If your property isn’t connected to the mains sewer, your wastewater has to go somewhere. For most rural and semi-rural homes in the UK, that means either a cesspit or a septic tank buried in the garden. Both handle sewage, but they do it in very different ways, and the running costs, maintenance demands, and legal obligations that come with each one are worlds apart.
Understanding the difference matters whether you’re buying a property with an existing system, building a new home, or thinking about upgrading what you’ve already got. Get it wrong and you could be looking at thousands of pounds a year in emptying costs, or worse, a system that breaks environmental regulations.
What is a cesspit?
A cesspit (also called a cesspool or cesstank) is a sealed underground tank that collects and stores all the wastewater from your property. Everything that goes down the sinks, toilets, baths, and washing machines ends up in the cesspit, and it stays there until a tanker comes to pump it out.
That’s all a cesspit does. It doesn’t treat the waste, doesn’t filter it, and doesn’t discharge anything into the surrounding soil. It’s simply a holding tank. When it fills up, you call a licensed waste carrier who arrives with a vacuum tanker, empties the entire contents, and takes it away for treatment at a sewage works.
For a typical household of three or four people, a cesspit will fill up surprisingly quickly. Most families find themselves booking an emptying every four to six weeks, sometimes more often if the tank is on the smaller side. That frequency is the single biggest drawback of owning a cesspit, because every emptying costs money.
What is a septic tank?
A septic tank also sits underground and receives all the wastewater from your property, but unlike a cesspit, it actually treats the waste before discharging the liquid portion. The tank is split into two chambers. Raw sewage enters the first chamber, where solids settle to the bottom and form a layer of sludge. Lighter material like grease and soap scum floats to the top as a crust.
The liquid in the middle, called effluent, flows through a dividing wall or baffle into the second chamber, where further settling takes place. Anaerobic bacteria (bacteria that work without oxygen) break down organic matter throughout the process, partially treating the wastewater. The clarified effluent then flows out of the tank through an outlet pipe and into a drainage field or soakaway, where it percolates through the soil for final treatment.
Because the liquid is constantly leaving the tank, a septic tank doesn’t fill up anywhere near as fast as a cesspit. You still need it emptied periodically to remove the accumulated sludge, but that’s typically once every one to three years rather than every month.
The key differences
The fundamental difference is treatment. A cesspit stores everything and treats nothing. A septic tank partially treats the waste and discharges the liquid. That one distinction drives almost every other difference between the two systems.
Emptying frequency
A cesspit needs emptying roughly every four to six weeks for an average household. A septic tank needs desludging every one to three years. Over the course of a decade, that’s the difference between around 100 tanker visits and perhaps four or five.
Running costs
Each cesspit emptying costs around £300 to £600 depending on the tank size and your location. If you’re emptying monthly, that’s somewhere between £3,600 and £7,200 per year. Septic tank emptying costs a similar amount per visit, but because you only need it every one to three years, the annual cost is a fraction of what a cesspit demands.
Installation cost
Cesspits are generally cheaper to buy and install because they’re simpler, just a sealed tank with an inlet. A basic cesspit might cost £1,000 to £3,000 installed depending on size and ground conditions. Septic tanks cost more upfront because they have internal baffles, two chambers, and require a drainage field to be constructed alongside them. A full septic tank system with drainage field typically costs between £3,000 and £8,000 installed, though the exact figure depends on the tank size, soil conditions, and the length of the drainage field needed.
Environmental impact
Cesspits carry a higher environmental risk because they store large volumes of untreated sewage underground. If the tank develops a crack, the seal fails, or it overflows because an emptying was missed, raw sewage can leak into the surrounding soil and contaminate groundwater. Septic tanks release treated effluent gradually through a drainage field, which is a controlled and regulated process that, when working properly, poses far less risk to the environment.
Space requirements
Cesspits need to be large enough to hold all of a household’s wastewater between empties. A family of four might need a tank of 18,000 litres or more. Septic tanks can be smaller because liquid is continuously leaving through the outlet, but they need additional space for the drainage field, which can be 30 to 50 metres of perforated pipe laid in trenches across the garden.
UK regulations you need to know
Off-mains drainage is regulated, and the rules differ slightly depending on where you live in the UK.
England and Wales
The General Binding Rules, introduced in 2020, set out the requirements for septic tanks and small sewage treatment plants. Under these rules, septic tanks must discharge to a drainage field (not directly to a watercourse), the system must be maintained and emptied regularly, and the drainage field must be designed to handle the volume of effluent your household produces. If your septic tank currently discharges into a ditch, stream, or river, you’re required to either replace it with a sewage treatment plant or connect to the drainage field. The Environment Agency enforces these rules.
Cesspits are legal in England and Wales, but under the Public Health Act 1936, allowing a cesspit to overflow or leak is a criminal offence. You’re responsible for ensuring it’s in good condition, properly maintained, and emptied before it reaches capacity. Local authorities can serve notice requiring you to repair or replace a cesspit that poses a health or environmental risk.
