Single Storey vs Two Storey Extension: Which Is Right?
A two-storey extension costs 50–60% more than single-storey but gives you double the space. The decision depends on what space you need, your planning constraints, and your budget.
The details
Cost per square metre
Single-storey: £1,800–£2,800/m². Two-storey: £1,500–£2,200/m² of ground floor area (effectively getting the upper floor at a 40% discount). For the same budget, a two-storey gives significantly more total space.
Planning differences
Single-storey rear extensions usually fall under PD. Two-storey extensions almost always need planning permission. This adds 8–12 weeks to the timeline, plus the council's application fee (set nationally and revised each year — the Planning Portal lists the current fees) and our fee, quoted after a free project review. Design constraints are also tighter — overlooking, daylight, and bulk all receive more scrutiny.
| Factor | Single Storey | Two Storey |
|---|---|---|
| Cost (4m × 5m footprint) | £55,000–£80,000 | £85,000–£125,000 |
| Total new space | 20m² | 40m² |
| Cost per m² | £2,750–£4,000 | £2,125–£3,125 |
| Planning | Usually PD | Usually needs permission |
| Build time | 10–16 weeks | 14–20 weeks |
| Neighbour impact | Minimal | More significant |
Our verdict
Choose single-storey if you only need ground-floor space (kitchen-diner) and want to avoid planning. Choose two-storey if you need both ground and upper floor space (kitchen-diner + bedroom) — the cost per m² is significantly better.
Common questions
Frequently asked questions
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