Single-Storey Rear Extension Cost in 2026 — UK Price Guide

Quick benchmarks

For 2026, a single-storey rear extension typically costs:

• Standard finish: £2,400 – £2,900 / m²
• Mid-range with bifolds: £2,900 – £3,600 / m²
• High-end / architect-led: £3,600 – £5,000+ / m²

A 25 m² extension at mid-range spec will therefore land around £72,000 – £90,000 before VAT, kitchen and landscaping.

Where the money goes

1. Foundations & groundworks — clay soil, trees or drains under the slab can add £5,000–£15,000.
2. Steel & openings — wide spans without internal columns mean heavy steels and engineered padstones.
3. Glazing — bifolds run £1,200–£2,500/m run; sliders are 20–40% more.
4. Roof — flat warm-roof with rooflights vs. lantern vs. pitched tile makes a big difference.
5. Kitchen integration — wiring, plumbing, MVHR and underfloor heating.

Red flags in a cheap quote

• No PC sum for bifolds (they've assumed £6k for a £15k spec)
• "Foundations TBC after dig" with no contingency
• No allowance for making good the existing kitchen wall
• Excludes scaffolding, skips and welfare

Sense-check before you sign

Run your project through the Build Price IQ cost checker to see the 2026 median for your postcode, then compare line-by-line against your builder's quote.