Building over a sewer: build-over agreements explained

Drains are the least glamorous thing on any extension drawing, and one of the likeliest to hold up a start date.

If a public sewer runs under the ground where your extension is going, or close to it, you need the water company's permission before you build. Across most of Greater Manchester and Cheshire that water company is United Utilities, though it is worth checking which one serves your address. This is a general guide. The water company's own current guidance is the one that counts.

Is the pipe a public sewer?

This catches a lot of people out. The shared drains behind most rows of houses used to be private, and each owner was responsible for their share. In 2011, most shared drains, and drains running outside a property's boundary, were transferred to the water companies in England and Wales and became public sewers.

So the pipe running across the back of a semi or a terrace, picking up next door's kitchen and bathroom as well as yours, is very likely a public sewer now, even though it looks like any other drain. A drain that serves only your house, and stays inside your boundary until it reaches the sewer, is usually still yours.

The water company holds sewer records, and they are the first place to look. They are not complete, particularly for the pipes transferred in 2011, so the map is a starting point rather than the answer. Lifting the manhole covers, seeing which way everything flows and, where needed, a CCTV survey of the pipe will tell you what is actually there.

When you need permission

United Utilities says you need its approval to build over or close to a public sewer, which usually means within 3 metres of it, even on your own land. Building control will also consult the water company. A building notice application generally isn't available when you are building over or close to a public sewer, so expect to be on the full plans route. There is more on that in what building control inspects on an extension.

The process depends on the pipe. Some straightforward cases can go through a simpler route alongside building control. Larger or deeper sewers need a formal legal agreement with the water company. Some can't be built over at all, and the only option is to divert the sewer, which is a separate application, slower and considerably more expensive. Water companies are also generally reluctant to see a manhole end up inside an extension, so one in the way may need moving.

None of this is unusual. It just needs to be found early, because it can change the design, the cost and the start date.

Find the drains before you finalise the drawings. A sewer under the extension is a design problem on paper and a programme problem on site.

What it means for the build

Water companies often ask for a CCTV survey of the pipe before work starts, and sometimes again afterwards, so there is a record of its condition. If the pipe is in poor shape, it may be sensible, or required, to replace that length before building over it.

The foundations are then designed around the pipe. Depending on its depth and position, they may be taken down below it, or bridge over it on lintels or a reinforced beam so the weight of the extension doesn't bear on it. Access to the sewer has to be kept. All of this goes through the structural engineer and building control, and it is part of why the ground is one of the biggest variables in what a house extension actually costs.

If it's ignored

Building over a public sewer without permission doesn't usually come to light until something goes wrong or the house is sold.

If the sewer fails, the water company has rights to get to it, and an extension sitting on top of a broken pipe is an expensive place to be standing. Who pays for what at that point is not a conversation you want to be having.

When you sell, the buyer's solicitor will normally order a drainage and water search, which asks about public sewers within the property and whether the water company has agreed to any building over them. A missing agreement then has to be sorted out after the event, often involving a CCTV survey and sometimes opening up, while the buyer waits.

Find out early

I would much rather look at the drainage before the design is fixed than find it with the digger. If a build-over is needed, it goes into the programme alongside building control and any party wall notices, rather than surprising everyone halfway through.

If you're planning an extension, send us the drawings and, if you can, a photo of each manhole with the cover lifted. We'll tell you whether the drains are likely to need water company involvement and what that means for the cost. Our working rates are on the cost guide, and more on our extension work is on our extension builders in Hale page.