Phase 2 · Pre-Construction · Task 07 of 12
Build-Over Agreement (Water Company Sewer)
If a public sewer runs within 3m of your extension, you need written consent from your water company before breaking ground. Here's how to check, apply, and what happens if you don't.
Your water company has the statutory right to demolish any structure built over their sewer without permission. Not "might seek an injunction." Not "could take you to court." Demolish. And they don't have to compensate you.
That's the legal position under the Water Industry Act 1991. It's not hypothetical. Homeowners have had extensions pulled apart to access blocked or cracked sewers. The fix is simple, but it has to happen before construction starts.
Do this first
A build-over agreement must be obtained before your groundworker breaks ground. Apply at the same time as your building control full plans submission, not when the builder is ready to dig. If a public sewer runs under or near your extension footprint, check the sewer map before finalising your extension design. It affects foundation design, and possibly the entire extension layout. The drainage leaf covers the wider drainage system and soakaway requirements for your extension.
Step One: Check Whether You Need One
Most extensions don't trigger a build-over requirement. But the ones that do can catch homeowners completely off guard, often because they had no idea there was a public sewer on their property at all.
A sewer is public if it serves more than one property. That's the rule. Your drains, from your house to the boundary, are private. But if your drain joins another property's drain before reaching the sewer main, that shared section is a public sewer. It became the water company's legal responsibility when they adopted it. And if that shared section runs under your garden, your driveway, or anywhere near your proposed extension footprint, you have a build-over issue.
The most common scenario: two neighbouring terraced houses share a manhole in the garden of one, then drain together into the road. Both feed the manhole. The shared section from that manhole to the public main is public. So is the manhole itself. Build over either of them without permission and you're in breach.
Tip
If there's a manhole cover in your garden or driveway that also takes drainage from a neighbouring property, assume the downstream section is public until your water company confirms otherwise. The fact that the manhole sits on your land is irrelevant to ownership.
How to Check
Your water company publishes a sewer map. Search "[your water company] sewer map" online. Thames Water, Severn Trent, United Utilities, Southern Water, Wessex Water, Anglian Water, and Welsh Water all have publicly accessible mapping tools. Some are more detailed than others, but all will show public sewer routes across your property.
Check the map as soon as you have an outline of your proposed extension. If a public sewer appears anywhere within 3 metres of the extension footprint, a build-over agreement is required. If the sewer runs directly under the footprint, the situation is more complex (more on that below).
Your architect should check the sewer map as standard practice. If yours hasn't mentioned it, ask. A sewer running under the design can force a complete redesign of the foundation layout, so the earlier it's identified the better.
Warning
A clear sewer map is not proof there's no public sewer. The map shows what the water company has on record, and the records are incomplete, particularly for the private sewers and lateral drains that became public in the 2011 transfer (see below). Water companies state plainly that sewers adopted in 2011 may not appear on the map. So a blank map means "nothing recorded here," not "nothing here." Treat it the same way you'd treat a clear LSBUD result before digging. Verify on the ground: look for manhole covers on your plot and your neighbours', work out which properties' drains feed them and which way the flow runs, and if anything looks shared, ask your water company to confirm the status in writing before you finalise the design. A shared manhole is public until they tell you otherwise.
The 2011 Transfer: Why Older Extensions Are Now a Problem
On 1 October 2011, most private sewers and lateral drains were transferred into public ownership under the Water Industry (Schemes for Adoption of Private Sewers) Regulations 2011. Sewers that served multiple properties, which had been maintained by the residents themselves, became the water company's responsibility overnight.
This matters now because extensions built before 2011 often built over drains that were private at the time. Private drains need no agreement. But the same pipe became public in 2011. The extension now sits over a public sewer with no record of approval. When those properties sell, the drainage search flags it. Buyers' solicitors ask for the build-over agreement. There isn't one.
If you're buying a property where an older extension exists, check whether the drainage search raised this. If you're extending an older property, check whether any previous extensions are already over public sewers.
What the Approval Actually Is
A build-over agreement is a formal legal consent from your water company confirming that your extension design adequately protects their sewer and preserves their right to access it for maintenance and repair. It is not optional and it is not covered by planning permission. The planning department and the water company are entirely separate bodies. A planning approval does not grant permission to build over a sewer.
The approval is classified by sewer size:
Class 1 applies to sewers up to 175mm diameter. This covers the vast majority of domestic extensions. The process is streamlined: you submit drawings, they assess, they respond within around 21 days. No pre-construction CCTV survey is required (though the water company may inspect at their discretion).
Class 2 applies to sewers between 176mm and 350mm diameter. This requires pre- and post-construction CCTV surveys of the sewer, which the water company commissions and you pay for. Processing takes longer.
Class 3 covers sewers over 350mm diameter. These are major infrastructure assets. Class 3 applications require an engineer inspection and can take up to 6 months. If your extension is near a Class 3 sewer, the design process becomes significantly more involved and may require a structural engineer from the outset.
The legal basis sits in Approved Document H4 (Building Regulations 2010): "The erection or extension of a building or work involving the underpinning of a building shall be carried out in a way that is not detrimental to the building or building extension or to the continued maintenance of the drain, sewer or disposal main."
Building control cannot issue your completion certificate without evidence that a required build-over agreement was obtained.
What this guide covers
- 01The Application Process
- 02What It Costs and How Long It Takes
- 03When the Sewer Is Under the Foundation Line
- 04What Happens If You Don't Get One
- 05What to Ask Your Solicitor Before You Exchange
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