Extension Cost in London (2026)
A extension in London typically costs between £40,800 and £61,200 - roughly 28% above the UK average. Higher local labour rates and strong demand are the main factors driving prices up.
£40,800–£61,200
Typical range
£51,000
Average cost
+28%
vs national avg
10–16 weeks
Typical duration
Free, no obligation. Quotes from vetted tradespeople in London.
Important: Building Regulations approval is required for all house extensions. Most single-storey rear extensions up to 6m (detached) or 3m (semi/terrace) fall under Permitted Development, but you should always check with your local council before starting work.
London extension prices
Prices below reflect the typical cost of an extension in London, including materials and labour.
| Type | Details | London range | London avg |
|---|---|---|---|
| Single-storey rear | 3m x 4m basic build, plastered and decorated | £40,800 – £61,200 | £51,000 |
| Single-storey side | Infill or wraparound, converting dead space | £36,700 – £55,100 | £45,900 |
| Double-storey rear | Two floors, extra bedroom and living space | £71,400 – £107,200 | £89,300 |
All prices include materials and labour. Based on contractor submissions and market data, April 2026.
What affects extension costs in London
Extension costs are heavily influenced by local build rates - the cost per square metre for structural work. In London, builders typically charge £60–£90 per hour for general construction labour, which affects the per-square-metre rate for your build.
London has a huge range of property types, from Victorian terraces to modern new-builds. Older properties in inner London boroughs often require more preparatory work due to age and access constraints. The type of property you are extending matters. A straightforward rear extension on a detached house is simpler than extending a mid-terrace or a property on a slope.
Foundation costs can vary significantly. Ground conditions in London vary - clay soil is common and can require deeper foundations, adding £2,000 to £5,000 to the cost.
Planning and Building Regulations fees are set nationally, but if your extension requires full planning permission rather than falling under Permitted Development, the design and application process adds time and cost. Many London boroughs have additional planning constraints, particularly in conservation areas.
How London compares
London extension costs compared to other UK regions and the national average.
| Region | Average cost | vs national avg |
|---|---|---|
| LondonYou are here | £51,000 | +28% |
| National average | £40,000 | - |
| South East | £45,000 | +13% |
| East of England | £42,600 | +7% |
How to get your extension for less in London
Use Permitted Development rights
If your extension qualifies under Permitted Development, you skip the full planning application - saving £200 to £500 in fees and potentially months of waiting. Check with your London borough council before assuming you need planning permission.
Get a fixed-price contract
Extensions are complex and costs can escalate. Insist on a fixed-price contract from your London builder that covers all structural work, roofing, plastering, electrics, and plumbing. Vague quotes lead to expensive surprises.
Build during quieter months
Builders in London are typically busiest from April to September. Starting your build in autumn or winter can sometimes get you a better price, though weather delays are more likely.
Frequently asked questions
How much does a house extension cost in London?
Do I need planning permission for an extension in London?
How long does it take to build an extension in London?
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