New Build & Design
Private Drainage System Design
How private drainage systems get designed and sized, and what developers, self-builders and architects need to plan for.
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
- Site assessment — ground conditions, water table, access, and proximity to watercourses and boundaries.
- Percolation test — establishes how well the ground can support a drainage field. See our percolation test guide.
- System selection — septic tank with drainage field, sewage treatment plant, or (rarely, for constrained sites) a cesspit. See septic tank vs sewage treatment plant.
- Sizing — calculated from population equivalent (expected occupancy), following BS 6297 and British Water’s Code of Practice.
- Regulatory route — confirming whether the design falls under the General Binding Rules or needs an Environment Agency permit.
- 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
Should I get a percolation test before or after buying land? + −
Ideally before, or as a condition of purchase — discovering the ground can't support a drainage field after completion can be a costly surprise.
Does every unit in a small development need its own septic tank? + −
Not necessarily — a single, appropriately sized shared system is often more practical and cost-effective than multiple individual tanks, subject to design and regulatory requirements.
Who signs off a private drainage design? + −
Building control sign-off is required, and depending on the discharge point, the Environment Agency (or equivalent body outside England) may also need to be involved.