How much does a house extension cost?

A single-storey rear extension costs £1,400–£2,200 per m² (or £30,000–£65,000 for a typical 20–30m² extension). A two-storey extension costs £1,200–£1,800 per m² (cheaper per m² because the roof and foundations are shared). A side return extension costs £25,000–£50,000, and a wrap-around extension costs £40,000–£80,000+. Costs vary significantly by region — London and the South East are 20–50% more expensive. These figures include building costs but typically exclude fees, kitchen/bathroom fit-out, and landscaping.

Do I need planning permission for an extension?

Many extensions can be built under permitted development rights without planning permission, as long as they meet certain size, height, and position criteria. Single-storey rear extensions up to 4m (detached) or 3m (terraced/semi) can qualify, or up to 8m/6m under prior approval. Two-storey extensions, side extensions visible from the road, and extensions to listed buildings or in conservation areas typically need planning permission. Always check with your local planning authority before starting work.

How long does a house extension take to build?

A single-storey extension typically takes 8–14 weeks from breaking ground to completion. A two-storey extension takes 12–20 weeks. Add 4–12 weeks before building starts for design, planning permission (if needed), building regulations, and party wall agreements. The total process from initial design to moving in is usually 6–12 months. Weather, supply chain delays, and complexity can all extend the timeline.

Does an extension add value to a house?

A well-designed extension typically adds more value than it costs — but this depends on location, quality, and what you add. Adding a bedroom or bathroom has the highest impact. A single-storey kitchen-diner extension typically adds 5–10% to a property's value, a two-storey extension 10–20%, and an extra bedroom 10–15%. However, over-extending (making your home the most expensive on the street) can reduce the return. Get an estate agent's opinion before committing.

House Extension Costs UK 2026 — Prices by Type & Size | PocketWise

An extension can transform your home and add significant value. Here’s what it costs in 2026.

Costs by Extension Type

Extension type	Cost per m²	Typical size	Total cost
Single-storey rear	£1,400–£2,200	15–30m²	£25,000–£65,000
Two-storey rear	£1,200–£1,800	30–60m²	£40,000–£100,000
Side return (terraced house)	£1,500–£2,500	10–20m²	£25,000–£50,000
Wrap-around (rear + side)	£1,400–£2,200	25–40m²	£40,000–£80,000
Over-garage	£1,200–£1,800	15–25m²	£20,000–£40,000
Kitchen extension	£1,500–£2,500	15–30m²	£30,000–£75,000 (incl. kitchen)
Conservatory	£800–£1,500	10–20m²	£10,000–£30,000
Orangery	£1,500–£2,500	15–25m²	£25,000–£60,000

Cost Breakdown — Typical 20m² Single-Storey Extension

Item	Cost
Foundations	£3,000–£6,000
Walls	£4,000–£8,000
Roof	£3,000–£6,000
Windows and doors (bi-fold/sliding)	£3,000–£8,000
Flooring	£1,000–£3,000
Plastering and decoration	£1,500–£3,000
Electrics	£1,500–£3,000
Plumbing (if kitchen/bathroom)	£2,000–£5,000
Heating (radiators, underfloor)	£1,000–£3,000
Building Regulations	£400–£1,200
Structural engineer	£400–£1,000
Architect/designer	£2,000–£5,000 (8–12% of build cost)
Party wall surveyor (if applicable)	£700–£1,500 per neighbour
Total (building only)	£25,000–£50,000

Regional Price Variations

Region	Cost per m² (single storey)	Multiplier
London	£2,000–£3,500	1.3–1.5×
South East	£1,600–£2,500	1.1–1.3×
South West	£1,400–£2,000	1.0–1.1×
Midlands	£1,300–£1,800	1.0× (baseline)
North of England	£1,200–£1,700	0.9–1.0×
Scotland	£1,200–£1,800	0.9–1.0×
Wales	£1,200–£1,700	0.9–1.0×

Professional Fees

Fee	Typical cost
Architect	8–12% of build cost (or £2,000–£8,000 fixed)
Architectural technician	£1,500–£4,000
Structural engineer	£400–£1,500
Planning application (if needed)	£258 (householder, England)
Building Control fees	£400–£1,200
Party wall surveyor	£700–£1,500 per adjoining property
Quantity surveyor (cost management)	£500–£2,000

Planning Permission

Permitted Development (No Planning Needed)

Rule	Single storey	Two storey
Max depth (detached)	4m (or 8m with prior approval)	3m
Max depth (semi/terrace)	3m (or 6m with prior approval)	3m
Max height	4m (within 2m of boundary)	Eaves no higher than existing
Max eaves height	3m (within 2m of boundary)	Must be same as existing house
Side extensions	Half the width of the original house	Not permitted development if facing a highway
Coverage	Extension + outbuildings must not cover more than 50% of the garden

When You NEED Planning Permission

Situation	Why
Exceeds permitted development size limits	Too large
Listed building	Need listed building consent
Conservation area	Additional restrictions apply
Article 4 direction	Local council has removed PD rights
Flat or maisonette	PD rights generally more limited
Two-storey side extension visible from road	Usually needs planning permission

Building Regulations

Almost all extensions need Building Regs approval for:
Structural safety (foundations, walls, roof)
Fire safety (escape routes, fire-resistant materials)
Energy efficiency (insulation, glazing)
Ventilation
Drainage
Electrical safety
Accessibility

Timeline

Phase	Duration
Design and drawings	2–6 weeks
Planning permission (if needed)	8 weeks (standard householder application)
Building Regulations submission	2–5 weeks
Party wall notices (if applicable)	1–3 months
Getting quotes and appointing a builder	2–6 weeks
Building work — single storey	8–14 weeks
Building work — two storey	12–20 weeks
Total (design to completion)	6–12 months

Value Added

Extension type	Typical value added	Cost vs value
Extra bedroom (two storey)	10–15% of property value	Usually profitable
Kitchen-diner extension	5–10%	Usually profitable
Two-storey rear	10–20%	Often profitable
Loft conversion	10–20%	Usually the best ROI
Conservatory	3–5%	Rarely recoups full cost
Garage conversion	10–15%	Good ROI

Budget-Saving Tips

Tip	Potential saving
Stay within permitted development (no planning fee)	£258+
Use a building designer instead of an architect	30–50% on design fees
Get at least 3 quotes from builders	Can save 10–20%
Avoid bespoke — use standard window sizes	£1,000–£3,000
Project-manage yourself (if experienced)	10–15% of build cost
Build in autumn/winter (quieter period for builders)	Sometimes better pricing
Choose standard roof tiles to match existing	Saves vs bespoke materials