New Build & Design
Private Drainage for New Builds
What to plan for when a new build or conversion won't connect to the mains sewer, from site assessment through to building control sign-off.
In short: if a new build, extension or conversion won’t connect to the mains sewer, private drainage needs to be designed in early — ground conditions determine what’s feasible long before foundations go in, and getting this wrong is expensive to unpick later.
Why this needs early planning
Unlike retrofitting a system to an existing property, a new build gives you the chance to design the drainage system into the site plan from the outset — but only if it’s considered early. Percolation results, drainage field space and access all affect where a building can realistically sit, not just where the tank goes.
What’s involved
- Site assessment and percolation test — establishes what the ground can support. See our percolation test guide.
- System choice — a septic tank with drainage field, or a sewage treatment plant, depending on space and discharge options. See septic tank vs sewage treatment plant.
- Design against BS 6297 — sizing based on the number of bedrooms/occupants the finished building will have.
- Building Regulations and environmental permissions — approval is required before the system is used, and a permit may be needed depending on the discharge point.
- Installation, typically alongside groundworks — sequencing this with the wider build programme avoids delays.
Common new-build scenarios
- Barn and agricultural building conversions — often no existing drainage connection at all
- Self-build homes on rural plots — private drainage is frequently the deciding factor in site feasibility
- Small developments — may need a larger, shared system designed to serve multiple units
- Holiday accommodation — needs sizing for peak, not average, occupancy
Can I get planning permission before confirming private drainage is feasible? + −
It's possible, but risky — some local authorities and lenders want evidence drainage is workable before committing, and discovering it isn't after purchase can be costly.
Does a new build need a bigger system than an equivalent existing home? + −
Sizing is based on occupancy (bedrooms), not whether the building is new — but new builds sometimes have higher water-use fixtures, worth factoring into the design.
Who signs off a new private drainage system? + −
Building control sign-off is required, and depending on the discharge point, the Environment Agency (or equivalent body in Scotland, Wales and Northern Ireland) may also be involved.