GUIDE
Why extension £ per m² is a blunt tool
Last checked September 2026 · Published by Rodway Labs
Not a quote. Figures in this guide are illustrative UK cost bands compiled for planning. They are not a tender, valuation or building-regulation ruling. Get written quotes on the same drawing. Project Cost Lab (Rodway Labs) is not a contractor, architect or quantity surveyor. The mid figure is a band, not a price; the VAT toggle is a display choice, not a ruling.
The headline rate and what it hides
Published 2026 UK guides often quote roughly £2,000–£3,400 per m² for a single-storey extension. That spread is wide for a reason: glazing ratio, kitchen fit-out, drainage route, ground conditions and steelwork move the job more than the floor area alone. A cheap-looking £/m² on a simple brick box is not transferable to a highly glazed wraparound on made-up ground.
Compiled mid points in this lab (before factors)
- Rear single-storey — about £2,650 / m²
- Side return — about £2,800 / m²
- Two-storey — about £2,300 / m² (foundations and some structure shared across floors — still not “cheap”, just a different geometry)
- Wraparound — about £2,750 / m²
These are compiled mid illustrations before specification, region, access, VAT display, contingency and extras. They are planning bands, not contractor prices.
Limits of pricing by floor area
Openings and glass. Bi-folds, roof lanterns and corner glazing change both materials and structural steel. Two extensions with the same footprint can sit in different thirds of the band.
Kitchen and services. Moving a kitchen into the extension, upgrading electrics or relocating boilers is often a large share of the lived cost — and may be omitted from a bare shell £/m².
Ground and drainage. Soft ground, trees, shared drains and pumped solutions are classic overruns. Optional extras in the calculator are allowances only.
Party walls and neighbours. Awards, surveyors and access scaffolding sit outside a simple area rate unless you add them deliberately.
Region and access still apply
Inner London in this model is illustrated at +30% on the works mid; Outer London +22%; South East outside London +12%, with lower factors further north and in the devolved nations. Tight terraces without side access add another uplift. See the region & access guide. None of these multipliers is a postcode price book.
VAT display is not a ruling
Most work on an existing dwelling is standard-rated. Some projects involving new dwellings or qualifying energy-saving measures can differ. The calculator’s VAT control only changes how the band is shown — it does not decide HMRC treatment for your supplier. Read the VAT on works guide and confirm with your accountant or contractor.
Practical way to use a £/m² band
- Measure the proposed internal floor area of the extension, not the whole house.
- Start from the matching form (rear, side, two-storey, wraparound).
- Set spec to match finishes and glazing honestly.
- Tick kitchen, bi-folds, steel, underfloor heat, planning and fees only if they apply.
- Compare two or three written quotes on the same drawing — not against a blog mid point.