Building Regulations · Planning
Building Over a Public Sewer: Build-Over Agreements Explained
A public sewer running through a London garden can shape an extension or basement, and building over or near one needs a build-over agreement from the water company. This guide explains when it is required and why it is separate from planning permission.
What a build-over agreement is
A build-over agreement is the water company’s consent to build over or close to a public sewer or lateral drain. It is a separate approval from planning permission and building regulations — planning permission does not give you the right to build over a sewer — and it is easy to miss, because the sewer is underground and often shared with neighbours. In much of London, a public sewer runs along the rear of the gardens, exactly where a rear extension or basement wants to go.
When you need one
You will generally need a build-over agreement where building works are within 3 metres of a public sewer, or within 1 metre of a public lateral drain. The water company assesses whether the work would restrict its access to maintain and repair the pipe, and whether the weight of the new structure could damage it. Foundations near a sewer often need to be designed specifically — deeper, bridging the pipe, or with the sewer diverted — which is why the position is best established at design stage.
Why it matters
Beyond protecting the sewer, the agreement has a practical sting: without it, you may not be granted a building regulations completion certificate — and a missing completion certificate is one of the things a buyer’s solicitor picks up, which can hold up or complicate a future sale. Starting construction over a sewer without the agreement in place is a risk to both the works and the resale of the home.
How it fits your project
The practical step is to check for public sewers early — a drainage search or the water company’s records will show what runs under or near the property. Where a sewer affects an extension or basement, we design the foundations and drainage around it and coordinate the build-over application alongside the planning and building regulations work, so it is resolved before construction rather than discovered on site.
Frequently asked questions
Do I need a build-over agreement for my extension?
What is the distance rule for building near a sewer?
Who grants a build-over agreement?
Does planning permission cover building over a sewer?
What happens if I build over a sewer without an agreement?
How do I find out if a sewer runs under my property?
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