How Much Does a Single Storey Extension Cost in the UK?

A single storey extension in the UK costs between £1,600 and £3,200 per m² of floor area. For most projects, the total cost falls between £25,000 and £80,000 depending on size, specification, and location.

Size	Budget Spec	Mid Spec	Premium Spec
15m²	£24,000 – £33,000	£33,000 – £45,000	£45,000 – £60,000
20m²	£30,000 – £42,000	£42,000 – £58,000	£58,000 – £78,000
25m²	£36,000 – £50,000	£50,000 – £70,000	£70,000 – £95,000
30m²	£42,000 – £58,000	£58,000 – £80,000	£80,000 – £110,000

All costs exclude VAT (20%) and the kitchen or other fit-out items inside the extension.

What’s Included in These Costs?

These estimates cover a standard single storey extension with:

Strip or trench foundations on normal ground
Cavity brick and block external walls
Flat roof (EPDM rubber) or pitched roof (tiles to match existing)
Double glazed UPVC windows and rear/side doors
One structural opening into the existing property (RSJ beam)
Internal insulation and plasterboard
Plastered and ready-to-decorate walls and ceilings
First and second fix electrics
Extension to central heating (radiators)
Concrete floor slab with screed
Scaffold and waste removal

Key Cost Factors

Roof type: flat vs pitched

A flat roof is typically 15–20% cheaper than a pitched roof. However, a pitched roof matching the existing house often looks more cohesive and adds more value. Flat roofs have improved significantly with modern EPDM rubber systems — a good EPDM flat roof should last 20–25 years.

Ground conditions

Standard strip foundations assume normal bearing ground. If ground investigation reveals soft spots, clay shrinkage zones (common under trees in London clay), or poor fill, engineered foundations (piles, reinforced raft) can add £3,000–£20,000.

Wall construction

Brick and block cavity walls are the standard and most cost-effective. Timber frame construction is faster to build but typically costs the same or slightly more for single-storey work. Structural Insulated Panels (SIPs) are faster still and highly insulated but carry a premium.

Opening size into the original house

A single standard RSJ beam opening (up to 3.5m) is included in most base estimates. Wider openings, steel frames, or multiple openings add cost. A wide open-plan opening using a steel portal frame can add £5,000–£15,000.

Specification upgrades

Common items that push costs up:

Aluminium bifolds or sliding doors: +£4,000–£8,000 per set
Roof lantern: +£2,500–£5,500
Underfloor heating (electric): +£800–£1,500
Underfloor heating (wet): +£2,000–£4,500
Premium tiles: +£1,500–£4,000 on materials alone

Typical Cost Breakdown: 22m² Single Storey Extension

For a mid-specification single storey rear extension at national average rates:

Trade / Cost Category	Typical Cost
Demolition and enabling works	£1,200 – £2,000
Excavation and groundworks	£2,500 – £4,500
Foundations (concrete strip)	£2,500 – £4,500
External walls (cavity brick/block)	£5,500 – £9,000
Structural steelwork (one beam)	£2,000 – £3,500
Flat roof structure (timber)	£2,500 – £4,000
EPDM flat roof covering	£2,000 – £3,500
External doors and windows (UPVC)	£2,500 – £5,000
Internal insulation and plasterboard	£2,000 – £3,500
Plastering	£1,500 – £2,500
Electrical (first + second fix)	£2,500 – £4,000
Plumbing and heating extension	£1,800 – £3,500
Floor screed / tiling (basic)	£1,500 – £3,000
Internal decoration	£1,500 – £2,500
Scaffold	£1,500 – £2,500
Skip and waste disposal	£500 – £900
TOTAL (approx)	£34,000 – £58,000

Regional Costs Per m²

Region	Cost Per m² (mid-spec)
Inner London	£2,600 – £4,000
Outer London	£2,100 – £3,000
South East	£1,900 – £2,700
South West	£1,800 – £2,600
East of England	£1,800 – £2,500
West Midlands	£1,700 – £2,400
East Midlands	£1,650 – £2,300
North West	£1,600 – £2,200
Yorkshire	£1,550 – £2,200
North East	£1,500 – £2,100
Scotland	£1,650 – £2,400
Wales	£1,550 – £2,200

How to Price a Single Storey Extension

Getting an accurate quote for a single storey extension the traditional way means a site visit, careful measurements, supplier quotes for every material, and building up each trade from scratch. Done properly, this takes 4–8 hours per estimate.

PricingPro does it in minutes. Describe the job (“22m² single-storey rear extension, flat roof, brick/block, Birmingham, mid-spec”) or upload your drawings, and the AI produces a fully itemised estimate using live prices from 100+ UK suppliers. Every trade, every material — including waste factors and regional price adjustments.

Generate a single storey extension estimate →

FAQs

What is permitted development for a single storey extension? Under permitted development rights (no planning permission required), a single storey rear extension can be up to 4m deep on a detached house or 3m deep on a semi-detached or terraced house — and no higher than 4m at the ridge or 3m at the eaves. Side extensions, larger rear extensions, and extensions on flats require planning permission.

What Building Regulations apply to a single storey extension? All extensions require Building Regulations approval, which covers structural integrity, thermal performance (insulation), fire safety, drainage, and ventilation. Your local authority building control or an approved inspector will inspect at key stages and issue a completion certificate.

How long does a single storey extension take? Typically 12–18 weeks from breaking ground to handover, assuming normal weather and no significant groundwork surprises. Planning permission (if required) typically adds 8–12 weeks before work can start.

Should I get more than one quote? Yes — for a project of this size (£35,000–£75,000), getting 2–3 quotes is sensible. Be wary of quotes significantly below the others — they often indicate items have been missed or assumed as client supply.

What is the best time of year to build an extension? Spring and early summer are ideal — long days and lower risk of frost or rain delays. Starting in autumn is common but winter building slows groundwork and roofing. Most good contractors are booked 3–6 months ahead, so plan accordingly.

Single Storey Extension Cost UK 2026: Accurate Pricing Guide