Extension costs are one of the most searched and most misleadingly quoted figures in UK home improvement. A builder’s quote of “around £2,000 a square metre” can be meaningless without knowing whether that includes VAT, professional fees, kitchens, or even the glazing. This guide unpacks the real 2026 figures, by extension type, specification level, and region, so you can build a realistic budget before approaching contractors.

What the Per-m² Rate Actually Covers

A credible per-m² build cost for an extension should include: demolition and strip-out, groundworks and foundations, structural frame, external walls, roof, windows and external doors, first-fix services (electrics, plumbing, heating), internal walls, insulation, first and second fix joinery, plaster, and decoration. It typically excludes VAT (20%), professional fees (architect, structural engineer, planning), kitchen or bathroom fit-out, furniture, landscaping, and Party Wall surveyor costs.1

Always clarify scope when comparing quotes.

Cost per m² by Extension Type (2026)

Extension typeStandard spec (£/m²)Good spec (£/m²)High spec (£/m²)Notes
Single-storey rear (cavity wall)£1,800–£2,200£2,200–£2,800£2,800–£3,800Most common; straightforward groundworks
Single-storey rear (structural glazing / aluminium)£2,400–£3,000£3,000–£3,800£3,800–£5,000Steel frame, bifolds or sliders, complex roof
Two-storey rear£1,600–£2,000£2,000–£2,600£2,600–£3,400Lower per m² as foundation/roof costs spread
Side return (infill)£2,200–£2,800£2,800–£3,600£3,600–£5,000Often requires structural steelwork; London common
Wrap-around (rear + side)£2,000–£2,600£2,600–£3,200£3,200–£4,500Complex geometry; single contract
Over-garage / above existing£1,800–£2,400£2,400–£3,000£3,000–£4,200Must check existing structure; often lighter construction
Basement extension£3,500–£5,000£5,000–£6,500£6,500–£9,000+Tanking, underpinning, pumped drainage

All figures exclude VAT and professional fees. London adds roughly 15–30% to build costs (outer to inner London) and the South East and East about 10–15%; Wales and the North East may be 5–10% below the midpoint figures, while Yorkshire and the North West sit within about 5% of them (see the regional table below).

For comparison, the HomeOwners Alliance puts a single-storey extension in London at £2,300–£3,400 per m², against a UK average of £1,800–£3,000 per m².2

Worked Example: 25 m² Single-Storey Rear Extension

A typical 5 m × 5 m rear extension in the Midlands, standard specification:

ItemCost
Build cost (25 m² × £2,000/m²)£50,000
VAT @ 20%£10,000
Architect (design + contract admin, ~10% of build cost)£5,000
Structural engineer£900
Planning application fee (England, 2026)£5483
Building Regs application (Full Plans)£830
Party Wall surveyor (if required)£1,200–£1,500
Total£68,478–£68,778 (about £68,500)

In London, with build costs 15–30% higher and the same fixed fees, the same project would land at roughly £78,000–£89,000 including VAT and fees, and more where London professional fees are higher.

What Drives the Per-m² Cost Up

1. Structural complexity. Any extension requiring a steel RSJ or flitch beam to open up the rear elevation, typically needed when merging kitchen and dining space, adds £2,000–£8,000 in steel supply, installation, and temporary propping, which is hidden in the per-m² figure.

2. Groundwork conditions. Poor ground (made ground, clay soil near trees, which can push foundations deeper or onto piles, high water table) can add £5,000–£25,000+ depending on severity. A ground investigation report (£500–£1,500) is wise on any site with uncertainty.

3. Roof type. A pitched tiled roof costs less than a flat roof with standing-seam zinc or aluminium, and far less than a frameless glazed lantern roof. Allow an extra £200–£400/m² for premium roof finishes.

4. Glazing. Bifold or sliding door sets in aluminium cost £800–£2,500 per running metre depending on width and specification; a fully glazed wall with roof lights can add £15,000–£30,000 to a project.

5. Utility diversions. Drainage rerouting, gas pipe diversions, or adding underfloor heating can add £3,000–£10,000 to any project.

What Drives the Per-m² Cost Down

Adding floor area generally reduces the per-m² rate because fixed costs (scaffolding, foundation mobilisation, roofing setup) are shared across more space. A 15 m² extension is rarely meaningfully cheaper than a 20 m² one, but the 20 m² extension costs less per square metre.

Two-storey extensions share the same foundation and roof over twice the floor area, hence the lower per-m² rate, but the total project cost is of course higher.

Regional Adjustments (2026)

RegionAdjustment vs national midpoint
London (inner)+20–30%
London (outer)+15–20%
South East, East+10–15%
East Midlands, West Midlands0 to +5%
Yorkshire, North West−5 to +5%
Wales, North East−10 to −5%
Scotland−5 to +10% (varies by city)

Professional Fees to Budget

Professional fees are often under-budgeted. Allow:

  • Architect: typically 3–15% of construction cost, depending on the architect’s level of involvement, so a full service through planning, building regulations and contract administration costs more than drawings alone4
  • Structural engineer: £800–£2,000 for residential extensions5
  • Planning fee (England): £548 for a householder application (2026)6
  • Building Regs (Full Plans, England): charges are set by each local authority and vary. Solihull, for example, charges £830 including VAT for an extension of up to 40 m² in 2026/27.7 Hillingdon, in outer London, charges £1,122 including VAT for an extension under 40 m².8
  • Party Wall surveyor: £800–£2,500 if applicable. The HomeOwners Alliance puts an average party wall agreement at around £1,000 with one surveyor, and says the cost can double when each neighbour appoints their own.9

Getting Accurate Quotes

Per-m² rates are a planning tool, not a contract figure. To get accurate quotes:

  1. Have architectural drawings prepared before approaching builders, quoting from outline descriptions produces wildly varying prices.
  2. Get at least three competitive tenders from similar-tier contractors.
  3. Use a fixed-price JCT Home Owner or Minor Works contract rather than a day-rate arrangement.10
  4. Include a contingency of 10–15% in your budget for unforeseen groundwork issues, price variations, and design changes.
References (10)

Figures in this note were checked against the sources below on 2 October 2026. Superscript numbers in the text point to them. Every source the site cites, by topic.

