Build over agreements

Thames Water is responsible for all public sewers and lateral drains in our region. Private drains are the responsibility of the landowner.

You’ll need to apply for a build over agreement if you’re building:

• Within 3 metres of a public sewer or lateral drain
• Within 1 metre of the point where a pipe serving only your property crosses your boundary (where it becomes a public lateral drain)

This diagram shows the difference between public and private drains and public sewers.

Thames Water is responsible for all public sewers and public lateral drains in our region. We need to be certain your building work doesn’t restrict our ability to maintain and repair these pipes. And we also need to make sure the extra weight of your new building doesn’t damage the pipe.

Without a build over agreement, you may not be granted a Building Regulations completion certificate, which may cause you problems when selling your property. Planning permission may also be required from the local planning authority.

We have legal powers under the Water Industry Act 1991, giving us the right to maintain, repair, replace and renew public sewers. If any building work limits this, we could seek an injunction to remove the building and potentially recover damages.

Manholes

If there’s a manhole in the area you intend to build on, you’ll need to relocate it. Internal manholes aren’t permitted, to protect your property from odours and an increased risk of flooding. You must complete the build over application form to relocate a manhole.

Strategic sewers

Strategic sewers are the most important in our wastewater network and we’re unlikely to consent to building over them.

Rising mains

You can’t build over a pressurised sewer (which pumps foul sewage or rainwater). The alternative is to divert the sewer and upgrade the pump, which requires extensive construction works. Learn more about diverting a sewer.

There are two ways to apply for a sewer diversion, depending on the complexity of your requirements.

Build over: minor sewer diversions

For short diversions to smaller sewers required for domestic extensions or home improvements, you may be able to apply for a minor diversion. Your proposal must meet the following criteria: 

• The internal diameter of the sewer to be diverted is 150mm/6 inches or less 
• The total length of the sewer diversion is 20m or less 

A CCTV survey will be required before construction begins, to assess the condition of the sewer, and after it’s been completed, to ensure it’s been constructed to the agreed specifications and no damage has occurred. A refundable cash deposit of £2,000 is required and will be refunded once the completed diversion has been inspected by Thames Water and found to be in a satisfactory condition. 

To apply for a minor sewer diversion, complete the build over application online. 

Sewer diversions 

If you do not meet the minor sewer diversion, you’ll need to complete a full sewer diversion application. 

Read more about the process and apply online.

Current build over application costs

• Class 1: £130 - the internal diameter of the sewer is 150mm or less
• Class 2: £440 - the internal diameter of the sewer is 151mm to 375mm
• Class 3: £520 - the internal diameter of the sewer is over 375mm, plus additional legal costs starting at £1,000

These charges are updated on 1 April each year.