What Does a Rear Extension Cost?

Quick Answer

Single storey typically £1,500–£2,200 per m² (2026 UK)

Full spread across guides and specs £1,000–£3,000/m². A 20m² extension at typical rates: £30,000–£44,000 before fees and fit-out

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Extension Costs per m² by Type (2026 UK)

The table collates the major published 2025–2026 UK guides (MyJobQuote June 2026, Checkatrade 2026, HomeHow July 2025). Rates are build cost per m² of new floor area including labour and materials, excluding professional fees, planning and kitchen or bathroom fit-out. Where the guides disagree, the range shows the full spread.

UK extension build cost per square metre by type and spec, 2026
Extension type	Guide rate per m²	Notes
Single storey, basic spec	£1,000–£1,700	MyJobQuote’s basic-finish band (June 2026); standard off-the-shelf fixtures throughout
Single storey, good spec	£1,500–£2,200	The typical band: MyJobQuote £1,700–£1,900, HomeHow £1,500–£2,500, Checkatrade £1,800–£3,000
Single storey, high spec	£1,900–£3,000	Glazed gables, bi-folds and premium finishes; largely glazed extensions run £2,000–£4,500/m², typically around £3,000
Double storey	£950–£2,200	Cheaper per m² than single storey because foundations and roof are shared; typical good-spec band £1,500–£1,850
Side return	£1,500–£2,400	Guides disagree on totals: MyJobQuote quotes £15,000–£25,000 while Checkatrade’s average project is £30,000–£65,000, with London rates to £4,500/m²
Wraparound	£1,200–£3,300	Guides disagree widely: quality-tier guides run £1,200–£2,000+ per m² while London-focused firms quote £2,400–£3,300 nationally and up to £4,500 in London. MyJobQuote’s project band is £40,000–£100,000

Worked Examples at Typical Rates

Multiplying the typical good-spec single-storey band (£1,500–£2,200 per m²) gives realistic starting budgets. These are build costs only:

Worked example extension budgets at typical 2026 rates
Size	Build cost at typical rates
3m × 4m single storey (12m²)	£18,000–£26,400
4m × 5m single storey (20m²)	£30,000–£44,000
6m × 4m single storey (24m²)	£36,000–£52,800
4m × 5m double storey (40m² total)	£60,000–£74,000 (at £1,500–£1,850/m²)

MyJobQuote’s own standard-build totals corroborate these: around £52,000 for a typical single storey and £82,500 for a double storey, with side returns from £20,000. HomeHow prices a small 15m² single storey at £22,500–£37,500. Kitchen fit-out is always extra.

Professional Fees and Extras

UK extension professional fee guide prices, 2026
Item	Guide price	Notes
Architect / design package	£1,500–£5,000	MyJobQuote budgets £3,000–£5,000 including surveys and approvals; drawing-only packages run £1,500–£2,000
Structural engineer	£400–£2,500	A full design pack for a straightforward single storey runs £550–£1,400, side returns and wraparounds £1,000–£1,600, and double storey schemes £1,400–£2,500. Size the steel first with the extension steel calculator
Planning application, if needed	£548	England householder fee from 1 April 2026, index-linked annually; many rear extensions are permitted development
Building regulations (full plans + inspections)	£580–£950	Set by each council or approved inspector; single-storey full-plans applications typically £250–£550 plus £330–£400 of inspections (the range is the sum of those two bands)
Party wall agreement	£700–£2,700	Per agreement across guides (about £1,000 with a single agreed surveyor); check if notice is needed with the extension party wall calculator
Drainage surveys and works	£250 survey; works from £200	MyJobQuote’s bands; building over or diverting a public sewer needs a build-over agreement and can cost thousands, so check early with the extension drainage calculator

What Affects the Price

Specification: glazing, bi-fold doors, underfloor heating and kitchen fit-out move the total more than the shell does
Size: bigger extensions cost less per m² but more in total
Ground conditions: clay, trees and poor ground mean deeper foundations; scope them with the extension foundation calculator
Storeys: a double storey shares its groundwork and roof, so it is the cheapest way to buy floor area per m²
Access: tight terrace access slows everything and raises labour
Drainage: moving or building over sewers adds cost and lead time

London and the South East

MyJobQuote applies a 10–25% labour premium in London and the South East, and Checkatrade’s side-return rates run to £4,500 per m² in London against £1,500–£2,400 elsewhere. In London, treat the top of each band in these tables as the realistic starting point.

Permitted Development

Single-storey rear extensions up to 4m deep (detached) or 3m (semi and terrace) are usually permitted development, extendable to 8m and 6m under the larger-home prior approval route (£249 fee). Check your limits with the extension PD calculator or see what size extension you can build under PD.

Once you have a budget, price the quantities with our extension materials calculators for every build stage.

Important Notes

All figures are guide prices from published 2025–2026 UK cost guides, not quotes: get at least three itemised quotes
Confirm what each quote includes; kitchen fit-out, floor finishes and VAT are the usual gaps
Timescales: MyJobQuote quotes 9–14 weeks on site for most single storey types and 16–19 for double storey; HomeHow quotes 2–3 and 4–6 months, so agree a written programme

Figures cross-checked against published UK cost guides July 2026. Last updated: July 2026