Extension costs per square metre are one of the most searched figures in home renovation, and one of the most misquoted. Headlines often cite a suspiciously tidy number without explaining what that figure actually includes. This guide breaks down realistic 2026 UK costs, explains what drives the spread, and works through a concrete example.

£/m² Ranges by Specification Level

The cost per square metre of a UK extension varies enormously depending on structural complexity, finish quality, and what’s counted in the figure.

Specification LevelTypical Build Cost/m² (2026)What It Usually Includes
Basic / developer-grade£1,600–£1,900Shell only, walls, roof structure, windows, screed. No fit-out.
Standard residential£1,900–£2,600Shell + plaster, basic electrics, radiators, standard glazing.
Good quality£2,600–£3,100Above + fitted kitchen or bathroom, better glazing, underfloor heating.
High-spec / architect-led£3,100–£4,000+Premium materials, structural glazing, bespoke joinery, top-tier M&E.

These figures reflect Southern England averages. See the regional multipliers section below.

What’s Included: and What’s Not

The single biggest cause of budget shock is comparing quotes that don’t cover the same scope. A “£2,000/m²” build cost from one contractor may be shell-only; another may include full fit-out.

Typically included in a quoted build cost:

Typically NOT included (budget separately):

  • Architect fees: typically 3–15% of construction cost, depending on how much of the job the architect handles2
  • Structural engineer: £800–£2,500 for calculations and inspections
  • Planning application: £548 for a householder application (England, 2026)3
  • Building control: £800–£1,600 depending on the extension’s size and the local authority or registered building control approver
  • Party wall surveyor: about £900–£2,700 for most projects, and up to double if each owner appoints their own surveyor45
  • Kitchen or bathroom fit-out beyond allowances
  • Landscaping and reinstatement of garden
  • VAT: work to an existing home, including an extension, is normally standard rated at 20%; the 0% rate is for building new dwellings6

A fair total-project budget typically adds 25–40% on top of the raw build cost per m².

Finish Levels Explained

Finish level is the lever that moves cost most dramatically once the shell is up.

Plasterboard and paint, the cheapest route. Skimmed walls, plain ceiling, carpeted or laminate floor. Adequate for a utility room or modest living space.

Standard kitchen or bathroom, mid-range appliances and units (Wickes, Howdens, IKEA). Budget £8,000–£15,000 for a typical kitchen fit-out on top of build cost.

Premium fit-out, handleless cabinetry, quartz worktops, Neff or Siemens appliances, underfloor heating with smart controls, bifold or slide-and-turn doors. Add £20,000–£45,000 for a 20 m² kitchen.

Structural glazing / frameless, glass roofs, flush-threshold sliding doors, rooflights, each is a meaningful line item. A single 2 m × 2 m flat rooflight costs £1,500–£3,500 supplied and fitted.

Regional Multipliers

Building costs vary across the UK, though by less than is often claimed. The multipliers below are rescaled from the official BCIS location factors that the Valuation Office Agency publishes with its Rating Cost Guide 2026, where the UK average is 100 and a typical South East district is about 110 (taken here as 1.0).7

RegionOfficial factor (UK = 100)Multiplier vs South EastExample: 20 m² at £2,400/m²
Inner London123–1321.12–1.20£54,000–£58,000
South East (excl. London)105–1150.95–1.05£46,000–£50,000
South West99–1030.90–0.94£43,000–£45,000
Midlands95–1030.86–0.94£41,000–£45,000
North West101–1020.92–0.93£44,000–£45,000
North East89–930.81–0.85£39,000–£41,000
ScotlandNot coveredn/aGet local quotes
Wales88–960.80–0.87£38,000–£42,000

In the VOA table, inner London boroughs run from 123 to 132, Surrey is 115, Greater Manchester 101 and County Durham 89.7

Note: London jobs also carry costs that a multiplier hides, such as parking bay suspensions, skip licences and restricted access, so price those as separate lines. Always get three quotes locally rather than relying on national averages.

What Increases the Cost Per m²

Some design and site decisions push the cost per square metre up even at the same spec level:

Small footprints, fixed costs (skip hire, scaffolding, project management overhead) spread over fewer square metres. A 10 m² extension often costs more per m² than a 25 m² one.

Basement or semi-basement level, if the ground slopes or you’re extending below the existing damp-proof course level, excavation and waterproofing add £500–£1,200/m².

Structural steel, if load-bearing walls are removed or large spans crossed, exposed steel beams cost £1,500–£5,000 per beam installed, plus engineering fees.

