Expert Guide

Off mains drainage

This Guide to Off Mains Drainage will outline the different types of drainage systems available, the differences between them and the things you might need to consider.


What is off mains drainage?

In order for wastewater to be taken away from your property, it will be connected either to a network of pipes leading to a main sewer, or to its own private drainage system, also known as off mains drainage. It is estimated that approximately 4% of properties in the UK have off mains drainage systems, and these properties are typically situated in more rural locations where a connection to a main sewer is not possible.

Our Guide to Off Mains Drainage will outline the different types of drainage systems available, the differences between them, and the things you might need to consider.


What are the different types of off mains drainage systems?

An off mains drainage system typically comprises two main parts; a tank, and some sort of pipework or soakaway arrangement into which wastewater from the tank passes (the only exception to this being a cesspit, which is a tank with no outlet).

These are the three different types of tanks:

  1. Septic Tanks
  2. Cesspits
  3. Sewage Treatment Plants

Septic Tanks

A septic tank is the most common type of off mains drainage tank, and the majority of properties with off mains drainage will have one.

Septic tanks can come in various different shapes, sizes and designs - some are brick built, some are made of concrete, and more modern ones are made of glass reinforced plastic.

A septic tank does not have any mechanical parts, and it doesn't perform any 'treatment' of the waste. Instead, the waste enters the tank from the property, then separates into three different layers. The middle layer of separated waste water then leaves the tank, typically into a soakaway system.

A soakaway system is typically a network of slotted or perforated pipes, which allow the waste water to percolate through the sub soils without causing a pollution. There are other types of soakaway system, and you can find out more in our Guide to Soakaways.

The remaining contents of the tank are then removed at regular intervals by a tank emptying company.

Some septic tanks discharge through pipework straight to a watercourse or ditch. Property owners with this type of drainage system should be aware that discharging from a septic tank straight to a watercourse or ditch is no longer allowed, and legislation dictates that these types of systems must be upgraded or replaced.

You can find out more about the regulations affecting septic tanks in our Off Mains Drainage Regulations Guide.



A cesspit is literally just a holding tank for the waste from your property. Waste goes in and stays there until the tank is emptied. There is no treatment of the waste that goes in.

Cesspits are not very common for a couple of reasons - firstly because they pose a greater threat to the environment if they fail, but also because they require a lot of emptying which can become expensive for the property owner. Generally speaking, a cesspit is seen as the last resort for a property’s drainage system, if the size or nature of the land available does not allow for a septic tank or sewage treatment plant to be installed.


Sewage treatment plants

A sewage treatment plant is the most modern form of off mains drainage tank.

Powered by an electricity supply, a sewage treatment plant provides treatment of the waste. This means that the waste water that leaves the tank is cleaner than that which would come from a septic tank. For this reason, it is allowed for a sewage treatment plant to discharge straight to a ditch or watercourse.


What maintenance is required for off mains drainage?

You won't have to pay sewerage rates once you own a property that has off mains drainage. Sewerage rates can vary but are typically around £400-£500 per year. However, you will need to have your tank emptied regularly.

As a general rule of thumb, most septic tanks and sewage treatment plans need to be emptied annually. There are huge regional variations in prices for tank emptying, but most prices are somewhere between £120-£250. If you have a cesspit, you'll be looking at needing to get the tank emptied between 4 and 8 times a year.

In addition to regular emptying, sewage treatment plants also need regular servicing of the moving parts - this is usually done annually and usually costs around £150.

It is important to understand what type of drainage system you have in place at your property, as this will dictate the amount of emptying the system requires, and whether or not there are any regulations which affect it.

You can find out more in our Guide to What Type of Drainage System You Have.

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