Single Storey Extension Cost UK: 2025 Pricing Guide

Quick Answer: A typical UK single storey rear extension prices at £1,800-£2,400/m² in the North, £2,000-£2,800/m² in the Midlands, and £2,500-£3,500/m² in the South East and London (2025-2026 rates). Knocking through to the existing house typically adds £4,000-£8,000 for the structural steel and associated works. Building Regulations approval is required under the Building Regulations 2010 even where planning is exempt under permitted development.

Summary

A single storey extension is the bread-and-butter job for a UK general builder. Margins are predictable, the trades sequence is well-rehearsed, and the customer base is large. Where builders get it wrong on price is in two places: under-pricing the foundations and the knock-through, and forgetting that finishes inflate the rate per m² faster than anything else in the build.

The £/m² benchmarks above are for a "builder's finish" — shell complete, plastered, electrics first and second fix, kitchen-ready but no kitchen, decoration in mist coat. Once you add a high-spec kitchen, bifold doors instead of French doors, underfloor heating, and a vaulted ceiling with rooflights, you're rapidly into £3,500-£4,500/m² territory regardless of region. The customer rarely understands this. A clear, line-itemed quote is your defence.

This guide breaks down the standard package — foundations, superstructure, roof, openings, fit-out — with current day rates, materials cost ranges and the preliminaries and overhead allowance that make the difference between a profitable job and a break-even one. It assumes a typical 20-30m² rear extension on a traditional masonry house with a pitched roof or warm flat roof.

Key Facts

• Typical floor area — 15m² (small dining infill) to 40m² (large kitchen-diner); 20-30m² is the most common spec
• Programme — 12-16 weeks for a 25m² extension including drying time and finishes
• Building Regulations — full plans or building notice; structural calcs required for any opening >1.2m
• Planning permission — single storey rear extensions up to 4m depth (detached) or 3m (terraced/semi) under permitted development (Schedule 2 Part 1 Class A of GPDO 2015); larger Prior Approval scheme up to 8m/6m
• Foundations — typically 1.0-1.2m deep strip foundations to NHBC Chapter 4 or BS EN 1997 (Eurocode 7); £150-£300/linear m for excavation, concrete, blockwork to DPC
• Cavity walls — 102.5mm facing brick + 100mm cavity (75-100mm full-fill insulation) + 100mm aircrete inner leaf; £100-£140/m² fully built
• Roof options — pitched tiled (warmer, more headroom, more cost), warm flat roof with single-ply membrane, lantern + flat warm roof combo
• Steel beam (knock-through) — typically 152 × 152 UC or 203 × 102 UB, £600-£1,500 for the steel itself, £4,000-£8,000 all-in including padstones, propping, structural calcs, fire protection and making good
• Builder day rate — £200-£300/day; £180-£250 in lower-cost regions
• Bricklayer day rate — £200-£280/day, gang rate £400-£550/day for two-and-one
• Carpenter day rate — £220-£300/day for second fix and roofing
• Electrician — £35-£50/hour, £280-£400/day
• Plumber — £40-£60/hour, £320-£480/day; Gas Safe registered for any gas works
• Plasterer — £180-£250/day, ~30-40m² walls per day
• Roofer — £200-£280/day; tilers ~10-15m² per day on plain tiles
• Preliminaries — 10-15% (skip hire, scaffold, welfare, site protection, deliveries, equipment hire)
• Contingency — 5-10% — non-negotiable on extensions because foundations and existing fabric are unknowns
• Overhead & profit (margin) — 15-25% gross; 12-18% net after company overheads
• VAT — standard 20% rated; no reduced rate for extensions to occupied dwellings

Quick Reference Table

Detailed Guidance

Foundations — Where the Hidden Money Goes

Foundations are the most variable item on any extension job. The drawings will say "1m strip foundation" but the building inspector decides on site, and if you hit clay, made ground, tree roots, drains or a high water table the depth can double overnight. The NHBC guidance (Chapter 4 Foundations) is the practical reference — and Eurocode 7 (BS EN 1997) is the formal one — but for pricing what matters is having a realistic depth assumption and a contingency.

Typical strip foundation cost on a clear, easy-dig site: £150-£200 per linear metre at 1m deep, going to £250-£350/m at 1.5m or where you need to pump out. A 25m² extension has roughly 16-18 linear metres of foundation depending on shape, so £2,400-£6,300 just for the dig and pour, before the blockwork up to DPC.

Watch for the trigger items: trees within 30m on shrinkable clay (NHBC requires deeper, sometimes pile-and-beam at £8,000-£15,000+); existing drains crossing the line of the extension (lintel or divert at £400-£1,200); proximity to neighbours' boundary (Party Wall Act notices, possible award costs of £500-£2,500 if disputed).

The Knock-Through

The knock-through is often the most visible cost saving the customer asks about, and the easiest place for a builder to lose money. A typical knock-through between a kitchen and the new extension involves: structural engineer's calculation (£350-£600), supplying a 200-250mm deep UB or UC steel beam (£600-£1,500 supplied and delivered), padstones at each end (often concrete or grouted brick, £150-£300), Acrow props and strongboys for the temporary works (£250-£500 hire), labour for breaking out, propping, lifting the steel, building it in and making good (5-8 days for a bricklayer and labourer = £2,000-£3,500 in labour), plus fire protection if the steel is exposed (intumescent paint £200-£500 or boxing in with two layers of pink plasterboard £300-£600). All in, £4,000-£8,000 is a realistic price band. Don't quote less than £4,500 unless you've seen the wall and confirmed the steel size.

