House extension cost in the UK
A house extension in the UK typically costs between £1,800 and £3,200 per m², meaning a 20 m² single-storey rear extension lands around £36,000–£65,000 once VAT, professional fees and finishes are included. London and the South East routinely run £3,200–£4,500 per m²; the North and Midlands often deliver mid-range builds at £1,600–£2,200 per m².
Homeowners researching this typically also price up loft conversion cost in the uk, kitchen renovation cost in the uk and builder day rate in the uk. For wider context, browse our bathroom pricing, kitchen pricing and boiler & heating costs.
Cost table
| Item | Typical range (GBP) |
|---|---|
Single-storey rear (20 m²) | £36,000–£65,000 |
Single-storey side return (12 m²) | £24,000–£45,000 |
Double-storey rear (40 m²) | £65,000–£120,000 |
Wrap-around (30 m²) | £55,000–£100,000 |
Garage conversion (15 m²) | £9,000–£18,000 |
Labour costs
Labour is usually 35–45% of an extension build. The trades sequence runs groundworks → structure → roof → first fix → plaster → second fix → decoration.
| Item | Typical range (GBP) |
|---|---|
Groundworks & foundations | £4,500–£12,000 |
Brick & block walls | £5,000–£14,000 |
Roofing | £3,500–£9,000 |
Electrical first/second fix | £1,800–£4,500 |
Plumbing & heating extension | £1,500–£4,500 |
Plastering & decoration | £2,500–£6,500 |
Materials costs
Materials swing widely based on glazing area and specification. Bifolds, lanterns and structural steels are the biggest variables.
| Item | Typical range (GBP) |
|---|---|
Bricks, blocks, mortar | £2,500–£6,000 |
Steel beams & lintels | £1,500–£5,000 |
Insulation & membranes | £1,200–£3,500 |
Bi-fold or sliding doors | £2,500–£8,000 |
Roof lantern / lights | £800–£3,500 |
Underfloor heating system | £1,500–£4,000 |
Cost per m² (build cost only, ex. VAT)
| Item | Typical range (GBP) |
|---|---|
Basic spec (Midlands / North) | £1,600–£2,200/m² |
Mid spec (most of UK) | £2,200–£3,000/m² |
High spec (London / SE) | £3,000–£4,500/m² |
Factors affecting cost in the UK
- ·Size and storey count — double storey is cheaper per m² than single storey.
- ·Glazing — large bifolds and roof lanterns can add £8,000–£20,000.
- ·Ground conditions — clay soils or trees nearby may need deeper foundations.
- ·Region — London and SE add 30–50% on national rates.
- ·Spec of internal finishes — kitchen, flooring and underfloor heating.
Ways to reduce cost
- ·Keep the floor plan simple — square or rectangular shapes are cheaper to build.
- ·Standard glazing sizes are 25–40% cheaper than bespoke.
- ·Stay within permitted development to skip planning fees and delays.
- ·Use a single main contractor on a fixed-price JCT minor works contract.
- ·Tender to 3 contractors with identical drawings and spec for true comparison.
Frequently asked questions
- Do I need planning permission for an extension?
- Many single-storey rears fit permitted development. Double-storey, side and front extensions usually need full planning permission.
- How much value does an extension add?
- A well-designed extension typically returns 70–110% of build cost in added value, with kitchen-diners performing best.
- Are extension prices likely to rise?
- UK construction material inflation has averaged 4–6% per year since 2021. Labour rates rise with general wage inflation.
- What's the cheapest type of extension?
- A garage conversion at £600–£1,200 per m². A simple single-storey rear extension is next, around £1,800–£2,400 per m².
- How long does a typical extension take?
- 12–20 weeks on site for single-storey, 16–26 weeks for double-storey, plus 8–16 weeks for design and planning beforehand.
- Do I need a Party Wall Agreement?
- Yes if foundations come within 3 m of a neighbour's wall, or you build on the shared boundary.