How much does a house extension cost per square metre?

The short answer

Published 2026 guides put a single-storey extension at roughly £1,700 to £3,000 per square metre, depending on specification, size and region. Smaller extensions usually cost more per square metre.

What the guides say

Novuna's guide (published 8 July 2026, updated 23 July 2026), citing the Federation of Master Builders, gives £1,800 to £3,000 per square metre for a single-storey extension nationally, with London and the South East at the top of that range or above. It gives £36,000 to £60,000 before VAT, fees and fit-out as an example for a 20 m² single-storey extension.

MyJobQuote's guide (updated 19 October 2026) gives a general range of £1,500 to £2,000 per square metre for a basic finish and £2,000 to £3,000 for a high-end finish. Its table for a standard good finish puts single-storey at £1,700 to £1,900 per square metre (typical total £44,000 to £60,000 for 20 to 30 m²) and double-storey at £1,500 to £1,800 (typical total £80,000 to £120,000). The page notes that its two sets of figures do not fully match, which shows how much the assumptions matter.

An example breakdown

MyJobQuote breaks a £52,000 single-storey example down like this:

• Architect, surveys and approvals: £3,000
• Groundworks and foundations: £8,000
• Main structure and roofing: £18,000
• Windows and external doors: £4,500
• Plumbing and electrics: £6,000
• Internal finishes: £7,000
• Contingency: £5,500

What the rate usually leaves out

• VAT, which Novuna says is at 20%, and which not every guide includes in its headline figures.
• Architect and structural engineer fees, which Novuna puts at typically 7 to 15% of build cost, plus planning and building-control fees.
• Party wall surveyor fees, if needed.
• Larger windows, bi-fold doors, rooflights and extra structural steel.
• Kitchen or bathroom fit-out. MyJobQuote gives £3,000 to £10,000 for kitchen appliances and fittings and £2,000 to £6,000 for a full bathroom.
• Skip hire, waste disposal and utility upgrades.

Novuna says that once these are added, the all-in cost can be 20 to 30% higher than the headline per-square-metre figure.

Region and contingency

Novuna puts the London and South East uplift at 20 to 40%. It advises setting aside 10 to 15% of the budget as a contingency and treating it as untouchable unless something goes wrong. Our guide to setting a renovation budget explains how.

Check the rules first

Many extensions are permitted development and others need planning permission; all need building regulations approval. See our guide to finding out whether you need planning permission.

Picture it before you price it

Upload a photo of your house to GlowUpp to see AI-generated extension ideas, then use them to brief a designer and builders. These figures come from published UK cost guides, several of them from trade and quote-comparison sites, so treat them as planning benchmarks, not quotes. Prices vary by region, property, specification and time, and GlowUpp does not set or guarantee them. Always get itemised written quotes. Always ask a qualified professional before moving gas, electrics or plumbing, or changing anything structural, and check your lease or local rules if you rent or live in a conservation area.