August 23, 2025
Your guide to off-mains drainage and how to know what you have.
For homeowners who aren’t connected to the main public sewer, managing wastewater is a private responsibility. This is known as “off-mains drainage,” and it typically relies on one of two systems: a septic tank or a cesspit.
While they sound similar, they function in vastly different ways and have hugely different maintenance costs and legal responsibilities. Knowing which one you have is absolutely critical. So, if your property is off-mains, how do you tell the difference?
The First Clue: Your Water Bill
Before you even look in your garden, check your paperwork. Your water utility bill will have a detailed breakdown of your charges.
If you are connected to the public sewer, you will see a charge for “wastewater” or “sewerage services”.
If these charges are absent from your bill, it is the single biggest indicator that your property is off-mains and has its own private system.
What is a Septic Tank? The Mini Treatment Plant
A septic tank isn’t just a holding container; it’s a basic, private sewage treatment system.
How it works: Wastewater from your home flows into the tank. Solids sink to the bottom to form sludge, while fats and oils float to the top. The remaining liquid waste (effluent) then flows out of an outlet pipe and into a “drainage field” or “soakaway system,” where it is filtered naturally by the soil.
Key Identifier: A septic tank has an outlet. Because it treats and discharges liquid, it only needs emptying (desludging) periodically, typically once a year, to remove the solid waste.
What is a Cesspit? The Sealed Holding Tank
A cesspit is much simpler. It is a sealed, underground tank with no outlet.
How it works: It does nothing but collect and store all the wastewater from your property.
Key Identifier: A cesspit has no outlet. Because nothing can drain away, it needs to be emptied completely and very frequently. Depending on the size of the tank and the number of people in the house, this could be as often as every 4 to 8 weeks. The sheer frequency (and cost) of emptying is the most obvious sign of a cesspit.
Physical Clues on Your Property
Look for Lids: Both systems will have inspection covers or manhole lids in your garden. There might be one, two, or sometimes three lids clustered together. Frequency of Emptying: This is the easiest way to tell them apart. If you have a large tanker visiting to empty your system once a year, it’s a septic tank. If that tanker is a very regular visitor, it’s a cesspit. Signs of a Drainage Field: A septic tank must have a drainage field for the liquid to soak away. This area may not be obvious, but sometimes you can identify it as a patch of your garden where the grass is unusually green and fast-growing. A cesspit will not have this.
Why Knowing the Difference is Crucial
Cost: The running cost of a cesspit is many times higher than a septic tank due to the need for frequent emptying.
Legal Duties: Septic tank owners in the UK must comply with the Environment Agency’s “General Binding Rules,” which govern how the tank is used and maintained to prevent pollution.
Property Transactions: When buying or selling an off-mains property, the type, size, and condition of the drainage system is a major legal and financial consideration.
If you’re unsure about the system you have, or are considering buying a property off-mains, a professional inspection is essential. It can confirm the type of system, its condition, and whether it complies with current regulations.
