Can I connect to mains drainage if I have a septic tank?

There are many things which can affect whether you can connect a property with a septic tank to mains drainage, and other factors which you’d need to consider even if it is an option for you.

Here we’ll run through some of the reasons my you might want to connect your property to a main sewer, and the things which might influence whether it will be possible to do so. Whilst septic tanks are the most common type of off mains drainage system, this will also apply to other types of systems such as sewage treatment plants and cesspits.

There may be a number of reasons why a property owner who currently has a septic tank may want to connect to a main sewer:

Has your existing septic tank, sewage treatment plant failed?

Off mains drainage systems such as septic tanks and sewage treatment plants need some sort of outlet for the wastewater to discharge to. Both types of drainage tanks provide a certain amount of treatment of wastewater, to bring it up to an acceptable level so that it can be discharged locally. This will usually be through one of two main methods:

1. Pipework leading from the tank straight to a local ditch or watercourse (this is now only legal for sewage treatment plants).
2. Pipework leading from the tank to a drainage field or other permitted soakaway system.

If your property’s drainage system has failed, and this in turn has caused the drainage field or soakaway system that it discharges into to fail, it is typically not possible to repair it or to replace it in situ. This means that an additional footprint for a new drainage system will be required at the property, with the ground conditions appropriate for a new drainage field, and in some cases, this may not be possible. Other alternatives would need to be considered – such as other off mains drainage systems which would work within a smaller footprint, or possibly a connection to a main sewer.

If your property is now within 30m of an existing main sewer

It may be that when your off mains drainage system was installed, there were no main sewers in close enough proximity to the property. However, if one has since been installed, building regulations state that you must request permission from the local water and sewerage company to connect to the main sewer if any part of the building your treatment plant serves is within 30 metres of one.

It's worth noting that if your property shares an off mains drainage system with any other properties, the 30 metre requirement is multiplied by the number of properties that use it. So, if three properties connect to the off mains system, the rule would be that if any part of the 3 properties it served was within 90 metres of a main sewer, you must apply to be connected to it.

If your property already has an off mains drainage system such as a septic tank, treatment plant or cesspit, one of the key reasons for this is likely to be lack of proximity to a main sewer. Off mains drainage systems are self-contained, and typically used in more rural locations where perhaps a main sewer is too far away to make connecting into it a viable option. Typically, a septic tank or other off-mains drainage system is the most appropriate but also the only possible drainage solution for a rural property.

However, over time the shape of towns and villages can change, and additional property developments can bring utilities such as drainage within reach of more rural properties. It’s possible that a new development has been built locally, part of which includes creating a main sewer to serve the new properties, and this may present an opportunity to connect into it.

In the first instance, you can find out where main sewers are in your area by visiting your local water and sewerage company website. You will usually be able to access maps of local sewer pipework, often through a third-party website.

If there is a main sewer nearby, it doesn’t always mean that you will be able to gain permission to connect to it. There needs to be sufficient capacity within the system to allow for an additional connection, and there may be stringent criteria that need to be adhered to if you want to connect.

The only way to know whether a connection to a main sewer is possible is through submitting a detailed application to the water and sewerage company in the first instance, which would need to include a plan of the local area including the property and the location of the main sewer.

Connecting a property with a septic tank to mains drainage can be costly. There are lots of things which can affect how much it would cost specifically to connect your property, such as:

• The distance from your property to the main sewer, and therefore the extent of any excavation required.
• If the proposed pipework needs to be installed in a highway, this can involve traffic management and in some cases a road closure.
• The depth of your property’s drainage pipework compared to the depth of the main sewer. This is important because a connection to the main sewer can either be gravity fed (the depths are such that the waste can reach the main sewer under its own weight) or pumped in which the waste requires a pumping station to push it towards the main sewer. If a pumped solution is needed, a pump chamber is installed within the property boundary which needs a permanent power supply.

Your ongoing sewerage costs will be different moving forwards, and this can vary from property to property. You will no longer have to cover the costs of emptying your off mains drainage system, and any maintenance such as servicing which comes with it. If your off mains system is old and/or not in good condition, the costs of replacing it can be significant. However, by connecting to a main sewer instead, you will become liable to sewerage rates – you can find out what you might pay by contacting your local water and sewerage company, or by visiting their website.