Independent reference. UK figures verified 21 June 2026. No vendor affiliation.
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Knockdown rebuild vs extension

In raw build cost an extension is cheaper per sqm for small additions; once 20% VAT, party-wall costs, retrofit insulation, and structural ties land, KDR is the better number above roughly 60 sqm of new floor area on a sound suburban plot.

Oliver Wakefield-Smith · Updated 21 June 2026 · Reviewed against HMRC VAT Notice 708, BCIS Q2 2026, FMB House Builders Cost Guide

The £/sqm breakeven

Extension headline rates run £1,800 to £2,400/sqm for a mid-range single-storey rear extension in the South East[FMB 2026]. New-build replacement runs £2,000 to £2,500/sqm in the same tier[BCIS Q2 2026]. So in isolation the extension wins by £200 to £300/sqm. Then VAT lands.

The VAT inversion

Extensions are standard-rated for VAT at 20%[HMRC 708]. A new replacement dwelling is zero-rated under VAT Notice 708 section 3. On a 100 sqm addition that is a £36k to £48k delta. On 200 sqm of new floor area it is £80k to £100k. That is the line item that flips the calculation.

0% VAT. New dwelling (VAT Notice 708)

Hidden cost drivers in big extensions

Party Wall Act surveyors at £900 to £2,400 per affected neighbour. Retrofit insulation of the existing house to bring the building-control thermal-element test (50% rule) in line. Structural ties and steels at the junction. Roof reconstruction where the two pitches meet. Once an extension exceeds ~60% of the existing footprint, you are effectively building a new house around the old one and paying VAT on it.

When the extension still wins

Listed buildings (KDR is forbidden), conservation areas with strong character (KDR rarely consents), mortgage-locked properties that cannot be vacated for 12 months, and additions below 40 sqm where the VAT delta is small.

Primary sources

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