How Much Does a Double-Storey Extension Cost in the UK?
A double-storey extension adds rooms on two floors simultaneously, making it one of the best-value ways to significantly increase a home's square footage. Because foundations, roof, and scaffolding are shared across both floors, the cost per square metre is often lower than building two separate single-storey extensions at different times. Expect to pay £1,800–£2,800 per square metre across most of England.
Typical Cost (UK, 2026)
Approximately £1,800–£2,800 per m² including labour and materials
All costs are indicative estimates. Obtain at least 3 quotes from qualified builders. Prices exclude VAT (20%).
Factors Affecting the Cost
Size
Total floor area across both storeys drives cost. A 6m × 4m footprint yields roughly 48m² of new floor space.
Materials
Matching existing brickwork costs more than render. Stone or specialist cladding adds a premium.
Roof type
A simple pitched roof to match the existing house is most common. A flat roof or complex hipped design costs more.
Location
London and the South East attract a 20–30% premium. The North of England and Wales are typically at the lower end of the range.
Groundwork conditions
Clay soils, high water tables, or neighbouring trees may require deeper or reinforced foundations, adding £5,000–£15,000.
Internal layout
An en-suite bathroom or ground-floor WC adds plumbing and tiling costs. Open-plan layouts may require steel beams.
Cost Breakdown
| Item | Basic | Mid-range | High-end |
|---|---|---|---|
| Foundations | £5,000 | £9,000 | £18,000 |
| Ground floor structure (walls, slab) | £10,000 | £16,000 | £25,000 |
| First floor structure (joists, walls) | £8,000 | £14,000 | £20,000 |
| Roof construction | £6,000 | £10,000 | £16,000 |
| Windows and external doors | £4,000 | £8,000 | £18,000 |
| Internal finishes (plastering, insulation, flooring) | £8,000 | £14,000 | £22,000 |
| Electrics and plumbing | £4,000 | £7,000 | £12,000 |
| Architect and planning fees | £3,000 | £5,000 | £9,000 |
Costs are indicative and exclude VAT. Regional variations apply.
Typical Timeline
16–24 weeks on site
Frequently Asked Questions
Get a Personalised Cost Estimate
Use our interactive calculator to estimate your extension cost by size, type, and location.
Calculate Your Extension Cost →