Extension Cost Per Square Metre 2026: What to Expect
Cost per square metre is the standard way to compare extension quotes. But the figure varies dramatically depending on specification, region, complexity, and what is included. This guide explains what the numbers actually mean.
Cost breakdown
2026 rates by specification
Basic specification (standard bricks, flat roof, basic windows): £1,500–£2,000/m². Standard specification (quality bricks, bi-fold doors, underfloor heating): £2,000–£2,800/m². Premium specification (bespoke materials, structural glazing, high-end finishes): £2,800–£3,500/m². These are Dorset rates.
What affects the rate
Foundation depth (clay soil or trees nearby = deeper = more expensive), access difficulty (narrow lanes or terraced properties), specification level, bathroom inclusion (wet rooms cost more per m²), and the builder's current availability. A busy builder quotes higher.
| Region | Standard | High Spec |
|---|---|---|
| Dorset | £1,800–£2,200 | £2,500–£3,200 |
| London | £2,500–£3,500 | £3,500–£5,000 |
| South East | £2,000–£2,800 | £2,800–£3,800 |
| Midlands | £1,600–£2,200 | £2,200–£3,000 |
| North | £1,400–£2,000 | £2,000–£2,800 |
Prices based on Dorset rates, 2026
Frequently asked questions
Local cost guides
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