03/09/2026
If your extension will sit within 3 metres of a public sewer, you need Severn Trent's written approval before you build, and a build over agreement drain survey is usually part of that application. What most people get wrong is which survey, and when. There are cases where no pre-works survey is required at all, and one survey that is required every single time and gets forgotten.
The 3 metre rule, and what it is measured from
Severn Trent requires an application where any part of a proposed building sits within 3.0 metres of a public sewer, measured from the nearest point of the building to the centreline of the sewer. Not the edge of the pipe. The centreline.
That distinction catches people out on narrow Derby plots, where a 300mm difference decides whether you are applying or not.
The obligation comes from Part H4 of the Building Regulations 2010, which applies in England, and your building inspector will ask for the agreement.
https://www.inspectadrain.co.uk/build-over-agreement-drain-survey/build-over-agreement-drain-survey/