Guide To Building An Extension Over A Drain

Posted by Tim Foster on Feb 11, 2020 11:32:17 AM

Guide To Building An Extension Over A Drain

Drains often cause concerns for people considering building an extension.

There are a lot of things to consider when planning to build near or over drainage. Click here to download our FREE essential guide to extending your home!

Type Of Drain

Records can be obtained from the water authority, but they only show the main drains in the road and a few drains in private properties.

Water authority drainage records are very limited. Locations of drains are only approximate, but it is a good starting point.

Many drains within private properties have been adopted by the water authority but they don’t have records.

There are 2 type of drain. Foul and surface water. Usually these are kept separate but sometimes you can have a combined drainage system.

It is important to ensure that separate drains are kept separate. When connecting into the system the first thing to do is determine if the drain is foul, surface or combined.

Connecting a foul drain into a surface water drain (or the other way around) is called a cross connection. This can either pollute the surface water or it can increase the amount of foul water to be treated. Both should be avoided at all costs.

The existing system can be tested by flushing a toilet in the house or running a hose pipe into a gutter and observing the flow in the manholes. If you don’t have manholes in your own property you may need to obtain permission from neighbours to access manholes in adjacent gardens.

Private Or Public

All drains within your property will be either private or public.

Any drain that passes through your property but also serves another property will be classed as public.

Any drain that solely serves your own property will be classed as private.

You can build over a private drain. Building control will inspect the pipework and approve the works as part of your extension.

A public drain is a different matter.

Public Drains

Public drains are maintained by your local water Authority. They are responsible for the drains and may need access in the future. Every public drain has an easement which is a protected area either side of the drain.

Usually 5m for a pipe over 300mm diameter or 3m for a smaller pipe.

You therefore need to apply to the water authority to build within the easement.

You or your designer should always approach the Water Authority to check the easement and confirm if an application is required in your property.

In most cases the pipes close to your house will be either 100mm diameter or 150mm diameter in which case you can submit an application called ‘Application to build over or close to a public sewer’.

This application is required if you intend to build over a public sewer or within 3m of a public sewer.

The application needs to include a CCTV survey of the pipework before any work starts. They will also ask for a further CCTV survey on completion to confirm that no damage has been done.

If the entire public drainpipe is replaced with new then you will not need the CCTV survey.

The application requires details of the pipe size, depth and relationship to the foundations.

All foundations should be below the invert of the pipe.

Lintols should be used where the pipe passes through a foundation with a compressible material around the pipe.

Pipes under a floor should be surrounded by pea gravel and capped in concrete.

In most cases this type of application will be approved for an extension. However, the water authority will not approve a build over agreement for a detached building such as a garage. In this case the drains will need diverting instead. This requires an application to divert a public sewer.

Manholes & Junctions

On a public drain you cannot have a junction under a floor. The water authority will not accept this as there is a risk of blockage. All drains from the house or extension will need to pass to the external area where they enter a manhole.

You will need at least one manhole within your property along the public sewer.

The water authority will not accept an internal manhole on a public sewer.

Where you wish to build from boundary to boundary this may cause you to reduce the width of the extension so that you can have a manhole along the public sewer next to the boundary.

Planning Your Extension

It is important to survey the position of any public drains before finalising the size of your extension.

The above factors may affect your options with regards to width or length of extension.

It is not possible to have a public drain running along the length of a foundation. The foundation can only cross the pipe at an angle.

In this case the pipe may restrict the size of the extension so that the foundation is running parallel to the pipe one side or the other. Your extension may have to be larger or smaller to accommodate the pipe.


It is possible to divert a public sewer. The costs increase with larger pipes. A 100mm or 150mm pipe is generally cost effective. Pipes above 300mm are generally expensive to divert and the water authority may not permit the diversion.

The above is only a guide. You should take advice from your local water authority before commencing any work on public drainage.

Our architectural designers at BDS Architecture can check this out for you and advise best design solutions for your specific property. We can also arrange the CCTV surveys and prepare the application on your behalf.

We can also offer free telephone advice if you just want to talk through your options.

To go through your options and a get clear understanding of what to do next, call BDS Architecture to speak to one of our advisers.

The Essential Guide To Extending Your Living Space

Topics: Extensions

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