First investigate connection to the public foul sewer. If off-mains drainage is justified, assess flows, site levels, soil, groundwater, receptors and access before choosing a system. Obtain the relevant environmental authorisation, planning and building-control approvals, then commission and document the installation.
When this guide fits
Clues worth recording
- A new home, conversion or extension is planned
- Occupancy or daily flow will increase
- The discharge point will move or change type
- An existing system cannot serve the proposal
Start with mains feasibility
Regulators and planning policy prefer connection to public foul drainage where reasonable or practicable. In England, a new discharge must be connected to the public foul sewer if the Environment Agency decides that is reasonable, and the general binding rules cannot be used if a sewer is within 30 metres of the property boundary (30 metres for each property in a development). Natural Resources Wales uses the same distance for registration. Obtain evidence from the sewerage provider rather than assuming distance alone rules it out.
Survey the site
A designer needs intended occupancy and flows, levels, boundaries, access, soil and groundwater evidence, watercourses, wells, boreholes, flood risk, protected areas and space for maintenance or replacement.
- Topographical and drainage survey
- Ground investigation and percolation evidence
- Mains connection enquiry
- Environmental constraint check
- Access and ownership review
Select and authorise the route
The choice may be septic tank to drainage field, package plant to ground or permitted surface-water outlet, or a sealed cesspool where no other option is feasible. Each has siting and operational constraints. In England, a new discharge to ground since 2 October 2023 needs a permit if it is within 50 metres of another discharge operating under the general binding rules. Changes after approval should be checked again.
Install, commission and hand over
Use competent installers, protect the field from trafficking, commission mechanical and electrical equipment, and provide the operator with drawings, manuals, approval records, service schedule and emergency contacts.
If a new driveway or parking area is part of the same project, plan it around the field. Better Driveways compares driveway surfaces, including permeable options
Do not buy a tank before the discharge route, capacity and approvals have been established.
Questions homeowners ask
What people usually want to know
Do I need planning permission for a septic tank?
In England, you need building regulations approval to install a new septic tank or treatment plant, and you may also need planning permission. Environmental authorisation is separate; check all three before installation.
Can roof water enter the septic system?
It should not. Clean surface water can hydraulically overload foul treatment and drainage fields and should have a separate lawful route.
Does changing an outlet create a new discharge?
In England, yes if you change between ground and surface water, move the discharge more than 10 metres, or increase it above 2 cubic metres a day to ground (5 cubic metres to surface water). New discharges have extra rules.
Sources & review
Evidence behind this guide
Reviewed 13 September 2026. We use primary guidance where available and label trade-body advice clearly.