The short answer
A cesspit is a sealed holding tank with no outlet: everything that goes in stays there until a tanker takes it away. A septic tank separates solids and lets the liquid flow out to a drainage field, so it only needs emptying about once a year. That makes a cesspit cheap to install but very expensive to run.
On this page
People use “cesspit”, “cesspool” and “septic tank” as if they meant the same thing, and estate agents’ particulars do not always help. They are different systems with very different running costs, and if you are buying a house off mains drainage it is worth knowing which one you are taking on.
How a cesspit works
A cesspit, or cesspool, is a sealed underground tank. Waste water from the house flows in and stays there. There is no outlet, no treatment and no soakaway. When the tank is full, a tanker empties it.
Because nothing leaves the tank, a cesspit is not a “discharge” under the environmental rules. The Environment Agency’s general binding rules, SEPA registration and Natural Resources Wales registration do not apply to it in the same way. What matters is that it does not leak and that it is emptied before it overflows.
Approved Document H sets a minimum cesspool capacity of 18,000 litres for two people, plus 6,800 litres for each extra person. That sounds large, but a household of four can fill it in a matter of weeks.
How a septic tank works
A septic tank is a two or three chamber tank. Sewage enters the first chamber, where solids sink to form sludge and fats float to form a crust. Bacteria break down some of the solids without air. The liquid in the middle flows on, through the next chamber, and out of the tank to a drainage field.
The drainage field is where the real treatment happens: effluent seeps from perforated pipes into gravel-filled trenches and then into the soil, where bacteria and oxygen clean it. The septic tank itself only needs emptying of its sludge, usually once a year.
Approved Document H sets a minimum septic tank capacity of 2,700 litres for up to four people, plus 180 litres for each extra person.
The differences side by side
| Cesspit | Septic tank | |
|---|---|---|
| Outlet | None, sealed | To a drainage field (or, in England and Wales, never to a watercourse) |
| Treatment | None | Settlement in the tank, then treatment in the soil |
| Emptying | Every few weeks for a family | About once a year |
| Running cost | High | Low |
| Ground needed | None beyond the tank | Ground that passes a percolation test |
| Discharge rules | Must not leak; no discharge | General binding rules, SEPA or NRW |
Running costs
This is where the two part company. A septic tank emptied once a year costs, in published UK guides, roughly £120 to £350 a visit. A cesspit for a family can need emptying many times a year, and each visit is a full tanker load. Over a few years the emptying bill on a cesspit can exceed the cost of replacing it with a septic tank and drainage field or a treatment plant.
When a cesspit makes sense
Cesspits are mostly used where nothing else is possible: ground that will not take a drainage field, no watercourse for a treatment plant outlet, or a temporary building. Building control generally treats them as a last resort. NetRegs notes that the regulators prefer a sewer connection first, then a treatment system, with a cesspool only where neither is practical.
They can also suit buildings with very low or occasional use, where the tank fills slowly.
Replacing a cesspit
Many owners replace a cesspit to cut emptying costs. The usual options are:
- A septic tank and drainage field, if the ground passes a percolation test and there is room for the field.
- A sewage treatment plant to a watercourse that flows all year, or to a smaller drainage field.
- A sewer connection, if a public sewer has come within reach.
Any of these is a new discharge, so it must meet the current rules where you live and needs Building Regulations approval. Our septic tank replacement page covers the process, and the cost estimator gives a typical price range by bedrooms.
How to tell which one you have
If you are not sure:
- Ask the seller or check the property information form, which should describe the drainage.
- Look for an outlet pipe leaving the tank. A cesspit has only an inlet.
- Check the emptying receipts. Frequent visits point to a cesspit.
- Have a septic tank survey done, which will trace any outlet.
Is one better than the other?
For most homes, a septic tank (or a treatment plant) is the better system: cheaper to run, and it treats the waste rather than storing it. A cesspit is a holding tank, useful only where the ground or the location rules out everything else.
If you have a cesspit and the tanker bills are mounting, a septic tank installation company can tell you whether your ground will take a drainage field. Use the form below to ask for a site visit.