Off-mains drainage guide

Many thousands of properties – domestic homes and public or commercial buildings – rely on off-mains drainage systems to store, process and disperse wastewater.

Our off-mains drainage guide helps you understand how they work, how they must be maintained and how you can ensure they comply with regulations.

Click on one of the boxes to see a specific answer or scroll down to see all questions and answers in this guide.

What is off-mains drainage?

Off-mains drainage systems are installed where it is not practical to connect a property to the mains wastewater and sewage treatment system.

In most cases, each property will have its own wastewater storage and treatment system, with at least a proportion of the waste having to be collected at regular intervals for disposal at an authorised waste site.

Owners of properties with off-mains drainage systems are legally responsible for ensuring waste, including effluent and solid waste, is managed and disposed of in accordance with all relevant health and safety and environmental regulations.

These duties can be contractually passed to other parties, for example, those who lease or rent the property. Failure to fulfil these responsibilities can result in prosecution leading, potentially, to heavy fines and / or imprisonment.

In what areas are off-mains drainage systems most commonly used?

Most off-mains drainage systems are located in more remote rural locations, away from the wastewater network that collects sewage for processing at treatment plants. In some cases, a property (which may be a dwelling, a public building or a commercial property) may be close to a main sewer but there could be specific technical and geographical reasons why it would be very costly to connect it to the network.

Some properties located in urban areas that have a mains drainage network retain their off-mains drainage systems because it is the choice of their owners to do so.

Do property owners have to connect to the main sewer network if they can?

No, they do not. As the sewer network has developed, and expanded, properties with off-mains drainage systems will have had the opportunity to connect to it. However, as long as their off-mains system complies with regulations, they can choose to keep it, perhaps because they do not want to incur the connection cost.

What are the main types of off-mains drainage systems

There are three main types of off-mains drainage systems – cesspits (also known as cesspools), septic tanks, and sewage treatment plants, also known as small sewage treatment plants or mini sewage treatment plants.

What are cesspits?

A cesspit, also referred to as a cesspool, is a single underground holding tank with one opening in which wastewater (liquid waste and sewage) is temporarily stored.

The wastewater is not treated in a cesspool. Instead, all the liquid and sludge stored in one has to be regularly pumped out and taken away for safe treatment and disposal.

This increases the frequency with which the tank has to be emptied, compared with other off-mains tank systems (septic tanks and sewage treatment plants). In turn, it can make this off-mains drainage system more costly to maintain.