Flat roofs vs pitched, a flat roof with a decent warm-deck EPDM finish costs roughly £80–£140/m² for the roof covering alone; a pitched roof costs £100–£180/m² including tiles and insulation. Lantern or glass roofs push to £400–£900/m².

Poor ground conditions, chalk, clay-shrink risk (common in South East and London), or made ground can require engineered foundations (pile and beam or raft), adding £3,000–£8,000 over strip footings.

Worked Example: 20 m² Rear Kitchen Extension, South East

Scope: Single-storey, flat warm-deck roof, bi-fold doors, good-quality kitchen fit-out, underfloor heating.

ItemCost (exc. VAT)
Build cost: 20 m² at £2,400/m²£48,000
Architect (design + planning + contract admin) at 10%£4,800
Structural engineer£1,400
Building control (registered building control approver)£1,100
Kitchen fit-out (units, worktop, appliances)£18,000
Underfloor heating system£2,200
Rooflights (two 1.5 m × 0.75 m units)£3,200
Provisional: party wall (one adjoining owner)£1,600
Planning fee£5488
Contingency (10%)£8,100
Total exc. VAT£88,948
VAT at 20% (not charged on the planning fee)£17,680
Total inc. VAT~£106,600

At 20 m², that’s roughly £4,400/m² before VAT, or about £5,300/m² including VAT, well above the headline build-cost figure. This is why total project budgeting matters more than the per-m² build rate alone.

How to Use These Figures

A cost-per-m² figure is a useful sanity check, not a quote. It helps you identify if a contractor’s price seems wildly low (possible scope exclusions) or high (worth probing what’s included).

For a reliable budget:

  1. Agree scope in writing with your designer before going to tender.
  2. Get at least three competitive quotes from VAT-registered builders.
  3. Ask each tenderer to quote from the same specification and drawing set.
  4. Add 10–15% contingency on top, older houses rarely produce no surprises.

Building Regulations approval is needed for almost every extension, whether or not planning permission is required; the main exemption is a small ground-level conservatory or porch of up to 30 m², subject to conditions.9 Separately, permitted development under Class A of Part 1, Schedule 2 to the General Permitted Development Order 2015 allows a single-storey rear extension of up to 4 m on a detached house and 3 m on any other house (8 m and 6 m under the prior approval process).10 Buildings other than the original house must not cover more than half of the land around it (the curtilage).10

References (10)

Figures in this note were checked against the sources below on 30 September 2026. Superscript numbers in the text point to them. Every source the site cites, by topic.

