← All guides · Reviewed 2026-08-05
Cesspit vs septic tank: what's the difference?
People use “cesspit”, “cesspool” and “septic tank” as if they mean the same thing. They don’t, and the difference is expensive.
A cesspit stores. A septic tank treats. Everything else follows from that.
The actual difference
A cesspit (or cesspool — same thing) is a sealed underground tank with no outlet. Every drop of wastewater your house produces goes in and stays there until a tanker takes it away. It does not treat anything. It is a holding tank.
A septic tank has an outlet. Solids settle out and begin to break down; the liquid effluent flows out to a drainage field, where the soil finishes the treatment. It is a two-stage system, and the ground is the second stage.
That single structural difference — outlet or no outlet — is what drives everything below.
Why it matters for cost
Because a cesspit holds everything, its emptying frequency is set by how much water your household uses, not by how fast sludge builds up. A family that showers daily and runs a washing machine fills a cesspit steadily and predictably.
The practical result: cesspits typically need emptying several times a year, where a septic tank on similar occupancy is usually around once a year. Each visit is a tanker job at broadly similar cost, so the running-cost gap is large and it recurs forever.
If you are buying a rural property and the survey says “cesspit”, budget accordingly. Our cost calculator covers emptying costs, and how often should you empty a septic tank explains the frequency side for tanks specifically.
Why it matters legally
This is where people get caught out.
The General Binding Rules — the Environment Agency regime most off-mains guidance talks about — govern discharges to ground or surface water. A cesspit does not discharge. It has no outlet. So the General Binding Rules, in the sense that applies to septic tanks, do not apply to it.
That does not mean a cesspit is unregulated. It means the duty is different:
- It must be watertight. A leaking cesspit is polluting groundwater.
- It must be large enough for the property it serves.
- It must be emptied often enough that it never overflows. An overflow is a pollution offence, and “the tanker was late” is not a defence.
For a septic tank the duties are the ones set out in the General Binding Rules: a compliant discharge, a properly functioning drainage field, and — since 2020 — no discharge directly to a watercourse.
Different systems, different rules. Advice written for one does not transfer to the other.
Why it matters when you sell
Whichever you have, you must tell the buyer, and you must be able to say what it is. A septic tank discharging to a watercourse has to be upgraded or replaced on sale under the 2020 change. A cesspit does not fall under that rule — but a buyer’s surveyor will price in its running costs, and an undersized or leaking one will come up.
The most common problem we see is not non-compliance. It is uncertainty — an owner who genuinely does not know which system the property has, because nobody told them when they bought it.
How to tell which one you have
- Look for a drainage field. A septic tank discharges to one — usually a run of perforated pipe in gravel, often visible as a strip of ground that greens up differently or stays wetter. No drainage field anywhere on the plot points to a cesspit.
- Count the chambers. Septic tanks are typically two- or three-chamber. A cesspit is a single sealed vessel.
- Check the emptying history. Several empties a year on a normal household strongly suggests a cesspit. Roughly annual suggests a septic tank.
- Check the deeds and the last survey. They often say, and it is the cheapest place to look.
If it is still unclear, that uncertainty is worth resolving before it becomes a sale problem rather than after.
The short version
| Cesspit | Septic tank | |
|---|---|---|
| Outlet | None — sealed | Yes, to a drainage field |
| Treats sewage? | No, stores it | Partially, then the soil finishes it |
| Typical emptying | Several times a year | Around once a year |
| General Binding Rules | Do not apply (no discharge) | Apply |
| Main legal duty | Watertight, never overflows | Compliant discharge + working drainage field |
Not sure where you stand? The free compliance checker walks through your system and tells you what the rules require of it.
Sources: GOV.UK — Permits you need for septic tanks · GOV.UK — General binding rules: discharge to the ground
Common questions
Is a cesspit the same as a septic tank?
No. A cesspit is a sealed holding tank with no outlet — everything stays in it until it is emptied. A septic tank has an outlet and partially treats wastewater before discharging it to a drainage field. They have different costs and different legal duties.
Do the General Binding Rules apply to a cesspit?
No. The General Binding Rules cover discharges to ground or surface water, and a cesspit does not discharge — it stores. A cesspit must instead be watertight, big enough, and emptied often enough that it never overflows. Overflow is a pollution offence.
Is a cesspit more expensive to run than a septic tank?
Almost always, yes. A cesspit holds every litre you use, so emptying frequency is driven by household water use rather than sludge build-up — often several times a year against roughly annually for a septic tank.
Can I replace a cesspit with a septic tank or treatment plant?
Often, but not always. It depends on whether your ground can take a drainage field (a percolation test decides this) and how close you are to a watercourse. Where ground conditions rule out a drainage field, a sewage treatment plant discharging to a watercourse may be the alternative. Both routes need proper assessment.
Need this sorting in Cornwall?
Kernow Septic covers Cornwall's towns and rural parishes. Call or run the free tools.