Single vs Double Storey Extension: 2026 UK Cost Comparison
Single storey £30,000–£60,000 · Double storey £55,000–£110,000. Double-storey is cheaper per m² (£1,800–£2,600/m² vs £2,000–£3,000/m²) but harder to get planning. Side-by-side cost, ROI, planning and build-time breakdown for 2026 UK homes.
Planning the work? Check what it usually costs, or work out your own estimate. The free quote form is at the top of this page.
Planning differs significantly between the two
Most single-storey rear extensions are Permitted Development. Double-storey rear extensions almost always need full planning permission — this single rule shifts whole projects from 8 to 16+ weeks before site start.
Read the extension planning rules →Single vs Double Storey: Direct Comparison
All figures from BestBuilders 2026 UK pricing data, averaged across 519 towns.
| Factor | Single storey | Double storey |
|---|---|---|
| Total cost (20 m² footprint) | £40,000 – £60,000 | £70,000 – £104,000 |
| Cost per m² | £2,000 – £3,000 | £1,800 – £2,600 (cheaper) |
| Floor area added (20 m² footprint) | 20 m² | 40 m² (double) |
| Build time on site | 12 – 16 weeks | 18 – 26 weeks |
| Planning | Often Permitted Development | Almost always full planning |
| Disruption | Lower — ground floor only | Higher — affects bedrooms above |
| Garden lost | Same as double | Same as single (no extra) |
| Typical ROI | £1.10 – £1.40 / £1 | £1.30 – £1.70 / £1 |
| Resale uplift | Better kitchen / dining | Extra bedroom + bathroom |
Which Extension Should You Choose?
Single-storey wins when…
- You want a kitchen-diner extension (most common UK use)
- Speed matters (PD route shaves 8–12 weeks before start)
- Budget is the dominant constraint
- You're in an Article 4 area where double-storey planning is unlikely
- Roof works above would damage existing dormer or roof structure
Double-storey wins when…
- You need bedrooms (each bedroom typically adds 8–12% to value)
- You want best £/m² — extra storey is cheap because foundations and roof are shared
- Garden depth limits footprint expansion
- You're prepared for full planning timeline
- Long-term family home rather than 5-year project
Differences Beyond the Headline Price
Both extensions share most hidden costs — but a few are materially different.
📋 Planning fees & timeline
Single-storey under PD: £129 LDC and 8 weeks. Double-storey full planning: £258 fee and 12–16 weeks before site start.
🏗 Foundations
Same foundation work in both — you're reusing them on a double-storey, hence the better £/m². The structural margin needed for double-storey often pushes foundations slightly deeper.
🏛 Party wall surveyors
Both trigger PWA notices. Double-storey almost always means dissents on terraces and semis as the second storey impacts a much larger area of the neighbour wall.
First floor disruption
Double-storey requires opening the existing first floor for plumbing/electrics. Decoration of existing rooms is rarely in the quote and adds materially to total spend.
☕ Temporary kitchen / bathroom
Most homeowners stay in the property. Single-storey usually allows kitchen-in-place; double-storey often requires temporary kitchen + bathroom for 4–6 weeks.
📊 Contingency (always)
10% on a single-storey, 12–15% on a double-storey. Larger projects = more surprises.
Compare Extension Costs by Town
Each town page shows local extension specialists, town-specific cost ranges and free quote requests.