Extension Cost Per m² in the UK: What 2026 Rates Really Buy

The headline rates for 2026

Every extension quote in the country boils down to a rate per square metre, and in 2026 the bands are:

• Basic: £1,700–£2,000 per m² — simple form, flat roof, standard windows
• Standard: £1,900–£2,600 per m² — where the majority of family projects land
• Premium: £2,800–£3,500 per m² — structural glazing, complex rooflines, high-end finishes
• Two-storey blended: £1,700–£2,400 per m² — the second floor rides on foundations and a roof you have already paid for

Add 20–30% in London and the South East, and note that builder quotes to homeowners normally include VAT. To turn rates into project totals for your own house, start with the house extension cost guide or run your numbers through the extension cost calculator.

Where the money goes: the build, element by element

For a standard-spec single storey extension, the per-m² rate breaks down roughly like this:

• Foundations and groundworks (£350–£500 per m²): excavation, concrete and drainage connections — the least visible money you will ever spend
• Shell: walls and structure (£450–£650 per m²): masonry or frame, steels, lintels and insulation
• Roof (£250–£400 per m²): structure, covering and openings for rooflights
• Glazing and external doors (£200–£450 per m²): the widest swing of any element — bifolds alone run £3,000–£8,000
• First fix, plumbing, electrics and plaster (£300–£450 per m²)
• Finishes and decoration (£200–£400 per m²): flooring, joinery and paint

Those elements sum to roughly £1,750–£2,850 per m², which is exactly why the standard band sits where it does. A timber frame shell shifts money from wet trades to factory fabrication and can shorten the programme on site.

Extension cost per m² by build element (2026)

Typical element shares for a standard-specification single storey extension; glazing is the element most likely to break the range, and a new kitchen or bathroom sits on top of all of it.

What basic, standard and premium actually mean

Spec labels get thrown around loosely, so pin them down before comparing quotes. Basic means a simple rectangular form, warm flat roof, uPVC or entry-level aluminium glazing and painted plasterboard — a perfectly good room. Standard adds a better roof, a rooflight or two, quality glazed doors and underfloor heating. Premium means structural glazing, minimal sightlines, complex roof forms and joinery-grade finishes.

Whatever the band, the big-ticket contents sit on top: a kitchen in the new space adds £10,000–£25,000, a bathroom £4,000–£8,000, and bifold or sliding doors £3,000–£8,000. Two quotes can only be compared once you know which of these each one includes.

What pushes your rate up or down

The same drawings can price £500 per m² apart on different plots. The usual suspects:

• Project size: scaffold, setup and the steel opening cost broadly the same at 10m² as at 25m², so small projects always carry a higher rate — see the side return for the extreme case
• Ground conditions: clay, trees and old drains mean deeper, dearer foundations
• Glazing ratio: every extra square metre of glass replaces cheaper wall
• Roof form: flat is cheapest; pitched, parapet and lantern designs climb from there
• Access and region: tight terrace logistics and the London premium stack on top of everything else

Depth-for-depth comparisons for the most common project type are in our rear extension cost guide.

Planning permission, building regulations and the other 20–30%

Permitted development covers many extensions in England: rear additions up to 3m on a semi or terrace and 4m on a detached house (6m and 8m with prior approval), a 4m height cap, side extensions single storey only and at most half the width of the original house, and two-storey rear additions up to 3m deep provided they stay 7m from the rear boundary. Conservation areas and listed buildings are excluded — check your address on the Planning Portal. Building regulations always apply to a habitable extension.

Finally, remember the per-m² rate is a build cost, not a budget. Professional fees add 10–15%, contingency another 10–15% — so the build rate is only about four-fifths of the money you actually need.

FAQs: Extension Cost Per m² in the UK