Class Q 150 m² cost

Costs verified against BCIS Online (May 2026 release) and ONS Construction Output Price Index, April 2026. Class Q rules current to the 21 May 2025 GPDO amendment.

The 21 May 2025 GPDO amendment caps each Class Q dwelling at 150 m² gross internal area. A 150 m² mid-spec conversion typically scopes at £270,000 to £390,000 all-in before VAT431C reclaim.

What the 150 m² cap means in practice

The cap is per dwelling, measured as gross internal area (the warm envelope), not gross external. A barn with a 220 m² footprint can still be Class Q if it is sub-divided into a single 150 m² dwelling plus a non-residential ancillary, or two units of 100 and 120. The previous two-band 100 m² / 465 m² regime no longer applies. See SI 2025/579.

Worked £ example, 150 m² Class Q at mid spec

Shell works at £2,200/m² (BCIS stone mid)

£330,000

M&E (drainage, ASHP, electrics)

£42,000

Professional fees, 12%

£40,000

Contingency, 10%

£37,200

Less VAT431C reclaimapprox 7%

-£23,100

All-in, net of reclaim

£426,100

Designing within 150 m²

A 150 m² envelope comfortably supports a three-bedroom barn with double-height living, a snug, two bathrooms, plant room and bootroom. Floor-to-ceiling height is not capped under Class Q, so vaulted volumes do not consume floor area. Mezzanines do count, on the GIA convention used in the regulation.

What happens above 150 m²

The barn does not become unconvertible. It becomes a full-planning application. The economics flip toward higher fees, longer programme, and more design freedom (no 150 m² ceiling, no exterior restraint).

Sources cited on this page