Cost per m² benchmarks for UK residential extensions, Q1 2026

Live-adjusted reference rates for rear extensions, loft conversions and basement digs, by region and spec tier.

Trails Research·Updated 2026-04-18·6 min read
Single-storey rear extension, standard spec, UK average£2,550/m² Range across spec tiers: £2,200/m² to £3,200/m²

All figures live-adjusted to — via the ONS Construction Output Price Index. Change region below to apply regional factors.

These are indicative per-m² rates for UK residential projects, aggregated from publicly-available ranges (Turner & Townsend, Gleeds, Homebuilding & Renovating, Costmodelling) and inflation-adjusted to the latest ONS Construction Output Price Index. Regional premiums are applied on top using standard published factors.

They're a starting point, not a quote. Specification, ground conditions, planning constraints, logistics and the client's patience all shift real-world numbers by meaningful amounts. Treat these as a check. If your quote is a long way off these, there's usually a specific reason, and it's worth knowing what it is.

Benchmark table

Region:
Project typeEconomyStandardPremium
Single-storey rear extensionTypical 15–25 m², standard fit-out, no structural complications£2,200/m²£2,550/m²£3,200/m²
Double-storey rear extensionExtension over two floors — economies of scale per m²£1,800/m²£2,200/m²£2,900/m²
Loft conversion (dormer)Dormer or L-shaped, includes one new bedroom + en-suite£1,500/m²£1,900/m²£2,600/m²
Basement digNew basement excavation — most expensive per m² by a distance£4,000/m²£5,100/m²£7,500/m²
Side-return extensionTypical London infill; narrow footprint, glazing-heavy£2,400/m²£2,800/m²£3,500/m²
Whole-house refurbishmentVictorian / Edwardian terrace, full strip-out and rebuild£1,400/m²£1,800/m²£2,600/m²

All figures in £ per m² of built area, adjusted to — prices, applied at a regional factor of 1.00 (UK average = 1.00).

Work an example

Pick a project type, area and spec to see the indicative total. The calculation is: (baseline £/m² × area) × ONS adjustment × regional factor.

Indicative total£63,750
£2,550/m²·Single-storey rear extension·— prices

How to use these numbers

Three rules of thumb that save you time:

  • The range within a spec tier matters more than the midpoint. A "standard" rear extension can come in 20% under or over the midpoint depending on site specifics. If a builder's quote is 20% above the standard midpoint, they might be pricing a "premium" scope the client didn't realise they were asking for.
  • Basement digs are genuinely expensive. Per-m² rates roughly double a rear extension because underpinning, waterproofing, and muck-away costs are high whatever the finish. Be firm with clients who've seen a cheap number somewhere.
  • Regional factors are steady. London has been 12% above UK average for most of the last decade. That won't change meaningfully on the timescale of a single project. You can use the factor confidently in client conversations.

For a particular scenario, or to stress-test a historical £/m² quote against today's prices, the interactive Cost Tracker handles the full inflation adjustment including specific quarters.

Frequently asked questions

What is a reasonable cost per m2 for a single storey rear extension in the UK in 2026?
Standard spec single storey rear extensions sit in the range of roughly £2,400 to £2,800 per m2 at the UK average, with London adding a 10% to 12% premium. Economy spec can push below £2,200 where the design is straightforward and the contractor is working outside the home counties. Premium spec with structural glazing, bespoke joinery, or complex groundworks routinely runs above £3,200 per m2. These are live-adjusted to the current ONS COPI release shown in the table above. Figures are derived from Turner & Townsend and Gleeds published benchmarks, re-indexed each quarter.
Why is basement construction so much more expensive per m2 than an extension?
Basements typically run £4,800 to £5,500 per m2 UK average, roughly double a rear extension. The cost drivers are structural underpinning, waterproofing to BS 8102 Grade 3, mechanical ventilation for habitable rooms, excavation and spoil removal in constrained sites, and the Party Wall Act 1996 agreements with neighbours. Labour content is higher than a standard extension because of the trade mix: specialist groundworkers, waterproofing contractors, and structural engineers all carry premium rates. London basements routinely exceed £6,500 per m2 on difficult sites.
How do I adjust a 2022 or 2023 cost per m2 benchmark to today?
Take the original rate and multiply by the ratio of today's ONS Construction Output Price Index to the index at the reference period. For example, if a Q1 2022 benchmark was £2,100 per m2 and COPI has risen roughly 15% since, the re-indexed rate is around £2,415 per m2. This is what the tables on this page do automatically on every ONS release. For residential extensions specifically, consider blending New Housing and Repair and Maintenance indices, since the composition sits between the two.
Do these benchmarks include VAT and professional fees?
No. The per m2 figures shown are construction cost only: main contractor preliminaries, sub-contractor work, materials, overhead and profit. They exclude VAT (20% on most residential work, though new builds and some listed works are zero-rated or reduced-rated), architect and engineering fees (typically 8% to 15% of construction cost), planning and building regulations fees, party wall surveyor fees, and client-supplied items such as sanitaryware, kitchens, and appliances. To convert to an all-in client budget, uplift the construction figure by roughly 25% to 35% before VAT.
What spec tier does a typical UK residential extension sit at?
Most owner-occupier extensions in the £500k to £1.5m London house price range end up at standard to upper-standard spec. That means good quality double glazing but not full structural, pre-manufactured kitchen above IKEA tier, engineered oak or tile floors, bathroom spec around £5k fittings, and underfloor heating to the new floor area. Economy spec is rarely what architects specify but frequently what contractors actually build to when the QS is not involved. Premium spec is reserved for the top end of the market and carries a 20% to 35% cost premium across the build.

About this data

This article uses live data from Trails Cost Tracker, aggregating ONS Construction Output Price Indices, MHCLG Monthly Building Materials Statistics. Data is auto-refreshed each quarter.

→ See the full Trails Cost Tracker→ Methodology and sources

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