Extension cost per m² in the UK
A single-storey extension in the UK costs £1,500–£2,500 per m² of internal floor area, while a double-storey extension runs £2,000–£3,500 per m². Premium finishes, complex foundations (sloping sites, clay soil) and London postcodes push costs to £3,000–£4,500 per m². These figures exclude VAT, planning and professional fees.
Homeowners researching this typically also price up house extension cost in the uk, loft conversion cost in the uk and cost of building a garage in the uk. For wider context, browse our bathroom pricing, kitchen pricing and boiler & heating costs.
Cost table
| Item | Typical range (GBP) |
|---|---|
Single-storey, basic spec | £1,500–£2,000/m² |
Single-storey, mid-range | £1,800–£2,600/m² |
Single-storey, premium | £2,500–£3,500/m² |
Double-storey, basic | £2,000–£2,800/m² |
Double-storey, mid-range | £2,400–£3,200/m² |
Double-storey, premium / London | £3,200–£4,500/m² |
Basement conversion | £2,500–£4,500/m² |
Labour costs
Labour makes up 45–55% of the build cost. Groundworkers, bricklayers, carpenters, roofers, plasterers and decorators all contribute.
| Item | Typical range (GBP) |
|---|---|
Groundworker / foundations | £3,000–£8,000 |
Bricklayer / blockwork | £4,000–£10,000 |
Carpenter (roof, stairs, 1st fix) | £5,000–£12,000 |
Roofer | £3,000–£7,000 |
Plasterer / decorator | £4,000–£9,000 |
Electrician & plumber | £3,000–£7,000 |
Materials costs
Materials include concrete, blocks, timber, insulation, windows, doors, roofing, plasterboard, flooring and kitchen/bathroom fittings.
| Item | Typical range (GBP) |
|---|---|
Concrete & foundations | £2,500–£6,000 |
Blockwork & mortar | £2,000–£5,000 |
Timber (joists, rafters, stud walls) | £3,000–£7,000 |
Insulation (walls, floor, roof) | £2,000–£5,000 |
Windows & doors | £3,000–£8,000 |
Roofing (tiles, felt, lead) | £2,500–£6,000 |
Factors affecting cost in the UK
- ·Storey count — double-storey is more cost-efficient per m² than single-storey.
- ·Soil type — clay, chalk and sloping sites increase foundation costs.
- ·Access — narrow gardens or terraced houses make material delivery harder.
- ·Finish spec — underfloor heating, bifold doors and premium kitchens add cost.
- ·Location — London and the South East run 25–40% above the national average.
Ways to reduce cost
- ·Build double-storey rather than single-storey for better cost per m².
- ·Use standard-size windows and doors rather than bespoke.
- ·Keep the design rectangular — complex shapes increase labour and waste.
- ·Get fixed-price build contracts, not open-ended day rates.
- ·Schedule work in spring/summer for faster progress and lower weather risk.
Frequently asked questions
- Is it cheaper to build up or out?
- Building up (double-storey) is usually cheaper per m² because foundations and roofing are shared across two floors.
- Does the cost per m² include VAT?
- Usually quoted excluding VAT. Add 20% for VAT-registered builders. Some quotes include VAT — always confirm.
- What's the smallest extension worth building?
- From a value perspective, 15–20 m² is the sweet spot for a single-storey rear extension. Smaller extensions have a higher per-m² cost.
- How much do professional fees add?
- Typically 10–15% of the build cost for architect, structural engineer, Building Control and planning fees.
- Can I reduce cost by using timber frame?
- Timber frame can be 10–20% faster to erect and may reduce labour, but material costs are comparable to blockwork.