Guide
Replacing a septic tank with a sewage treatment plant: the rules
Updated
A septic tank that discharges straight to a ditch, stream or river must be replaced or upgraded in England, usually with a sewage treatment plant, and you must have plans in place within a reasonable timescale, usually 12 months, according to the Environment Agency's general binding rules.
What the rules say
- A discharge to surface water is limited to 5 cubic metres (5,000 litres) a day; above that you connect to the public foul sewer where it is reasonable, or apply for a permit.
- A septic tank discharging directly to a watercourse must be replaced or upgraded, for example by installing a drainage field so the water trickles through the ground.
- You must have plans to do this work within a reasonable timescale, usually 12 months.
- If you buy or sell a property with a septic tank that discharges directly into a watercourse, buyer and seller should agree who replaces or upgrades it.
Source: GOV.UK: general binding rules, small sewage discharge to surface water
A plant that discharges to the ground through a drainage field follows the rules for discharges to ground instead.
Source: GOV.UK: general binding rules, small sewage discharge to the ground
What it costs
A 6 person plant is a median £2,196 ex VAT across the UK sellers that publish one, read 28 October 2026, and Premier Tech puts the installation at £6,000 to £11,000.
Source: UK Sewage Treatment Plant Price Index: every row, with its source