Hidden costs of 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.
A barn conversion budget that quotes only £/m² shell-and-core misses £45,000-£110,000 of routinely-incurred ancillary spend. The audit-able ledger below itemises the lines.
The full ledger, mid-spec 200 m² barn
Topographical survey
£700 - £1,800
Measured building survey
£1,500 - £3,500
Structural engineer's appraisal
£1,200 - £2,400
Bat survey (Phase 1 + emergence)
£900 - £3,500
Ecology baseline + protected species
£2,000 - £6,000
Arboricultural impact assessment
£700 - £2,000
Heritage statement (if listed)
£2,500 - £6,000
BS 6297 percolation test (if drainage)
£800 - £1,600
Contamination Phase 1
£800 - £1,800
DNO three-phase upgrade
£8,000 - £40,000
BNG (10%, full planning only)
£2,000 - £12,000
Party wall (if shared)
£900 - £3,500
Building control fees
£900 - £2,200
10-year warranty (Premier Guarantee / LABC)
£3,000 - £6,500
Private water supply LA risk assessment
£350 - £800
Solicitor (conveyancing + warranties)
£1,800 - £4,000
Site insurance (during build)
£1,200 - £3,500
Hidden cost total
£28,250 - £100,300
Source
Figures cross-referenced to gov.uk, Bat Conservation Trust survey rate guidance, DEFRA BNG, RICS surveyor day rates, and confirmed quote ranges from 2026.
Sources cited on this page