Guide · Extensions

Single vs Double-Storey Extensions: How to Choose

Double-storey extensions cost more in absolute terms but cheaper per m² and add more value. Single-storey is faster, less disruptive, and often achievable under Permitted Development. Here's the honest comparison.

Last updated: May 2026 · By Jamie Pocock

Quick Answer

What each one delivers

Single-storey rear extension

One additional ground-floor room — almost always used as kitchen, dining and informal living combined into an open-plan space. Typical floor area 15–35m². Roof is usually flat (EPDM, GRP) or pitched with rooflights. Stays connected to the main house via knock-throughs from the existing kitchen and dining areas.

Double-storey extension

Two floors of additional space — ground floor as per single-storey above, plus an additional bedroom (typically with en-suite) on the first floor. Floor area usually 20–40m² per level, total 40–80m² added. Roof typically pitched to match the existing house. Significant impact on rear elevation appearance.

Cost comparison — the per-m² advantage

Single-storey extension: £2,200–£3,200/m² typical for standard build. Double-storey: £1,800–£2,500/m² typical because the foundations, roof, scaffold and project mobilisation costs are shared across twice the floor area.

For example, on the same plot a 25m² single-storey costs £55,000–£80,000 (~£2,500/m²); a 50m² double-storey costs £100,000–£140,000 (~£2,200/m²) — twice the floor area for less than twice the cost.

Full pricing detail in our extension cost guide.

Property value uplift

Single-storey extensions in Hertfordshire/Essex typically add 10–15% to property value. Double-storey extensions typically add 18–25%. The extra bedroom changes the home's "category" on the property market — a 3-bed becomes a 4-bed, which moves it into a different price bracket entirely. The value uplift on going from 4 to 5 beds is usually less dramatic than 3 to 4, so consider where your extension lands you in the local property hierarchy.

Planning permission

Single-storey rear extensions often fall under Permitted Development (4m projection on detached, 3m on semi/terrace; 4m height; volume limits apply). Larger Prior Approval extensions to 8m projection are also possible.

Double-storey extensions have stricter PD limits — 3m projection maximum, must not exceed existing roof line, 7m minimum from rear boundary. Many double-storey extensions exceed PD and need full planning permission (£258 fee, 8–12 week determination).

Factor	Single-storey	Double-storey
Cost (typical, 2026)	£45k–£100k	£80k–£160k
Cost per m²	£2,200–£3,200	£1,800–£2,500
Build time	14–18 weeks	16–22 weeks
Floor area added	15–35 m²	40–80 m²
Adds bedroom?	No	Yes (typically + en-suite)
Property value uplift	10–15%	18–25%
Planning	Often PD	Often planning
Disruption	Moderate	Significant
Family in residence?	Usually yes	Often move out final weeks
Party Wall Act	Sometimes	Almost always

Garden footprint loss. Both eat into garden space, but the same square-metres-of-extension applied to a double-storey loses half the garden of an equivalent single-storey. If garden space matters, double-storey gives more house per garden lost.

Rear elevation appearance. Double-storey extensions visibly change the back of the house. Single-storey extensions tuck under the existing first-floor windows. Conservation areas and properties on prominent street corners often face more planning scrutiny on double-storey schemes.

First-floor layout impact. Double-storey extensions usually require reconfiguring the existing first-floor layout — adding the new bedroom often involves a small landing extension, sometimes moving an existing bedroom door. Architect attention here is critical.

Loft conversion alternative. If your goal is "extra bedroom plus better kitchen", consider single-storey extension plus loft conversion rather than double-storey. Cost can be similar (£90,000–£150,000 combined) and you get three space gains rather than two: ground-floor kitchen, loft bedroom, plus the original first-floor bedroom kept intact.

Side return as alternative on terraces. On Victorian and Edwardian terraces, side returns deliver kitchen-extension benefits at lower cost than full single-storey rear extensions. Worth comparing all three: side return, single-storey rear, side return + rear (wrap-around).

Mortgage and re-mortgage. Double-storey extensions adding a bedroom typically support stronger re-mortgage valuations because the property moves into a higher local price bracket. Discuss with your lender or broker before committing — releasing equity can fund a meaningful share of the cost.

About the Author

Jamie Pocock — Owner & Lead Contractor

Jamie has 25 years' hands-on experience in roofing and building across Hertfordshire and Essex. He runs every J&Co Contractors project personally — from quote to completion — and writes these guides from real on-the-tools knowledge of what works, what doesn't, and what costs what in 2026.

• 25 years' hands-on roofing and building experience
• CRC Certified Roofer — self-certifies under Building Regs
• SafeContractor approved
• Insurance-backed workmanship guarantee on every job