Is Your Septic Tank Legal? The 5-Minute Compliance Checklist

Since the General Binding Rules came into force (2015, tightened January 2020), a septic tank that discharges to a ditch, stream or any watercourse is not legal to operate — it must have been replaced or upgraded by 2020. Plenty of Cornish systems still haven't been, and owners typically discover this during a house sale, when the buyer's solicitor asks for drainage paperwork nobody has.

Work through the checklist below. It takes about five minutes if you know your system, and it covers what a buyer's conveyancer — or the Environment Agency — would ask.

The checklist

• I know what system I actually have. Septic tank (two chambers, discharges to ground), sewage treatment plant (powered, cleaner discharge), or cesspit (sealed holding tank, no discharge). If you don't know, that's the first thing a specialist can tell you.
• My septic tank does NOT discharge to a watercourse. This is the big one. Septic tank effluent must go to a drainage field/soakaway in the ground — discharge to a ditch, stream or river was outlawed for septic tanks from January 2020. (Treatment plants may discharge to a watercourse.)
• My discharge is within the permitted volume. Under 2,000 litres/day to ground (typical for domestic) falls under the General Binding Rules; larger discharges need a permit.
• The tank is emptied regularly by a licensed waste carrier. "Regularly" for most households means every 6–12 months. Keep the waste transfer notes — they're your paper trail.
• The system isn't showing failure signs. No pooling or soggy ground over the soakaway, no sewage smells, no slow drains or gurgling, no effluent visible anywhere it shouldn't be.
• The tank is far enough from water. Rule of thumb: 10m+ from any watercourse, 50m+ from a well or borehole used for drinking water.
• I have (or can get) the documentation. Installation details if known, emptying receipts / waste transfer notes, and any survey reports. Buyers' solicitors ask for exactly this.
• If I'm selling: the system is either compliant or priced in. A non-compliant system doesn't stop a sale, but it becomes a negotiation item — usually £3,000–£15,000 off, the cost of replacement or upgrade.

Ticked everything? Good — stay that way

Keep emptying on schedule (what it costs), keep the receipts, and re-check after any building work or change of use — a granny annexe or holiday-let conversion changes the loading on the system.

Couldn't tick something?

Not sure what you have or where it discharges: a CCTV drain survey (£150–£250) answers it definitively — cheap certainty compared to guessing wrong in a sale.

Discharging to a watercourse: you'll need an upgrade — usually a treatment plant or a new drainage field. Costs range £3,000–£15,000 depending on the fix; see the installation guide. There's no grace period left; this was due by January 2020.

Failure signs: start with diagnosis and repair — most "failed" systems are a saturated soakaway, not a dead tank.

Want the deeper background on the rules themselves — discharge volumes, permits, the 2015/2020 timeline? Read our full guide: The General Binding Rules, explained for Cornwall homeowners.