Building over a sewer: build-over agreements explained
The most common reason a London extension stalls is a pipe nobody checked for. The process is manageable, but only if it starts early.
What it is
Formal consent from the water company to build over or close to a public sewer. It sets out how the pipe will be protected during and after construction and how access will be maintained, since they still have to be able to reach it.
It sits alongside, not instead of, Building Regulations approval. Both can be needed for the same extension.
Why it catches people out
The 2011 transfer. A great deal of pipework that owners assume is private became the water company’s, including runs that pass under private gardens. So a homeowner can be genuinely surprised to learn there is a public sewer beneath the lawn they are about to build on.
Sewer records are indicative rather than exact, which is the second surprise. They show what exists, not precisely where, so a site trace is still needed.
When you need one
Broadly, when building over a public sewer or within a defined distance of one, typically three metres, though the water company sets the detail. Depth, pipe size and the type of structure all affect what conditions are attached.
The practical answer is to establish early whether a sewer is present at all, because that single fact changes the design brief.
How to avoid the delay
- Request sewer records before the design is fixed, not after.
- Trace the run on site with a camera and locator, so you know the line and depth rather than an approximation.
- Design around it where you can. Moving an extension a metre at drawing stage costs nothing.
- Apply early. Consent takes time, and applying once the foundations are dug is the expensive order.
More in check your drains before the foundations.
What else the drainage design should cover
Capacity, since an extension usually adds fittings. Falls and access points that will still be reachable in ten years. And surface water, because a new roof and terrace produce run-off that should not be pushed into a foul sewer.
Below-ground work is one of the few things on a property that is genuinely difficult to correct later, which is the whole argument for doing it carefully once. See drainage installation, or coverage in Ealing, Merton and Richmond.
If you have already built over one
It happens, usually on properties bought with an extension already in place. There is no retrospective penalty in the ordinary case, but two things follow.
The water company retains its right of access. If the sewer fails beneath the structure, reaching it is your problem to accommodate, and the cost of reinstating whatever sits on top is not theirs.
And it surfaces at sale. A buyer’s solicitor raising the absence of a build-over agreement is a common late-stage delay, sometimes resolved by indemnity insurance, sometimes by a retrospective application. Either way it is cheaper to know before the property is on the market than after an offer. See pre-purchase drain surveys.