We use essential cookies to run this site, and optional cookies only with your consent. Cookie policy AcceptReject Open 7 days · Hours vary by day · 25 years' experience Office: 01279 216 215 | Mobile: 07932 442502 Services Areas About Our Work Blog Reviews Contact 01279 216 215 Free Quote - Home - Building & Construction - Extensions - Single vs Double-Storey Extensions: How to Choose 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 Choose single-storey if your priority is a kitchen-diner and the budget is £45,000–£100,000. Choose double-storey if you also need an extra bedroom and the budget is £80,000–£160,000 — the per-m² cost drops by 20–30% because foundations and roof costs spread across two floors. Double-storey adds significantly more property value too: typically 18–25% versus 10–15% for single-storey on the same plot. ## 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). ## Direct comparison FactorSingle-storeyDouble-storey Cost (typical, 2026)£45k–£100k£80k–£160k Cost per m²£2,200–£3,200£1,800–£2,500 Build time14–18 weeks16–22 weeks Floor area added15–35 m²40–80 m² Adds bedroom?NoYes (typically + en-suite) Property value uplift10–15%18–25% PlanningOften PDOften planning DisruptionModerateSignificant Family in residence?Usually yesOften move out final weeks Party Wall ActSometimesAlmost always ## Other things that swing the decision 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 - Written completion sign-off on every job FAQs ## Frequently asked questions ### svg]:rotate-180 text-left font-bold text-base hover:no-underline py-5" data-radix-collection-item=""> Is double-storey always better value than single-storey? ### svg]:rotate-180 text-left font-bold text-base hover:no-underline py-5" data-radix-collection-item=""> Will double-storey planning be harder to get? ### svg]:rotate-180 text-left font-bold text-base hover:no-underline py-5" data-radix-collection-item=""> Can I extend single-storey now and add a second floor later? ### svg]:rotate-180 text-left font-bold text-base hover:no-underline py-5" data-radix-collection-item=""> How much disruption does each cause? ### svg]:rotate-180 text-left font-bold text-base hover:no-underline py-5" data-radix-collection-item=""> Do I need separate quotes for each option? ### svg]:rotate-180 text-left font-bold text-base hover:no-underline py-5" data-radix-collection-item=""> What's most popular in Hertfordshire and Essex? Related Guides How much does a house extension cost? Dormer vs Velux loft conversions Back to Extensions ## Considering an extension in 2026? We'll quote both single and double-storey options where they're both viable on your plot — line-by-line comparison so you can choose with full information. Owner-led delivery, written fixed price. See our extensions service 01279 216 215 Page last updated: May 2026 · J&Co Builders Ltd · 1 Ugley Hall Cottages, Bishop's Stortford CM22 6JB Owner-led roofing and building contractors covering Bishop's Stortford, Hertfordshire and Essex. 25 years of hands-on trade experience. Open seven days. 01279 216 215 (office) 07932 442502 (mobile) info@jandco-contractorsltd.co.uk 1 Ugley Hall Cottages, Bishop's Stortford CM22 6JB Open 7 days a week - Sun 07:00–17:30 - Mon 07:00–17:00 - Tue 07:30–17:00 - Wed 07:00–17:30 - Thu 07:00–17:30 - Fri 06:00–18:00 - Sat 09:00–17:00 We accept VISAAMEX PayG Pay ### Roofing - All roofing services - Flat Roofing - Roof Repairs & Maintenance - Emergency Roof Repairs - Guttering, Fascias & Soffits - Chimney Repairs & Repointing ### Building & Construction - All building services - Building & Extensions - Loft Conversions - Renovations & Refurbishments - Brickwork & Masonry - Damp Proofing & Waterproofing - Plastering & Rendering - Patio & Driveway Installations - Fencing & Garden Walls - Property Maintenance ### Areas We Cover - Bishop's Stortford - Stansted Mountfitchet - Sawbridgeworth - Great Dunmow - Harlow - Saffron Walden - Epping - Ware - Hoddesdon - Broxbourne - Cheshunt - Welwyn Garden City - Stevenage ### Company - About J&Co - Our Recent Work - Blog - Customer Reviews - Contact Us © 2026 J&Co Builders Ltd · Trading as J&Co Contractors · All rights reserved. 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