Roof Choice — Warm Flat vs Pitched

Warm flat roofs (insulation above the deck, single-ply or EPDM membrane) are now standard on extensions to meet Part L1B without sacrificing internal headroom. A typical build-up is: ply deck → vapour control layer → 120-150mm PIR insulation → EPDM or single-ply membrane → trims and gutter. Cost £180-£300/m² fully fitted. They're quick (a roofer can deck and membrane 25m² in 3-4 days) and reliable but the finished appearance is plain, and the warm flat detail at the parapet/upstand is where leaks start if poorly done.

Pitched roofs cost more (£260-£420/m²) but look better, give you a vaulted ceiling option, and tie in cleanly to the existing house roof. The structure is usually a truss or cut-and-pitch. Tile cost varies enormously by style — concrete interlocking plain at £25-£35/m², clay plain tile at £55-£85/m², slate from £45/m² up.

Glazing — Where the Customer Spends

Glazing is the single fastest way to inflate the £/m² rate. A pair of UPVC French doors costs £1,000-£1,800. A 3-pane aluminium bifold of equivalent width is £4,500-£8,500. A 4-pane sliding patio in aluminium can hit £6,000-£10,000. A roof lantern adds £1,800-£3,500. Customers want all of these; pricing them as visible line items rather than absorbing them into the £/m² rate keeps you protected and helps the customer make trade-offs.

Glazing must meet Part L1B (u-value 1.4 W/m²K for windows, 1.4 for doors as of 2021), Part K (safety glazing in critical locations, BS 6206 Class C), and Part N. All glazing in extensions over a certain area must be FENSA or CERTASS notified, or notified via building control.

Finishes — The Final 30% of the Cost

After shell and rough-in is done, you'll have spent maybe 60-65% of the budget. The final 35-40% goes on: plasterboard and plaster, floor finishes, decoration, kitchen fit (if included), and second fix carpentry (skirting, architraves, internal doors). This is where customer choices have the biggest leverage and where a clear, item-by-item quote keeps you out of trouble.

A 25m² extension with a builder's finish (plastered, electrics installed, ready for the customer's choice of kitchen and flooring) typically costs £45,000-£55,000 in the Midlands. The same extension with a £15,000 kitchen, porcelain floor, bifold doors and lantern rooflight is £75,000-£85,000. Be clear in the quote which spec you're pricing.

Frequently Asked Questions

Should I quote a fixed price or a schedule of rates?

For an extension under £80,000 with a clear spec, quote a fixed price with a clear written schedule of works and a separately listed PC (prime cost) sum for kitchen, sanitaryware and floor finishes. The PC sum protects you against customer indecision — they choose, you fit, the cost varies, and the variation is built into the contract. For jobs above £80,000 or with complex existing-fabric unknowns, consider a Schedule of Rates with a target price.

What if the foundations are deeper than expected?

Build a contingency line into the quote (£1,500-£3,000) and define it clearly as "foundations contingency — applied if dig exceeds 1.2m or encounters trees/clay/made ground requiring deeper construction". This is fair to the customer and protects you. NHBC's Chapter 4 tables are the reference for trees-on-clay foundations depths; a copy in the file gives you something to point at.

Is 15% margin enough?

It's the floor, not the target. 15% gross covers your overheads and leaves a thin net margin. 20-25% gross is healthy for a small builder. Above 25% you're at risk of being undercut by a competitor; below 15% you're working for nothing if anything goes wrong. Margin should also reflect risk — a known customer on a clear-spec job can be 15-18%; a stranger with vague specs and a difficult site should be 25-30%.

Do I need to charge VAT on the extension?

Yes — extensions to existing dwellings are standard-rated at 20% under VAT Notice 708. The reduced 5% rate applies only to conversions of properties unoccupied for 2+ years, conversions changing the number of dwellings, and certain renovations. New build dwellings are zero-rated. Extensions don't qualify for any reduction.

When does the Party Wall Act apply?

The Party Wall etc. Act 1996 applies if the extension involves: working on the line of junction with a neighbour, excavating within 3m of a neighbour's structure and below the depth of their foundations, or building on/cutting into a shared wall. Notice must be served 2 months in advance for line-of-junction work, 1 month for adjacent excavation. Notices are typically £100-£250 to serve via a surveyor; an "award" if the neighbour dissents is £750-£2,500. Always check the boundary before quoting.

Regulations & Standards

• Building Regulations 2010 — statutory framework

• Approved Document A (2013) — structure

• Approved Document B (2019) — fire safety

• Approved Document C (2004) — site preparation, moisture

• Approved Document K (2013) — protection from falling, including safety glazing

• Approved Document L1B (2021) — energy efficiency in existing dwellings

• Approved Document F (2021) — ventilation

• Approved Document M (2015) — access to and use of buildings

• Approved Document P (2013) — electrical safety

• The Town and Country Planning (General Permitted Development) (England) Order 2015 — permitted development rights for rear extensions

• NHBC Standards Chapter 4 — foundations on shrinkable clay

• BS EN 1997-1:2004 (Eurocode 7) — geotechnical design

• BS 6206 — safety glazing

• Party Wall etc. Act 1996 — neighbour notification requirements

• Approved Document L1B (2021)

• Town and Country Planning (GPDO) 2015

• NHBC Standards

• Party Wall etc. Act 1996 Explanatory Booklet (DLUHC)

• VAT Notice 708

• double storey extension pricing guide — for two-storey alternative

• garage conversion pricing guide — when the customer is comparing options

• party wall act notices — notice procedure and award costs