Guide · Extensions

How Much Does a House Extension Cost in 2026?

Extensions in 2026 typically cost £2,200–£3,200 per m² for a standard build, with kitchen-extension fitouts pushing that higher. Here are the honest 2026 figures we quote on real Hertfordshire and Essex jobs.

Last updated: May 2026 · By Jamie Pocock

Quick Answer

Cost by extension type (2026)

Single-storey rear extension (£45,000–£100,000)

The most common extension type — open-plan kitchen-diner schemes added to the back of the house. Typical floor areas 15–35m². Standard build £2,200–£3,000/m²; with a fitted kitchen, bi-folds and premium flooring £3,000–£4,500/m² total. 14–18 weeks build time.

Side return extension (£55,000–£90,000)

Extension into the unused side passage of a Victorian or Edwardian terrace, typically widening a galley kitchen into a full kitchen-diner. Floor areas usually 8–15m² but the impact on layout is huge. Costs higher per m² because of party-wall complexity and structural openings. 12–16 weeks.

Double-storey extension (£80,000–£160,000)

Two-floor extension giving extra ground-floor living space plus an additional bedroom (often with en-suite) above. Typical floor areas 20–40m² per floor. Better cost per m² than single-storey because foundations and roof costs spread across two floors. 16–22 weeks.

Wrap-around extension (£100,000–£200,000+)

Combines side return and rear extension into a single L-shape. Typical floor areas 30–50m². Most space-creating option short of a full rebuild. Often combined with internal layout reconfiguration. 18–24 weeks.

Full scope detail in our extensions service page.

What's normally included in those figures

• Structural works (foundations, steel beams, openings, roof structure)
• Brickwork to match existing (or rendered finish)
• Roof — pitched tile or flat (EPDM/GRP), insulation to current Part L
• Windows and external doors (standard UPVC or aluminium specs)
• 1st and 2nd fix carpentry, electrics, plumbing, heating extension
• Plastering ready for decoration (mist coat included)
• Internal doors, skirting, architrave
• Building Control submission, inspections and final sign-off
• Insurance-backed workmanship guarantee

What's normally NOT included

• Architect and structural engineer fees (typically £4,000–£10,000 separate)
• Planning permission application fee (£258) and possible drawings revisions
• Party Wall Act notices and surveyor (£1,200–£3,500 if required)
• Kitchen cabinets, worktops and appliances (separate trade — £8,000–£40,000+)
• Bathroom suites and tiling (typically £3,500–£12,000)
• Floor finishes (engineered wood, tile, LVT) — typically £40–£120/m²
• Final decoration (paint and wallpaper)
• Bi-fold or sliding doors above standard spec (£3,500–£15,000+ uplift)
• VAT (most domestic extensions are standard rate 20%)

Spec level	£/m² (2026)	What you typically get
Standard	£2,200–£2,800	Brick or render, UPVC windows, pitched tile or flat EPDM roof, mid-spec internal finish.
Mid-range	£2,800–£3,500	Aluminium windows, lantern rooflight, underfloor heating, some bi-fold doors.
High spec	£3,500–£4,500	Premium aluminium glazing throughout, structural glass, designer kitchen package, full LED/smart-home wiring.
Luxury	£4,500+	Bespoke joinery, large-format porcelain flooring, premium worktops, whole-home integration with extension.

Most Hertfordshire and Essex domestic extensions land in the standard-to-mid-range bracket. The single biggest cost variable is glazing — bi-fold and sliding doors at 4m+ widths can swing the project total by £15,000–£25,000.

Architect and engineer. Most builders quote the build, not the design. Architect £3,000–£7,000 typical residential. Structural engineer £1,000–£3,000 typical. Add up early — these are real costs paid before the build starts.

Planning vs PD. Many single-storey rear extensions fall under Permitted Development. Larger extensions, double-storey work and conservation-area properties need full planning permission (£258 application fee, 8–12 week determination, possible drawings revisions). Confirm with your council before quoting.

Party Wall Act. Any work within 6m of a neighbour's foundations or affecting a shared wall triggers Party Wall notices. Surveyor fees typically £1,200–£3,500 if your neighbour appoints their own. Build this into the timeline — notices need 2 months to expire.

Kitchen and bathroom packages. Often the largest single cost outside the build itself. Mid-spec fitted kitchen £12,000–£25,000. Premium kitchen £25,000–£60,000+. Plan early — kitchen design influences services layout in the extension structure.

Living arrangements. Single-storey rear extensions usually allow you to stay in residence; the kitchen is normally out for 4–6 weeks during fitout. Double-storey and wrap-around extensions are more disruptive — some clients move out for the final 4–6 weeks.

Overspend buffer. Always carry a 10–15% contingency above the build quote. Variations always emerge — even on tight, well-managed projects. Build it into the budget rather than borrowing emergency funds at month 4.

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