Calculate 2026 UK double-storey (two-floor) house extension costs per square metre (m²), taking advantage of shared foundation and roof economies of scale across ground and first floors.
In the UK in 2026, a double-storey extension costs £1,650 to £2,450 per square metre (m²) of total floor area across both floors—about 25% cheaper per m² than a single-storey extension because both floors share the same foundation and roof. A 20 m² footprint × 2 floors (40 m² total) costs £68,000 to £98,000.
📐 Engineering Formula Used: Total Cost (£) = (Footprint m² × 2-Floor Combined Rate × UK Region) + En-Suite + Steelwork + Architect/Planning
The table below provides verified 2026 material, labor, and installed turnkey cost ranges for double storey extension cost calculator UK projects. Compare these figures alongside live industry benchmarks such as ChatToYourBuilder.co.uk ↗.
| Footprint Size | Total Added Space (2 Floors) | 2026 Cost Per Total m² | UK Average Total Cost | London / South East Total |
|---|---|---|---|---|
| 15 m² Footprint (3m × 5m) | 30 m² Total | £1,750 – £2,300/m² | £52,500 – £69,000 | £65,000 – £88,000 |
| 20 m² Footprint (4m × 5m) | 40 m² Total | £1,700 – £2,250/m² | £68,000 – £90,000 | £85,000 – £115,000 |
| 25 m² Footprint (5m × 5m) | 50 m² Total | £1,650 – £2,150/m² | £82,500 – £107,500 | £104,000 – £138,000 |
| 30 m² Footprint (5m × 6m) | 60 m² Total | £1,600 – £2,100/m² | £96,000 – £126,000 | £122,000 – £162,000 |
Project Scenario: 20 m² Footprint × 2 Floors (40 m² Total Space) — Kitchen Extension Downstairs + Master Bedroom & En-Suite Upstairs
| Itemized Quote Line Item | 2026 Estimated Cost |
|---|---|
| 40 m² Total Two-Storey Shell, Roof, Floor Joists, Windows & Finishes (20 m² footprint @ £3,850) | £77,000 |
| Upstairs En-Suite Bathroom Fit-Out & Soil Stack Connection | £6,500 |
| Structural Steel RSJ Beams Across Ground & First-Floor Openings | £5,800 |
| Architect Plans, Structural Calculations & Building Control Fees | £4,200 |
| Estimated Total UK Double-Storey Extension Cost | £93,500 (£2,337.50 per total m²) |
The two most expensive elements of any UK house extension are digging the concrete trench foundations and constructing the insulated roof structure. Because a first-floor bedroom sits directly on top of the ground-floor extension walls and beneath the same roof, adding the second storey typically adds only 50% to 60% more cost for 100% more floor space.
Most UK local planning authorities apply the 45-degree rule: an imaginary 45-degree line drawn from the centre of your neighbour’s nearest habitable ground-floor or first-floor window must not be crossed by your two-storey extension, protecting neighbouring daylight.
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