If your property isn’t connected to a mains sewer, your septic tank or treatment plant is covered by the Environment Agency’s general binding rules. Most people only discover them when they sell a house, extend one, or apply for planning permission. Here is what they mean in practice.
Septic tanks that discharge to a stream or ditch
Since 1 January 2020, a septic tank can no longer discharge directly to a watercourse such as a stream, river or ditch. If yours does, you must do one of these, normally within 12 months:
- connect to the public foul sewer, if one is close enough
- replace the septic tank with a small sewage treatment plant, which treats the effluent to a higher standard
- install a drainage field, so the effluent soaks into the ground instead
If you’re selling, the buyer and seller should agree who will carry out the upgrade as a condition of sale. The buyer must also be told in writing about the system, where it is, and its maintenance records.
Discharging to the ground: the drainage field
Discharging to ground is allowed under the general binding rules when:
- the discharge is 2 cubic metres a day or less
- the effluent goes through a septic tank or treatment plant and then a properly designed drainage field
- the drainage field is designed to BS 6297
- it isn’t in a groundwater Source Protection Zone 1, or within 50 metres of a private water supply
- for new discharges since October 2023, it’s at least 50 metres from any other small sewage discharge
Old soakaways, wells and boreholes can’t be used to take sewage. If your system discharges to an old soakaway, it needs upgrading to a proper drainage field.
Larger discharges, sites near protected wildlife areas, and single properties within 30 metres of a public foul sewer need an environmental permit or a sewer connection instead.
Why a percolation test comes first
A drainage field only works if the ground can take the effluent at the right rate:
- Too slow, and effluent ponds and breaks out at the surface.
- Too fast, and it reaches groundwater before the soil has treated it.
BS 6297 sets out the infiltration test used to find the ground’s percolation value (Vp), and the drainage field is sized from that figure. Building Control and the Environment Agency expect to see it.
This is a different test from the BRE 365 soakaway test used for rainwater. Many rural sites need both, and we can carry out both on the same visit.
What to do next
- If you’re buying or selling: find out where the system discharges. Ask for the maintenance records.
- If it discharges to a watercourse: plan the upgrade now.
- If you’re installing a new system or replacing an old one: get the infiltration test done before choosing the treatment plant, so the drainage field design is based on your ground, not on guesswork.
We carry out BS 6297 infiltration testing across the Midlands and beyond. See foul infiltration testing (BS 6297) or request a quote.
This guide summarises the Environment Agency’s general binding rules for England. For the full rules, see GOV.UK: “Small sewage discharges to the ground” and “… to a surface water”. Wales, Scotland and Northern Ireland have their own rules.