The short answer
You need one if the extension will sit over, or within 3 metres of, a public sewer. On a London terrace that is common, because one shared sewer often runs across every back garden in the row. Ask the water company before the design is fixed.
A build over agreement is permission from the water company to build over or close to a sewer it owns. It exists so that the pipe is not damaged by your foundations and so that the company can still reach it for repairs.
Why so many London gardens have a public sewer
Until 2011, the pipe that served a row of terraced houses before reaching the main sewer in the road was usually a private shared drain. On 1 October 2011 most of those shared drains became public sewers, owned by the water company. In a Victorian terrace the shared run often passes behind the houses, across each back yard, and out at the end of the row.
So a pipe you may have always thought of as “our drain” is very likely public, and an extension at the back of the house is very likely near it. In most of London the sewerage company is Thames Water.
How to tell if your extension is affected
- Look at the manholes. A manhole cover in your yard or your neighbour’s is the clearest sign of a shared run. Note where it sits against the planned footprint.
- Order a sewer map. The water company’s asset map shows public sewers, though older shared runs are not always drawn accurately.
- Have the drains surveyed. A CCTV survey shows the route, depth, diameter and condition of the pipes. A builder planning a modular extension will want this anyway, because the foundations go in before the modules arrive.
If the new walls or foundations are over the pipe, or within 3 metres of it, you need to apply.
What the water company looks at
The company wants to know the pipe will survive and remain accessible. It usually considers the size and depth of the sewer, its condition, whether a manhole would end up inside the extension, and how the foundations will be designed around it. Smaller pipes in good condition are often approved with conditions, such as a CCTV survey before and after the work. Larger sewers, or a manhole that would end up under the new kitchen floor, can mean a redesign or a diversion, which takes much longer and costs much more.
Building control also checks how the work affects drains under the Building Regulations, and will expect the agreement to be in place.
When to apply
Apply as soon as you have a footprint. For a modular extension this matters more than for a brick one: the modules are made to fixed dimensions, and the foundations have to be right the first time because the crane day is booked. A late answer that moves a wall by half a metre is expensive.
Common outcomes on a London terrace
- Pipe beside the extension, not under it. Foundations are designed so the load does not bear on the pipe. Usually straightforward.
- Pipe under the extension, no manhole inside. Often approved, sometimes with the pipe relined or replaced first.
- Manhole inside the footprint. The manhole may be moved, or replaced with a sealed access cover. The water company decides which is acceptable.
If you are planning modular extensions London builders can price, raise the drains at the first visit. It is one of the few things that can stop a factory-built project on the day.