premium finishes) can increase cost by 50–100%.
Glazing and bi-folds
Large glass walls or bi-fold doors are popular for garden extensions. A quality 3-panel bi-fold costs £2,500–£5,000 installed. Roof lights add £500–£1,500 each.
Ground conditions and foundations
Standard strip foundations are cheapest. Clay soils may require deeper or piled foundations, adding £3,000–£10,000 to the groundwork cost.
Typical cost breakdown
Estimates are based on UK average rates in 2025. Prices in London and the South East are typically 20–30% higher.
Frequently asked questions
Do I need planning permission for an extension?
Single storey rear extensions up to 4m (detached) or 3m (semi/terrace) deep fall under Permitted Development. Larger extensions or side/double storey additions usually need planning permission. Always confirm before work starts.
How long does a house extension take to build?
A single storey extension of 20–30m² typically takes 12–16 weeks from groundworks to final decoration. Larger or more complex projects can take 6–12 months.
How much value does an extension add?
A well-built extension typically adds 10–20% to property value, depending on location and quality. In high-demand areas, the return can exceed the cost of the build.
Do I need an architect?
For simple single-storey extensions, a good builder with a draughtsperson may be sufficient. For planning applications, larger projects, or complex designs, an architect adds value and can save money through good design decisions.
What is a Party Wall Agreement?
If your extension is near a shared boundary or attached to a neighbour's wall, you'll likely need a Party Wall Agreement under the Party Wall Act 1996. This is a legal notice process — budget £500–£1,500 if a surveyor is needed.