Timber Dutch barn conversion cost
Timber Dutch barns convert at £1,650 to £2,180/m² mid spec, BCIS May 2026. They are the cheapest of the three archetypes when the curved roof is retained and the timber frame is sound.
The frame and the roof
A traditional Dutch barn is a five-bay open structure with a curved (often corrugated steel) roof. The frame is the asset; if it is sound, the conversion is essentially a new envelope hung within the frame. Replacement of the corrugated roof with insulated standing-seam metal (typically aluzinc or zinc) costs £160 to £240 per m² of roof area.
Insulation and Part L
Achieving the Part L elemental U-values on a timber Dutch barn typically uses warm-roof PIR (140-180mm) and an inner stud wall with mineral wool plus PIR plasterboard. Airtightness is the bigger challenge than U-value; expect the design team to specify a wraparound membrane.
Class Q on Dutch barns
Many Dutch barns fail Class Q on the "permanent and substantial construction" test, particularly older corrugated examples. The 21 May 2025 amendment did not relax that test. Allow for a 30% to 40% refusal probability on first submission unless the building is exceptionally well-preserved.