  1. 1technicalvery strongMinistry of Housing, Communities and Local Government.RN-L0FCJXThe in-force U-value tables a DIY insulation or door note must quote, with the exact numbers. Table 4.2, limiting U-values for new fabric elements in existing dwellings, sets roof 0.15, wall 0.18, floor 0.18, swimming pool basin 0.25, window 1.4 or Window Energy Rating Band B minimum, rooflight 2.2, doors with more than 60 per cent of the internal face glazed 1.4 or Doorset Energy Rating Band C minimum, and other doors 1.4 or Doorset Energy Rating Band B minimum. Table 4.3, for existing elements, gives a roof threshold of 0.35 and an improved value of 0.16, which is the figure a loft insulation note needs: paragraph 4.12 says a retained element whose U-value is higher than the column (a) threshold, for example through a loft or garage conversion, should be upgraded to the column (b) value. Paragraph 4.13 allows a lesser upgrade only where the improved value is not technically or functionally feasible or would not achieve a simple payback of 15 years or less. Paragraph 4.8 requires a replacement element to be no worse than the one it replaces as well as meeting Table 4.2. Paragraph 4.10 gives the heritage exception: where character must be maintained, a centre pane U-value of no more than 1.2 or low-emissivity secondary glazing. Note 9 records that the timber window concession of 1.6 or Band C ended on 14 June 2023. The 2021 edition came into force on 15 June 2022; a 2026 edition exists at https://assets.publishing.service.gov.uk/media/69c122a6cfa346b9d4704a55/ADL1_2026.pdf.
  2. 2costfairHomeOwners Alliance, guidance.RN-FTIV36A price, true at 2026-09. HomeOwners 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.
  3. 3technicalvery 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.
  4. 4costfairHomeOwners 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.
  5. 5costsupplier figureSurvey of Party Wall.RN-JLADBQA price, true at 2026-09. a London party wall surveying practice publishing its own fixed fee, £950 to £1,450 per adjoining owner for a standard extension, broken into the stages it covers: site appraisal, serving the statutory notices, tracking the 14 day response, the schedule of condition, negotiating access, drafting the award and delivery, with VAT added on top. It is also the source for the point the notes most often get wrong, that the fee is per ADJOINING OWNER rather than per job, and that a neighbour who dissents and appoints their own surveyor is paid separately again by the building owner under section 10(11)
  6. 6technicalvery strongHM Revenue & Customs, guidance.RN-KBNHMBHMRC's main VAT notice for building work, last updated 26 August 2026. Sets out that construction of a new building and work to an existing building is normally standard rated at 20%, with exceptions: 0% for new qualifying dwellings and for housing association conversions of non-residential buildings, and 5% for a 'changed number of dwellings' conversion (section 7) and for renovation or alteration of a dwelling not lived in during the 2 years immediately before work starts (section 8, conditions at 8.1.2 and 8.3). Cites the law: VAT Act 1994 section 30 and Schedule 8 Group 5 for zero rating, section 29A and Schedule 7A Groups 6 and 7 for the reduced rate. Also explains the developer input tax block on goods that are not building materials (VAT (Input Tax) Order 1992 articles 2 and 6) and points self builders to the DIY housebuilders refund scheme.
  7. 7technicalvery strongValuation Office Agency, guidance.RN-L37ZLPGuidance Note edition 9 to the Valuation Office Agency's Rating Cost Guide 2026. Explains the methodology behind the estimated replacement costs in the guide and, critically, the adjustments applied to them: the location factor, the contract size adjustment and how the unit rates are meant to be used. Read this before quoting any figure out of the cost guide itself, because a raw ERC without its adjustments is the wrong number.
  8. 8costvery 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.
  9. 9technicalvery strongThe National Archives (legislation.gov.uk), legislation.RN-Y7MVZKClass 7 is the conservatory and porch exemption every extension note leans on: the erection at ground level of an extension consisting of a conservatory, porch, covered yard or covered way, or a carport open on at least two sides, is exempt from the building regulations where the floor area does not exceed 30 square metres. Where a conservatory or porch is wholly or partly glazed, the glazing must still satisfy Part K4 and K5.1 to K5.4 and Part N of Schedule 1. Class 6 exempts small detached buildings up to 30 square metres single storey and up to 15 square metres otherwise, which is the garden room threshold.
  10. 10technicalvery strongThe National Archives (legislation.gov.uk), legislation.RN-Z0XQ8VParagraph A.1 sets the householder extension limits: buildings may not cover more than 50 per cent of the curtilage (A.1(b)); a single-storey rear extension may not extend more than 4 metres beyond the rear wall of a detached house or 3 metres for any other house, nor exceed 4 metres in height (A.1(f)), rising to 8 metres and 6 metres under the larger home extension route (A.1(g)); an extension of more than one storey may not extend more than 3 metres beyond the rear wall or be within 7 metres of the rear boundary (A.1(h)); eaves within 2 metres of a boundary may not exceed 3 metres (A.1(i)); a side extension may not exceed 4 metres in height, have more than one storey or be wider than half the width of the original house (A.1(j)). Paragraph A.2 removes cladding and side or multi-storey rear extensions on article 2(3) land such as conservation areas. A.3 requires materials of similar appearance and obscure-glazed, non-opening upper-floor side windows unless the opening part is more than 1.7 metres above the floor, and A.4 sets the neighbour consultation (prior approval) procedure with its 42-day determination period.

  1. Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments, Ministry of Housing, Communities and Local Government, accessed 14 September 2026. assets.publishing.service.gov.uk

  2. Architect Fees - And How To Cut Them In 2026, HomeOwners Alliance, accessed 30 September 2026. Priced at 2026-09 and left there. hoa.org.uk

  3. 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

  4. Party Wall Surveyor Costs 2026, HomeOwners Alliance, accessed 30 September 2026. Priced at 2026-09 and left there. hoa.org.uk

  5. Fixed-Fee Party Wall Surveyor Cost London 2026 Guide, Survey of Party Wall, accessed 30 September 2026. Priced at 2026-09, so no calibration is needed. surveyofpartywall.co.uk

  6. Buildings and construction (VAT Notice 708), HM Revenue & Customs, accessed 30 September 2026. gov.uk

  7. Rating Cost Guide 2026, Guidance Note, Edition No. 9, Valuation Office Agency, accessed 30 September 2026. assets.publishing.service.gov.uk

  8. A guide to fees for Planning Applications in England, 1 April 2026, V2026, Planning Portal, accessed 21 September 2026. Priced at 2026-04, so no calibration is needed. ecab.planningportal.co.uk

  9. The Building Regulations 2010, Schedule 2: Exempt buildings and work, legislation.gov.uk, accessed 30 September 2026. legislation.gov.uk

  10. The Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2, Part 1, Class A, legislation.gov.uk, accessed 30 September 2026. legislation.gov.uk