The short answer
A build over agreement is the water company's consent to build over, or within 3 m of, a public sewer. Many rear extensions need one, because shared drains behind terraced and semi-detached houses became public sewers in 2011. Apply once the drawings are fixed and before any digging. Building control will not sign the extension off without it where it is needed.
Why so many extensions need one
Until October 2011 the drain that ran behind a row of houses was usually private, shared by the neighbours whose waste it carried. That year the law in England and Wales transferred most of those shared drains and sewers to the water companies. A pipe that serves more than one property is now, in most cases, a public sewer.
The back of the house is exactly where those pipes run, and exactly where a rear extension goes. Building over a public sewer, or putting a footing within 3 m of one, needs the water company’s consent. Building control checks for it under Part H of the building regulations, and a completion certificate can be held back until it is in place.
A drain that serves only your house is still yours. You can build over it without the water company, though building control will still want it protected and accessible.
Find the pipe before you draw the room
Every water company keeps a map of its public sewers. You can ask for the record for your address, sometimes for a small fee, and some companies show it online. The map is a guide. It shows where the pipe is meant to be, not always where it is, and older private connections are often missing.
On site, lift the manhole covers in the garden and follow the direction of flow. A manhole with pipes coming in from next door is a strong sign that the run is shared and therefore public. If the position matters, a drainage firm can run a camera down the pipe and mark it on the surface.
Do this before the drawings are finished. Moving the extension half a metre on paper costs nothing. Moving a sewer costs a great deal and needs the water company’s agreement as well.
What the water company will ask for
The application goes to the water company that owns the sewer, not to the council. Expect to provide:
- a site plan showing the sewer, any manholes and the outline of the extension;
- a section through the new footing where it meets or crosses the pipe;
- the pipe’s size, depth and material, if known;
- a camera survey of the pipe’s condition before work starts, on many schemes.
Small domestic pipes under a single-storey extension are commonly approved through a simpler standard route. Larger sewers, pipes in poor condition and anything that would put a manhole inside the new room get a closer look, and some cannot be built over at all. Manholes generally have to stay outside the extension and accessible, which can mean relocating one at your cost.
Allow several weeks for a decision, more if the company wants changes. Some ask for a second camera survey once the work is finished, to show the pipe was not damaged.
What it changes in the ground
Consent comes with conditions about how the footing treats the pipe. The usual ones:
- footings near the pipe taken down below its level, so the weight of the extension does not bear on it;
- where a wall crosses the pipe, a lintel or a sleeve over it, so the wall bridges the pipe instead of resting on it;
- flexible joints or a protective surround where the pipe passes under the floor slab.
Each of these adds digging and concrete, and the extra depth can turn a simple strip footing into trench fill or a short run of piles. Price it before you commit to the layout.
Where a kit fits in
Extension kits do not touch the ground at all. The frame stands on whatever base you provide, and the supplier designs from the assumption that the base is right. The sewer is entirely on your side of that line.
What a kit does change is the order. The panels are made to the drawing, so if the water company’s conditions move a wall or change the floor level at the base of the frame, the drawing has to change before the kit is cut. Apply for the build over agreement as soon as the layout is settled, and give the supplier the final foundation drawing, not the first one.
Scotland
Scottish Water owns the public sewers in Scotland and runs its own consent process for building near them. The principle is the same: find the pipe, apply before you dig, and expect conditions about depth and access. Your building warrant application will need to show how the sewer is dealt with.
If you have already built without one
It happens, usually because nobody knew the pipe was public. Contact the water company, explain, and ask what they need. A retrospective application, often with a camera survey to show the pipe is undamaged, is the normal fix. Leaving it means the question resurfaces when you sell, usually at the worst moment.