House Extension Cost Guide 2026
House extensions remain the most popular way to add space without moving. Whether you're considering a single storey rear extension or a two-storey side return, costs depend heavily on size, complexity, and location.
This guide covers both single and double storey extensions with 2026 pricing. Double storey extensions don't cost twice as much as single storey — you share foundations, roof structure, and site setup costs, making the per-square-metre cost roughly 40–60% more rather than double.
Cost summary
| Size | Basic | Standard | Premium |
|---|---|---|---|
| 30m² (single) | £22,020 | £39,300 | £60,930 |
| 40m² (single) | £29,360 | £52,400 | £81,240 |
| 60m² (double) | £44,040 | £78,600 | £121,860 |
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
Single vs double storey
Double storey extensions cost 40–60% more than single storey (not double) because foundations, roof, and site setup are shared across both floors.
Structural complexity
Open-plan ground floors with large openings require significant steelwork. Removing load-bearing walls adds £2,000–£5,000 per opening.
Roof design
Flat roofs are cheaper than pitched. Complex roof junctions where the extension meets the existing house increase cost and risk of leaks.
Services relocation
Moving gas meters, drainage runs, or electrical mains can add £2,000–£8,000 to the budget.
Boundary proximity
Building close to boundaries may require special foundations, party wall agreements, and more expensive construction methods.
Frequently asked questions
How much does a single storey extension cost?
A 30m² single storey extension costs £20,000–£55,000 in 2026 depending on specification and region. Standard spec in the South East averages around £38,000.
How much does a double storey extension cost?
A 60m² double storey extension (30m² per floor) costs £40,000–£110,000. Per square metre, it's significantly cheaper than building two separate single storey extensions.
What is the cheapest type of extension?
A single storey rear extension with a flat roof is typically the cheapest option. Lean-to conservatories are cheaper still but have limitations on insulation and usability.
How long does a house extension take to build?
Single storey extensions take 10–16 weeks. Double storey extensions take 14–22 weeks. Allow 4–12 weeks additional for planning permission if required.
Do I need an architect for an extension?
While not legally required, an architect or architectural designer ensures your extension meets Building Regulations, maximises space, and typically delivers better value. Fees are usually 5–10% of the build cost.
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Single Storey Extension Cost Guide 2026
Single storey extension costs from £1,200 to £2,400 per m² in 2026. Detailed breakdown by trade, size, spec level, and UK region.
Read guideDouble Storey Extension Cost Guide 2026
Two storey extension costs from £1,400 to £2,800 per m² in 2026. Full trade breakdown, regional adjustments, and what affects the price.
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