Off-grid drainage cost for a barn
A domestic sewage treatment plant for a barn conversion typically costs £8,000 to £18,000 installed, including the BS 6297 percolation test, plant supply, excavation and outfall. Septic tank installation is permitted under the General Binding Rules where infiltration discharge is feasible.
Treatment plant vs septic
A sewage treatment plant discharges effluent to a clean enough standard to release into a watercourse; a septic tank produces effluent that must discharge to ground via a drainage field. The General Binding Rules restrict septic discharge to surface water (illegal since 2020). For most barn sites, a treatment plant is the practical choice.
Cost stack
Ongoing
Annual service is £180 to £350 from a manufacturer-certified engineer. Emptying every 12 to 24 months is £200 to £400. General Binding Rules require evidence of service to remain compliant.