Extension Cost Per m²: What You'll Really Pay

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.

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:

• Substructure (foundations, damp-proof course)
• Superstructure (walls, roof structure, insulation to Part L)¹
• External windows and doors
• First-fix electrics and plumbing (cables, pipes to connection points)
• Plastering and screeding
• Provisional sums for glazing and roofing

Typically NOT included (budget separately):

• Architect / designer fees: 5–12% of build cost
• Structural engineer: £800–£2,500 for calculations and inspections
• Planning application: £548 for a householder application (England, 2026)²
• Building control: £800–£1,800 depending on local authority or registered building control approver
• Party wall surveyor: £700–£1,500 per surveyor, per owner
• Kitchen or bathroom fit-out beyond allowances
• Landscaping and reinstatement of garden
• VAT: standard rated at 20% for most work (new builds are 0%, but extensions aren’t new builds)

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

Labour rates vary significantly across the UK. These multipliers approximate local costs relative to a South East England baseline of 1.0.

Note: London contractor rates have risen particularly sharply since 2022 due to high demand and the general cost of operating in the capital. 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.

At 20 m², that’s roughly £4,400/m² all-in, 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 mandatory for any extension regardless of whether planning permission is required. Permitted development rules (Class A of the Householder Order) allow rear extensions up to 4 m on detached houses and 3 m on semis/terraces at single-storey, but only if the extension doesn’t exceed half the total curtilage area and meets the Part L energy and Part B fire requirements.⁴