Single Storey Extension Cost Guide 2026
A single storey extension is the most common way to add ground-floor space to a UK home. Whether it's a rear kitchen-diner, side return extension, or wrap-around, the costs follow a predictable pattern driven by foundations, structure, and finishing.
Single storey extensions often benefit from permitted development rights, meaning you can build without full planning permission. This guide provides 2026 trade-by-trade pricing to help you understand what your builder's quote should include and where costs can vary.
Cost summary
| Size | Basic | Standard | Premium |
|---|---|---|---|
| 15m² (small) | £11,010 | £19,650 | £30,465 |
| 25m² (medium) | £18,350 | £32,750 | £50,775 |
| 40m² (large) | £29,360 | £52,400 | £81,240 |
Costs shown are direct trade costs (national average). Apply regional factors below for your area. Main contractor route adds 20–35%.
Trade-by-trade breakdown
Standard specification, mid estimate. Per-sqm rates for direct trades.
Regional cost variations
| Region | Factor | Adjusted cost |
|---|---|---|
| Inner London | ×1.35 | £1,769/m² |
| Outer London | ×1.22 | £1,598/m² |
| South East | ×1.08 | £1,415/m² |
| South West | ×1.00 | £1,310/m² |
| East Midlands | ×0.92 | £1,205/m² |
| North West | ×0.90 | £1,179/m² |
| North East | ×0.85 | £1,114/m² |
Multiply the national average cost by the regional factor for your area.
Key cost factors
Foundation type
Standard strip foundations suit most conditions. Clay soils, tree proximity, or poor ground may require trench-fill or piled foundations, adding £3,000–£10,000.
Party wall requirements
If you're building on or near a boundary with a neighbour, the Party Wall Act applies. Surveyor fees typically cost £700–£1,500 per neighbour.
Roof type
A flat roof is cheaper than a pitched roof by £50–£100/m². However, pitched roofs last longer, match the existing house better, and may be required by planning conditions.
Kitchen or bathroom inclusion
Adding a kitchen or bathroom to the extension significantly increases plumbing, electrical, and fit-out costs — budget an additional £5,000–£20,000 depending on specification.
Site access
Restricted rear access, narrow side passages, or no space for skip/scaffold storage can add 10–15% to labour costs due to manual handling and logistics.
Frequently asked questions
How much does a single storey extension cost per m²?
In 2026, expect £730–£2,030 per m² depending on specification. Standard spec averages around £1,310/m² for direct trade costs. Adding a main contractor markup of 20–35% brings the total to £1,570–£1,770/m².
Can I build a single storey extension under permitted development?
Most rear single storey extensions qualify under permitted development if they don't extend more than 6m from the original house (8m for detached). Side extensions must be single storey and no wider than half the original house. Prior notification may be required for larger extensions.
How long does a single storey extension take?
A typical 20–30m² single storey extension takes 10–14 weeks from groundworks to completion. Allow additional time for planning permission (8–12 weeks) and Building Regulations if applicable.
Is a flat or pitched roof cheaper for an extension?
A flat roof is typically £50–£100/m² cheaper and faster to build. However, pitched roofs have a longer lifespan (40–60 years vs 20–25 years for felt flat roofs), better match most houses, and may be required by planning. Modern EPDM or GRP flat roofs last 30+ years.
Do I need an architect for a single storey extension?
An architect isn't legally required, but a good designer ensures your extension meets Building Regulations, maximises space and light, and adds value. Architectural technologists offer a more affordable alternative. Budget 5–8% of build cost for design fees.
Will a single storey extension add value?
A well-designed single storey extension typically adds 5–10% to your property's value. Open-plan kitchen-diners with good natural light and garden access tend to deliver the best returns.
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