Build Over Agreement UK 2026 | What Somerset Owners Must Do

Last week a customer in Worle rang me about a rear extension he's been planning for months. Builder's booked, drawings are done, he's excited. Then his architect mentioned something about a rule change and a water company, and suddenly he's not sure he can start when he planned.

He wasn't the first call like that I'd taken this month. He won't be the last.

From 13 July 2026, the Build Over Self-Declaration route is gone. Closed. Southern Water confirmed it, citing a rise in operational incidents caused by structures built over or close to public sewers. If you're a homeowner in BS22, BS23, BS24 or anywhere across Somerset planning an extension, a garden room, or anything that sits within three metres of a public sewer, that change matters to you directly.

Here's what it actually means, and why, honestly, you should be relieved rather than annoyed.

What the self-declaration route was

For years, if your extension was modest and sat close to (but not directly over) a public sewer, you could self-certify the work. You'd fill in a form declaring your project met certain criteria, hand it to your water company, and crack on. No formal agreement, no water-company sign-off, no lengthy approval process.

It was a shortcut. And like most shortcuts in drainage, it caused problems.

Southern Water has been picking up the pieces: sewers damaged by foundations, access chambers buried under new slabs, pipes crushed or offset by loads they were never designed to carry. The incidents mounted up. So they pulled the route entirely.

What you need now instead

A formal Build Over Agreement. That's not a new invention, it's just that the shortcut bypassing it no longer exists.

To get one, you'll typically need:

• A CCTV drain survey of the public sewer in the vicinity of the proposed works, so Southern Water can see its current condition before your builder goes anywhere near it
• Proper structural drawings showing how the foundations will relate to the sewer
• Confirmation of the sewer's position, which may differ from what's shown on older maps
• Formal submission to Southern Water and their written approval before work starts

Approval takes weeks, sometimes longer if there are queries. Build that into your programme now. If your builder is booked for September and you haven't started the process, you may be having an uncomfortable conversation with them.

Why this is actually good news for homeowners

I'll be straight with you: the self-declaration route was a liability in disguise.

I've been into drains under extensions built using that shortcut. Some were fine. Others were a mess. Foundations bearing down on pipes never designed to take that load, access chambers permanently sealed under concrete, defects no-one knew were there until something went wrong. And by then, someone else owns the house.

That's the part people miss. An unauthorised or improperly agreed build-over doesn't just cause drainage problems. It creates a legal liability that conveyancers find every single time a property is sold. Buyers' solicitors now routinely ask for Build Over Agreement documentation. If you can't produce it, the sale stalls, the buyer negotiates hard on price, or they walk.

I've had customers ring me mid-sale, suddenly needing a CCTV survey to satisfy a solicitor because the extension their predecessor built twenty years ago has no paperwork. That survey costs money. Sorting the problem, if there is one, costs more. All of it could have been avoided.

The formal agreement process isn't bureaucratic nonsense. It's a record that your extension was built properly, in relation to infrastructure that serves your whole street.

The Weston-super-Mare picture

A lot of older properties across Weston-super-Mare, particularly in areas like Milton, Uphill, Bournville and parts of Worlebury, have public sewers running through back gardens. They're not always where you'd expect. The sewerage maps aren't always accurate to the inch, especially on Victorian-era systems that were extended piecemeal over decades.

That's why the CCTV survey isn't just a formality. It tells you exactly where the pipe runs, what condition it's in, and whether there's anything Southern Water is going to push back on before they approve your agreement. Better to find a root-infiltrated joint or a cracked section now, before your foundations go in, than six months after.

If a defect shows up on survey, Southern Water may require it's repaired before they'll approve the agreement. That sounds like more work. It is. But you'd have had to deal with it eventually, and it's considerably easier when there's no extension sitting on top of it.

What to do if you're mid-planning

If you've got drawings, a builder, and planning permission, do this now:

Find out if a public sewer runs within three metres of your proposed works. Your architect may already know. If not, check the sewer records via the Southern Water map or ask a drainage engineer to do it.

If there's a sewer in range, commission a CCTV survey. Get the footage and the written report, because Southern Water will want to see it.

Submit your Build Over Agreement application with your drawings and survey report. Don't start groundworks until you have written approval.

And if you've already built something without going through this process, get a survey done before you sell. Find out what's there and deal with it on your terms, not under pressure from a buyer's solicitor.

One more thing

Some builders will tell you it's fine, they've done it before, no-one checks. Maybe they have. Maybe no-one checked then. The route is now closed, which means there is no version of 'self-certify and carry on' that's legitimate. Southern Water will not retrospectively recognise a self-declaration made after 13 July 2026, because the mechanism doesn't exist any more.

If something goes wrong with the sewer later, and Southern Water can demonstrate an unauthorised structure over their asset, the costs come back to the property owner. Not the builder who's long gone.

Get the agreement. It's not a hoop. It's a paper trail that protects you.

Drainsco covers Weston-super-Mare and across the BS postcodes, with engineers available 24/7. If you need a CCTV drain survey to support a Build Over Agreement application, we'll give you a clear price before we start and there's no call-out fee. Give us a ring and we'll tell you where you stand.