Two Storey Extension Cost Guide 2026
A two-storey extension costs roughly 50–60% more than a single-storey on the same footprint — not double, because you share foundations and a roof. The additional cost buys you double the floor area, making it significantly better value per square metre.
Cost breakdown
Cost per square metre
Two-storey extensions cost £1,500–£2,200 per m² of ground floor area. This effectively gives you two floors for 50–60% more than the ground floor alone. A 4m × 5m footprint (20m² ground) gives you 40m² of total space for £50,000–£80,000.
Why not double the cost?
You share the foundations (the most expensive element per m²) and the roof. The upper floor structure (joists, plasterboard, staircase) costs significantly less per m² than foundations and roofing. The windows, exterior finish, and upper-floor fit-out are the main additional costs.
Planning costs
Two-storey extensions almost always need planning permission. Budget £2,200 for our planning service plus £258 council fee. The design needs to be planning-ready from the outset — a refused application costs time and money to redesign and resubmit.
| Footprint | Total Area | Build Cost | Estimated Total |
|---|---|---|---|
| 3m × 4m | 24m² | £38,000–£55,000 | £55,000–£80,000 |
| 3m × 5m | 30m² | £45,000–£66,000 | £65,000–£95,000 |
| 4m × 5m | 40m² | £60,000–£88,000 | £85,000–£125,000 |
| 4m × 6m | 48m² | £72,000–£106,000 | £100,000–£150,000 |
Prices based on Dorset rates, 2026
Frequently asked questions
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