Updated April 2026
How Much Does a House Extension Cost?
A house extension is one of the biggest home improvement projects you can undertake. Costs per square metre vary based on the type of extension, the finish level, and your location.
What you can expect to pay
| Description | Low | Mid | High |
|---|---|---|---|
| Single storey rear (per m²) | £1,200 | £1,500 | £1,800 |
| Double storey (per m²) | £1,000 | £1,250 | £1,500 |
| Side return / infill (per m²) | £1,500 | £2,000 | £2,500 |
Prices are UK averages for 2026 including VAT where applicable. Your actual cost may vary based on location and specific requirements.
What affects the price
Planning permission and building regulations approval add to the timeline and cost. Many single storey rear extensions fall under permitted development, avoiding planning fees.
Foundations are a major cost. Ground conditions vary and sometimes require deeper or more complex foundations than expected.
The internal finish level makes a huge difference. A basic plastered shell with standard fittings is very different from underfloor heating, bifold doors, and a high-spec kitchen.
Double storey extensions cost less per square metre than single storey because you share the roof and foundation costs across two floors.
How to keep costs down
- 1
Check whether your extension falls under permitted development before paying for a full planning application. A lawful development certificate costs less and gives you the same certainty.
- 2
Get a structural engineer involved early. Their fees (£500 to £1,500) are small relative to the overall project but prevent expensive surprises with foundations and steelwork.
- 3
Consider a party wall agreement timeline if you share a wall with neighbours. The process can take 2 to 3 months, so start it early to avoid holding up the build.
- 4
Get a detailed, itemised quote rather than a lump sum. This makes it easier to compare builders and to track costs as the project progresses.
Frequently asked questions
A single storey rear extension typically takes 10 to 16 weeks from breaking ground to completion. A double storey extension takes 14 to 24 weeks. Add 2 to 4 months before that for planning, design, and building control approval.
For a simple rear extension, an architectural technician or a builder's in-house designer may be sufficient and cheaper. For anything complex, involving listed buildings, or where design quality is important to you, a qualified architect adds real value.
Permitted development is a general grant of planning permission that covers certain types of work without a formal application. Single storey rear extensions up to certain sizes often qualify. If your extension exceeds the limits or you're in a conservation area, you'll need a full planning application.
A well-designed extension typically adds more value than it costs, especially if it adds a bedroom or a larger kitchen-living space. The return depends on your area and the quality of the build. In general, extending is more cost-effective than moving to a larger property.
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