Modern portal-frame barn conversion
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.
Modern steel portal-frame barns scope at £1,750 to £2,250/m² mid spec. The clear-span structural advantage gives M&E layout freedom that stone barns cannot match. Class Q applies if the building was in agricultural use on 24 July 2023.
The structural advantage
A steel portal frame gives 12 to 24 metres of clear span. The conversion essentially wraps a new insulated envelope around the steel and slips a domestic plan inside, with no internal load-bearing walls forced on the layout. M&E first-fix is straightforward.
Class Q on portals
Class Q on a steel portal works if the building was put up for agricultural use and was in agricultural use on 24 July 2023. gov.uk Class Q guidance does not exclude steel structures; the same 150 m² per dwelling and 1,000 m² site caps apply.
Envelope choices
- Insulated panel infill: aluzinc standing-seam outer, mineral wool / PIR composite, plasterboard inner. £180-£260 / m² of envelope.
- Timber clad infill: ventilated rainscreen on breather membrane and frame. £210-£300 / m².
- Brick or block skin: closer to a new-build cost; £250-£340 / m².
Sources cited on this page