On most properties when you are planning an extension whether it be to the rear or the side of your house you will need to get the build over agreement from your water company, in the area we cover this is normally Thames Water.

The water company own the sewer pipes that run onto your land and if you are going to be building near or over these you will need to seek their permission in order to get your extension signed off by building control.

In most cases your proposed extension doesn’t need to be compromised to accommodate this agreement but it is important to look into this before you start the build as it could result in changing the size or shape of your extension which will result In you having to submit a new planning application.

What is a build over agreement?

It should really be a Build Over or Building Near Agreement. As the water company own the pipe works if your extension is being built within 3 metres of a sewer pipe or is being built directly over the pipe you will need to get the permission for the works from Thames Water.

The main requirement for doing this is that if the drains become blocked Thames water will need access to the sewer pipes and they also want to know that measures will be put in place to protect their pipe work.

On some detached properties it is possible to build an extension without the need for this agreement as your extension could be over 3 metres away from the sewer pipe. On most semi-detached and terraced properties, you will have to apply. Of course, if you’re planning a rear extension and all the drainage is at the front of the house you can avoid this, also loft conversions do not require an application to be made.

If you have a manhole within the footprint of your planned extension or within 3metres it is likely you will need to make an application. However, if the drainage only serves your plot then this is classed as a private drain and does not need permission to build over.

If you want to check this you can lift your manhole, have a look at what direction the pipes run, if they look like they head towards your neighbour’s house it would suggest that it is a public drain.

You can request plans from Thames water which show roughly where your drainage is on your plot, they don’t always have records but can be worth a phone call to check.

How do I apply for a Thames water build over agreement?

You will need to fill out a form which can be found on the Thames Water website. You will also need plans to show where the drain runs and where your extension will be located in relation to the sewer pipe.

They will also require a section showing how deep the sewer pipe is and how close it will be to your proposed extensions.

When we carry out our measured site surveys we will locate any manholes on your site and measure where these are in relation to your property, we will also lift the manholes and make a note of where the pipes run, with this information we can make the application to Thames Water for you.

When do I need to apply for a build over?

As you will need plans of the existing house and the proposed extension it is best to make the application once you have secured your planning permission.

The reason for this is if you get the agreement first and then have to make changes to the plans during the planning process you may then need to re-apply for the build over agreement as the proposals would have changed.

What if I haven’t applied for a build over and I have already started building?

More often than not this is not an issue although it is best to consider it when designing your extension to make sure your extension can be built to comply with the requirements set out by Thames Water. Normally you can meet these requirements without having to make changes to you design.

You can still apply for the agreement during the build or even after works have been completed. It will just mean that you cannot get the extension signed off by building control unless this agreement is in place.

Depending on workloads once an application is decided is issued within 2 weeks so doesn’t have to add huge delays to your build.

How much a build over agreement cost?

Thames water currently charge £299 + VAT to make the application. In terms of the cost to the build it does not have a huge impact. View the application here.

It normally results in an existing manhole being blocked up and replaced with a Y section of pipe which runs to a new manhole or inspection chamber which will need to be located outside of the extension.

If the extension is over a sewer pipe you will need to have a break in the foundations and a concrete lintel installed to bridge the gap in the footings.

The largest cost will arise if one of your new walls is running directly above the sewer pipe, if this is the case you can end up needing to have large pad foundations and ground beams rather than traditional strip foundations which will add a fair amount to the ground works package and increase the length of the build slightly.

If this is the case, you will likely need to engage a structural engineer to calculate the size and strength of any required steelwork.