Scotland
Here’s where it gets different. Cesspits are illegal in Scotland. If your Scottish property currently has a cesspit, you’re required to replace it with a septic tank and drainage field, or a sewage treatment plant. SEPA (the Scottish Environment Protection Agency) regulates off-mains drainage north of the border, and their rules are stricter than those in England and Wales. All new septic tank installations in Scotland need to meet specific standards and may require a CAR (Controlled Activities Regulations) licence from SEPA.
Buying a property
If you’re buying a home with a cesspit or septic tank, get it surveyed before you exchange contracts. A drainage survey can tell you the condition of the tank, whether it’s compliant with current regulations, and what it’ll cost to maintain. Properties with cesspits come with a significant ongoing running cost that should be factored into your budget, and properties with older septic tanks may need upgrading to meet the General Binding Rules if they currently discharge to a watercourse.
When a cesspit might be the right choice
Despite the higher running costs, there are a few situations where a cesspit makes practical sense:
- The soil on your property has very poor percolation (heavy clay) and can’t support a drainage field
- You’re in a location where the water table is too high for a septic tank to discharge safely
- The property is a holiday home or second residence that’s only occupied for a few weeks a year, meaning the emptying frequency is much lower
- Space is extremely limited and there isn’t room for a drainage field alongside a septic tank
For a full-time family home, though, the maths rarely works in a cesspit’s favour. The ongoing emptying costs will outstrip the savings on installation within the first year or two.
When a septic tank is the better option
For most permanently occupied properties without access to the mains sewer, a septic tank with a properly designed drainage field is the more sensible long-term choice. The upfront cost is higher, but the dramatically lower running costs mean it pays for itself relatively quickly. A household spending £5,000 per year on cesspit emptying would recoup the cost of a septic tank installation within one to two years of switching.
Septic tanks are also the system that regulators prefer. The General Binding Rules are designed around septic tanks and treatment plants, not cesspits, and the direction of travel in UK drainage regulations is clearly towards systems that treat wastewater rather than just storing it.
What about sewage treatment plants?
There’s a third option worth mentioning. A sewage treatment plant (sometimes called a package treatment plant) goes further than a septic tank by actively aerating the wastewater, which encourages aerobic bacteria to break down organic matter more thoroughly. The effluent that comes out of a treatment plant is clean enough to discharge directly into a watercourse like a stream or ditch, which means you don’t need a drainage field at all.
Treatment plants cost more than septic tanks (typically £4,000 to £12,000 installed), but they’re the only legal option if your property can’t support a drainage field and you need to discharge to a watercourse. They’re also required as a replacement if your existing septic tank currently discharges to a river or stream, under the 2020 General Binding Rules.
Frequently asked questions
Is a cesspit the same as a cesspool?
Yes. Cesspit, cesspool, and cesstank all refer to the same thing: a sealed underground tank that stores wastewater without treating it. The terms are interchangeable, though “cesspit” is the most commonly used in the UK.
How do I know if I have a cesspit or a septic tank?
The easiest way to tell is to check whether your system has an outlet pipe leading to a drainage field. A cesspit has only an inlet and no outlet, because it’s a sealed tank. A septic tank has both an inlet and an outlet. If you’re not sure, a CCTV drain survey can confirm what you’ve got and check its condition.
Can I convert a cesspit into a septic tank?
Not directly. The two systems are built differently, and a cesspit can’t simply have an outlet pipe added. Converting from a cesspit to a septic tank means installing a new tank and constructing a drainage field, which is a full installation project. However, the long-term savings on emptying costs usually justify the investment.
How long does a septic tank last?
A well-maintained septic tank can last 20 to 40 years or more. Concrete tanks tend to last longer than plastic ones, though both will eventually need replacing. Regular desludging, avoiding pouring fats and chemicals down the drain, and keeping the drainage field in good condition all extend the tank’s lifespan.
What happens if my cesspit overflows?
A cesspit overflow is a serious problem and a legal offence under the Public Health Act 1936. Raw sewage can contaminate your garden, neighbouring properties, and local watercourses. If your cesspit overflows, you should call a tanker for an emergency emptying immediately and then have the tank inspected to check it hasn’t cracked or lost its seal. Your local authority can take enforcement action if a cesspit is causing pollution.
Getting professional advice
Whether you’re maintaining an existing system, upgrading a cesspit, or installing off-mains drainage from scratch, getting a professional assessment is the sensible first step. Clark Drainage carries out drainage surveys and drainage installations across the UK, and our engineers can advise on the right system for your property, soil type, and budget.
If you’re unsure what’s buried in your garden or whether your current setup meets regulations, we can survey the system and give you a clear picture of where you stand. Call us on 0800 085 1219 for a free quote, or visit our contact page to book a survey.