  1. 1technicalvery strongHM Revenue & Customs (GOV.UK), guidance.RN-4W7T33Sets the three UK VAT rates: standard 20% on most goods and services, reduced 5% on some goods and services such as children's car seats and home energy, and zero 0% on zero-rated goods such as most food and children's clothes. The standard rate rose to 20% on 4 January 2011 from 17.5%. Some supplies are exempt rather than rated, including postage stamps and financial and property transactions. This is the reference for converting a merchant's ex-VAT trade quote into the figure a homeowner actually pays.
  2. 2costfairHomeOwners Alliance, guidance.RN-XKP48OA price, true at 2026-09. HomeOwners Alliance's breakdown of extension costs by type, size and location, together with the fees on top: design, structural engineer, planning, building control and party wall. A consumer body rather than a cost consultancy, so the value is in listing every cost a homeowner forgets rather than in the accuracy of any one band.
  3. 3costvery strongPlanning Portal, guidance.RN-M2KUCTA price, true at 2026-04. The Planning Portal's consolidated guide to planning application fees in England from 1 April 2026, built from the 2012 fees regulations as amended and MHCLG's annual indexation. More usable than the regulations themselves because it puts each application type next to its current fee and its definition. The statutory amounts are in the regulations; this is the readable index to them.
  4. 4technicalfairHomeOwners Alliance, guidance.RN-FTIV36HomeOwners Alliance on architect fees, giving a range of roughly 3% to 15% of construction cost and explaining what moves it: the RIBA work stages taken on, whether the architect runs the build, and the size of the job. Useful for the shape of the fee, because a percentage quoted without saying which stages it covers is not comparable with another percentage.
  5. 5technicalfairHomeOwners Alliance, guidance.RN-6O3Y7XHomeOwners Alliance on structural engineer costs in 2026, around £100 an hour, with typical fees for common jobs: calculations for a beam over a knocked-through wall, an extension design, a structural survey. A consumer body's figures, so treat the hourly rate as indicative and the list of what needs an engineer at all as the useful part.
  6. 6technicalvery strongMinistry of Housing, Communities and Local Government (GOV.UK), a government page.RN-JM9I1NThe schedule of planning application fees in England from 1 April 2026. Fees rise annually with the September CPI under the 2023 amendment regulations; September 2025 CPI was 3.8%, so every fee from April 2025 rose by that and is charged from 1 April 2026. The table lists the new amount for each application type, which is the statutory figure a council must charge rather than a guide price.
  7. 7coststrong locallySolihull Metropolitan Borough Council.RN-XSQV70A price, true at 2026-09. Solihull Metropolitan Borough Council's published price for the item the cost register files as "building Regulations application fee, plan charge, England", which is the figure a cost footnote on this page points at.
  8. 8coststrong locallyLondon Borough of Hillingdon.RN-XNWC03A price, true at 2026-09. Hillingdon's 2026-27 building control fee schedule lists a regularisation charge for each type of domestic work: £1,210 for an extension under 40 m² and £440 for minor works such as chimney breast removal or a simple structural opening.
  9. 9costfairHomeOwners Alliance, guidance.RN-XA4DWQA price, true at 2026-09. HomeOwners Alliance on party wall surveyor costs in 2026: what an award costs, what pushes it up, and the rule that the building owner doing the work normally pays both sides' surveyors. That last point is the one homeowners are caught by, and it is the reason a neighbour appointing their own surveyor is a cost to the person building.
  10. 10technicalfairThe Joint Contracts Tribunal, guidance.RN-R4944ZThe Joint Contracts Tribunal's page for its Home Owner contracts, which it describes as written for homeowners appointing contractors or consultants for building work.

  1. VAT rates, GOV.UK, accessed 7 September 2026. gov.uk

  2. House extension costs, HomeOwners Alliance, accessed 2 October 2026. Priced at 2026-09 and left there. hoa.org.uk

  3. A guide to fees for Planning Applications in England, 1 April 2026, V2026, Planning Portal, accessed 15 September 2026. Priced at 2026-04, so no calibration is needed. ecab.planningportal.co.uk

  4. Architect fees – and how to cut them, HomeOwners Alliance, accessed 7 September 2026. hoa.org.uk

  5. Structural engineer costs in 2026, HomeOwners Alliance, accessed 7 September 2026. hoa.org.uk

  6. Fees for planning applications in England from 1 April 2026, Ministry of Housing, Communities and Local Government (GOV.UK), accessed 7 September 2026. assets.publishing.service.gov.uk

  7. Guidance notes on Building Regulation Charges, effective from 1 April 2026, Solihull Metropolitan Borough Council, accessed 2 October 2026. Priced at 2026-09 and left there. solihull.gov.uk

  8. Building Control Fees 2026-27, London Borough of Hillingdon, accessed 2 October 2026. Priced at 2026-09 and left there. pre.hillingdon.gov.uk

  9. Party wall surveyor cost, HomeOwners Alliance, accessed 2 October 2026. Priced at 2026-09 and left there. hoa.org.uk

  10. Home Owner Contracts, The Joint Contracts Tribunal, accessed 2 October 2026. corporate.jctltd.co.uk