Private Drainage System Design

In short: private drainage system design starts with the ground, not the tank — a percolation test and site assessment determine what’s actually feasible, and that should happen before a site layout or building position is finalised, not after.

Who this matters most to

• Developers planning multiple units without a mains sewer connection
• Self-builders on rural plots
• Architects specifying drainage for a rural or off-grid project
• Landowners assessing a plot’s development potential

The design process, step by step

1. Site assessment — ground conditions, water table, access, and proximity to watercourses and boundaries.
2. Percolation test — establishes how well the ground can support a drainage field. See our percolation test guide.
3. System selection — septic tank with drainage field, sewage treatment plant, or (rarely, for constrained sites) a cesspit. See septic tank vs sewage treatment plant.
4. Sizing — calculated from population equivalent (expected occupancy), following BS 6297 and British Water’s Code of Practice.
5. Regulatory route — confirming whether the design falls under the General Binding Rules or needs an Environment Agency permit.
6. Documentation — drainage design forms part of the Building Regulations submission.

Designing for multiple units or shared systems

Developments serving several properties often use a single, larger shared system rather than individual tanks per plot — this changes the sizing calculation, the regulatory route (larger discharges may fall outside the General Binding Rules), and typically requires more detailed design input than a single dwelling.

Common design mistakes worth avoiding

• Fixing the building or site layout before confirming ground conditions support the intended drainage approach
• Undersizing for future occupancy (extensions, converted outbuildings, holiday letting)
• Assuming a drainage field will work without a percolation test
• Overlooking access requirements for future emptying or servicing