Comparison Guide · Updated 2026 · Real UK Data

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.

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Real DB pricing across 519 towns
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Updated 2026
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Side-by-Side Calculator · 2026

Compare Single vs Double Storey for Your Home

Pick footprint and region — see both extension costs side-by-side instantly.

Single storey — 2026 estimate
£40,000 – £60,000
Cheaper, faster, easier planning
Double storey — 2026 estimate
£70,000 – £104,000
Better £/m², double the floor area
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Standard 20 m² · Standard spec · UK average
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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 →
Side-by-Side

Single vs Double Storey: Direct Comparison

All figures from BestBuilders 2026 UK pricing data, averaged across 519 towns.

FactorSingle storeyDouble 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 m²40 m² (double)
Build time on site12 – 16 weeks18 – 26 weeks
PlanningOften Permitted DevelopmentAlmost always full planning
DisruptionLower — ground floor onlyHigher — affects bedrooms above
Garden lostSame as doubleSame as single (no extra)
Typical ROI£1.10 – £1.40 / £1£1.30 – £1.70 / £1
Resale upliftBetter kitchen / diningExtra bedroom + bathroom
When To Choose

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
Hidden Costs

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.

Double-storey adds: 8–12 weeks pre-build

🏗 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.

Double-storey adds: £1,500 – £4,000

🏛 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.

Double-storey adds: £2,000 – £6,000

🛘 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.

Double-storey adds: £2,500 – £6,000

☕ 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.

Double-storey adds: £800 – £2,500

📊 Contingency (always)

10% on a single-storey, 12–15% on a double-storey. Larger projects = more surprises.

Both: 10–15% of total
Local Extension Costs

Compare Extension Costs by Town

Each town page shows local extension specialists, town-specific cost ranges and free quote requests.

LondonManchesterBirminghamLeedsLiverpoolBristolGlasgowEdinburghCardiffNottinghamBrightonReadingCambridgeOxfordLeicesterSouthamptonPortsmouthYorkBathMilton KeynesWatfordLuton

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Builders will quote single AND double-storey for the same footprint, side-by-side. BestBuilders matches you with up to 3 vetted local extension specialists, free.

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Frequently Asked Questions

Single vs Double Storey FAQs

Significantly. A 20 m² footprint single storey costs about £45,000 average; the same footprint double-storey adds 40 m² of floor area for around £85,000. Per-m² you save 15–20% by going double on the same plan, because foundations, roof, party wall and overhead costs are shared.
Double-storey typically adds more in cash terms because the second storey usually creates a bedroom — each extra bedroom adds 8–12% to UK property values in 2026. Single-storey generally returns better in percentage of project cost, particularly when it creates an open-plan kitchen-diner.
Single-storey rear extensions usually fall under Permitted Development if up to 4m on detached homes, 3m on others (or up to 6m/8m via the Larger Home Extension prior approval scheme). Double-storey rear extensions are limited to 3m projection, must be 7m from the rear boundary and must not exceed the existing roof height — conditions almost no double-storey design meets, so full planning is required in nearly all cases.
Single-storey: 4–6 months total (1–2 months design + planning, 12–16 weeks on site). Double-storey: 7–10 months (3–4 months design + planning, 18–26 weeks on site). The pre-build phase is usually where double-storey loses time — the actual additional build time is only about 6–8 weeks for the second storey.
Most homeowners do, but expect significant disruption. Bedrooms above the working floor become unusable for 6–10 weeks during structural work. Many families use the school holidays as a strategic move-out point or rent locally for 4–8 weeks. Single-storey is much easier to live through.
No — it's usually 30–40% more expensive overall. You pay for the same foundations and roof twice, plus a second mobilisation, party wall round and design fee cycle. If you can see yourself wanting the second storey within 5 years, build it now.