TL;DR
A septic tank settles solid waste and lets the treated liquid soak away through a drainage field, so it typically needs emptying about once a year. A cesspit has no outlet at all — it is a sealed tank that holds everything until it is pumped out, so it fills far faster and needs emptying much more often. The quickest way to tell which one you have is to look for a soakaway or a run of perforated pipe leading away from the tank; no outlet at all usually means a cesspit. If you are not sure which system serves your property, request a quote online and we will check it for you on site.
What's actually different between the two
A septic tank is a two-stage system. Solids settle to the bottom and are broken down slowly, while the liquid that separates out flows through a baffle and away into a drainage field, where the surrounding soil does the final treatment. Because liquid is constantly leaving the tank, only the settled sludge builds up, which is why emptying is an occasional job rather than a constant one.
A cesspit (sometimes spelt cesspool) is a much simpler idea: a sealed underground tank with an inlet and no outlet whatsoever. Everything that goes down the drain stays in the tank until a tanker removes it. There is no drainage field, no soakaway and no treatment — just storage. That single design difference is why cesspits fill so much faster than septic tanks and need emptying so much more often.
A third system, a package sewage treatment plant, sits between the two: it mechanically treats the effluent before discharging a cleaner liquid, usually where a drainage field isn't suitable or the site is close to a watercourse.
How to tell which one you have
Start in the garden. Follow the access cover and look for signs of a soakaway close by — a shallow, slightly raised or boggy strip of ground, or a visible run of gravel-backfilled trench. If you can see or trace a drainage field, you almost certainly have a septic tank. If the cover simply sits over a tank with no pipework leading away from it, that points to a cesspit.
Emptying frequency is another good clue. If a tank has been going a year or more between visits, it is behaving like a septic tank. If it needs pumping out every few weeks, particularly in a household with several people, that is classic cesspit behaviour, because the whole volume has to be removed each time rather than just the settled sludge.
Paperwork can settle any doubt. Building control completion certificates, planning permission notes, home reports from when you bought the property, or records from Wessex Water can all state which system is installed. Where none of that survives, a CCTV inspection of the chamber and any connecting pipework will confirm it.
Why the difference matters for cost and maintenance
The type of system you have is the single biggest factor in how often you need to budget for emptying, because it changes what is actually being removed each time — sludge from a septic tank, or the entire contents of a cesspit. Tank size, access for the tanker and the distance to a licensed disposal site also affect each visit, which is why a fair quote is always based on your actual property rather than a flat rate. Our guide to septic tank emptying costs in Somerset sets out what genuinely moves the price for both systems.
The 2020 rules and what they mean for each system
Since January 2020, the Environment Agency's General Binding Rules have required any septic tank that discharges directly to a watercourse — a ditch, stream or the River Avon itself — to be upgraded, typically to a package treatment plant or a proper drainage field. Cesspits are unaffected in one sense, because they have no discharge at all, but they are more tightly regulated in other ways precisely because everything they hold has to leave site by tanker. Knowing which system you have is the first step to knowing which rules actually apply to you.
Common around Bath's off-mains villages
Off-mains drainage is a fact of life in the villages south of Bath, where the mains sewer network run by Wessex Water simply doesn't reach. Properties in Wellow and Farmborough, along with neighbouring hamlets, commonly rely on a septic tank or a cesspit rather than a connection to the public system, and older properties in particular may have inherited a setup nobody has fully documented.
Frequently Asked Questions
**What's the difference between a septic tank and a cesspit?**
A septic tank treats wastewater and discharges the liquid to a drainage field, so it only needs the settled sludge pumped out occasionally. A cesspit is a sealed tank with no outlet at all, so it holds everything and needs emptying much more frequently.
**How can I tell if I have a septic tank or a cesspit?**
Look for a soakaway or drainage field near the tank — if you can see or trace pipework leading away from it, it's a septic tank. A tank with no visible outlet, and one that needs emptying every few weeks rather than roughly once a year, is almost always a cesspit.
**Do septic tanks and cesspits need emptying at the same frequency?**
No. A septic tank only needs its settled sludge removed, so an average household empties one about once a year. A cesspit has to have its entire contents removed every time, which is why it fills and needs emptying far more often.
**Does the 2020 septic tank rule apply to cesspits too?**
The General Binding Rules introduced in January 2020 specifically target septic tanks that discharge to a watercourse. A cesspit has no discharge point, so that particular rule doesn't apply in the same way, though the waste it holds must still be removed by a registered carrier and taken to a permitted site.
