Building over a drain or sewer: when you need a build-over agreement
If your extension will sit over a public sewer, or within 3 metres of one, you almost certainly need a build-over agreement from your water company before you start. Get it wrong and building control can withhold your completion certificate — which is exactly the thing a buyer's solicitor asks for later.
The short answer
A build-over agreement is permission from the water and sewerage company that owns the pipe. You need one when your work is built over a public sewer, or within 3 metres of it. For a public lateral drain — the stretch of a single-property drain that runs beyond your boundary — the trigger is tighter, within 1 metre. If the pipe is genuinely private, serving only your house and staying inside your land, you don't need an agreement, but building control will still want to see that your foundations don't wreck it.
This sits under requirement H4 of Approved Document H, the drainage part of the Building Regulations. It's separate from planning permission and separate from the rest of building control sign-off. You can have full planning consent and permitted development rights and still be stopped by a sewer you didn't know was there.
The pipe under your garden is probably not yours
Here's the part most homeowners miss. On 1 October 2011 the majority of private sewers and lateral drains in England and Wales — anything connected to the public network on 1 July 2011 — transferred into the ownership of the water and sewerage companies. Overnight, millions of pipes that people thought of as "their" drain became public sewers.
So the shared drain running along the bottom of a Victorian terrace, or the branch that leaves your garden to join next door's before it reaches the road, is very likely public now. It looks like your pipe. It runs through your soil. But the company owns it, and that ownership is what pulls a routine rear extension into build-over territory. The only reliable way to know is to look at the records, not the drain cover.
How to check before you commit to a design
Two things tell you what's underneath. The first is the water company's public sewer map, which many now publish online for free — useful for a quick look, though the plotted line is approximate and doesn't show depth. The second, and the one worth paying for, is a sewer asset location search (sometimes bundled into the drainage and water search, the CON29DW your conveyancer ordered when you bought). That gives you the mapped position of public sewers against your boundary.
Do this before your architect finalises anything. A sewer discovered at survey stage is a design tweak; a sewer discovered after the foundations are marked out is a stop-work notice. If a public pipe crosses where you want to build, sometimes the cheapest answer is to move the extension a metre, not to build over the sewer at all.
When an agreement is needed, and what it costs
| Situation | What applies |
|---|---|
| Building over, or within 3m of, a public sewer | Build-over agreement required |
| Building within 1m of a public lateral drain | Build-over agreement / permission required |
| Private drain, inside your boundary, serving only your house | No agreement; building control still checks it |
| Class 1 pipe — internal diameter 150mm or less | The usual domestic case; simpler route, lower fee, often no CCTV survey, roughly 21 days |
| Class 2 pipe — 151mm to 375mm | Full agreement with pre- and post-build CCTV survey; higher fee; a few weeks longer |
| Class 3 pipe — over 375mm | Engineered assessment; can take several months |
| Application fee | Set by each water company; broadly from around £80 up to £500+ depending on pipe size and route |
Sources: requirement H4, Approved Document H, Building Regulations 2010 (GOV.UK); the Water Industry (Schemes for Adoption of Private Sewers) Regulations 2011 (legislation.gov.uk); water and sewerage companies' published build-over charges and standards. Fees and pipe-class bandings vary by company — confirm the figure with the undertaker for your area.
On top of the application fee, a straightforward domestic case may need a pre-works CCTV drain survey, indicatively £150 to £350, based on BestBuilders' 2026 regional cost model (published national cost guides × a regional labour index, averaged across 519 UK towns). That's for the survey; any repairs it turns up are extra.
What the water company actually checks
The agreement is mostly about protecting access to the pipe and stopping your foundations from crushing or blocking it. Expect the company to look at how close your foundations come to the sewer and how deep they go. A common benchmark is keeping foundations clear of the pipe by around 600mm and taking them down to at least the pipe's invert level where they sit within a metre of it; where the sewer runs deeper than about 2 metres, they'll want more clearance. Where a pipe has to pass under the building line, the usual fix is a reinforced concrete lintel or raft bridging it, or piled foundations kept clear of the pipe, so the extension's weight never bears on the sewer.
Manholes are the other sticking point. You can't build over an inspection chamber or manhole on a shared sewer — the company needs to reach it. Either the design changes to keep it accessible, or the chamber is relocated with the company's agreement and at your cost. It's the kind of detail that's cheap to solve on a drawing and expensive to solve in a trench.
Two routes, and why the smaller pipe is easier
For the everyday case — a single-storey rear extension over a small-diameter public pipe with no manhole in the way — most companies offer a lighter-touch route: you submit drawings showing the pipe and your foundations, pay the fee, and get sign-off without a full survey. Larger pipes, or anything near a manhole, drop you into the full agreement, with CCTV before and after so the company has evidence the sewer wasn't damaged by the work.
Either way, build the sewer into your extension drawings from the start. And remember this is one approval among several: it doesn't replace building regulations sign-off, and it has nothing to do with whether the extension is permitted development or needs planning permission. If your work is near a shared boundary you may also be into party wall territory, which is a separate process again.
What happens if you skip it
Two problems, one now and one later. Now: the water company has a statutory right to protect access to its sewers, and can require you to remove work that obstructs a public pipe — and it takes no responsibility for damage to your building or theirs if you built without consent. Later: building control won't issue the completion certificate while the agreement is missing, and that gap shows up in the drainage and water search when you come to sell or remortgage. A missing build-over agreement is a routine conveyancing query, and a routine reason for a sale to stall until it's sorted retrospectively — which is slower and dearer than doing it first.
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