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Do I need a build over agreement?

What is a build over agreement?

A build over agreement is a contract between you and your sewage provider, when you’re building over, or near a sewer. It’s in place to protect the sewers and drains supplying you, and your neighbours. The agreement also makes sure Thames Water has access to maintain or repair the sewer network.

You shouldn’t start construction work before getting an agreement. We might need to make changes to your design, which could cause you additional costs or delays. Planning permission does not grant you permission to build over a sewer or drain.

Do I need a build over agreement?

Sewers often run beneath back gardens, particularly in urban areas. It’s likely that you’ll need to make an application if you’re building within 3 metres of a public sewer, or within 1 metre of a public lateral drain. 

For extensions to an existing property, if your sewer has a diameter of 150mm/6 inches or less, you can potentially apply for a free self-certify build over agreement.

If you’re building a new property, you must apply to have the sewer diverted rather than a build over agreement (including new car ports or sheds that are not attached to your property). 

Liability for any damage caused to the sewers or drains during the construction lies with you.

See the full self-certify specification.

How do I know if there’s a sewer or drain in my property boundary?

Your builder or a drainage specialist can conduct a survey to identify location and size of sewers or drains within your property boundary.

If you'd like to review maps detailing our infrastructure and assets, you can view these free of charge by calling 0345 263 7332 or contacting us by email.

You'll be offered an appointment to visit our public online access terminal. The terminal is located at:


Thames Water
Clearwater Court, 
Vastern Road,

Online sewer maps are also available to purchase from all utility searches companies, or from our property searches department.

If your sewers aren’t mapped, this video will help your builder identify sewers in your property boundary.

Am I eligible for a build over self-certificate?

You may be eligible for a self-certify build over agreement if:

  • Your application relates to a domestic property
  • The sewer you’re building over is 150mm/inches or less
  • You do not need to relocate a manhole 

See the full self-certified build over agreement specifications.

It’s important that you answer the questions thoroughly and honestly. Incorrect answers may result in your agreement being invalid. You may have issues selling your property without a valid build over agreement.

Download self-certification declaration

You don't need to print the form, you can type your answers into the boxes provided. Once completed, please email your application form to

If you have any questions, we recommend asking your builder or architect.

If you’re not eligible for a self-certified build over agreement, read more about how to make a full build over application.