Do I need a build-over agreement? A guide for extensions

23 October 2026 · Blog

Many homeowners only find out about public sewers when their extension plans are already drawn. If you are building over, or close to, a public sewer, you will usually need permission from the water company before work starts. Here is what you need to know.

What is a build-over agreement?
It is the water company’s consent to build over or near a public sewer or lateral drain. The water company needs to be sure your building won’t damage the pipe, and that the pipe can still be reached and repaired in the future. Building Regulations (Part H4) also require this to be dealt with, so Building Control will ask about it.

When do you need one?
Usually when you build over, or within 3 metres of, a public sewer. Extensions, conservatories, garages and outbuildings can all be affected. Rules and thresholds vary between water companies, such as Thames Water, Southern Water and Anglian Water, so always check with the one that serves your property.

Is the sewer public?
Often, yes, even if it runs under your garden. In 2011 most private sewers and lateral drains that serve more than one property, or that cross a neighbour’s land, were transferred to the water companies. A drain that only serves your own home and stays within your boundary is usually private.

How does the process work?
1. Find the sewer: a sewer map from the water company, often followed by a CCTV survey to confirm its position, depth and condition.
2. Design the foundations: the foundations need to keep load off the pipe, for example by bridging over it or going deeper than it.
3. Apply to the water company: with drawings, the survey and details of the foundations.
4. Build and record: the water company may ask for a CCTV survey before and after construction to show the pipe hasn’t been damaged.

What if I build without one?
It can cause problems when you sell, as buyers’ solicitors often ask for the agreement. If the sewer is damaged, or the water company needs to reach it, you could face costs or even have to remove part of the building. It is far easier to sort out before you build.

How we can help
Because we are both structural and civil engineers, we can design the foundations of your extension and handle the build-over application in one go. We also deal with new sewer connections (Section 106), sewer diversions (Section 185) and adoption (Section 104).

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Get in touch or call 020 3835 2633