Building Over a Sewer in Dorset: Build-Over Agreements
Updated: Oct 4
If you're planning a rear extension in Poole, Bournemouth or Christchurch, there's a fair chance a public sewer runs somewhere across the back garden. It's one of the first things we check on a site survey, because once your foundations come within three metres of that pipe, Wessex Water has a say in your project. That's where a build-over agreement comes in, and it catches out more homeowners than almost any other piece of paperwork we deal with.
Here's how it works, what it costs in 2026, and how we handle the whole thing so it doesn't hold your build up.
What counts as a public sewer, and why it matters
Since the private sewer transfer in 2011, almost any drain shared by two or more properties is a public sewer, even if it runs entirely through your garden. On the 1930s semis around Winton, Parkstone and Southbourne that's the norm: next door's foul drain usually joins yours somewhere behind the kitchen before heading out to the road. Wessex Water owns it, maintains it, and needs to be able to reach it.
Under Building Regulations (Approved Document H), any extension, conservatory or garage built within three metres of a public sewer needs the water company's approval before building control will sign the job off. Without it you won't get a completion certificate, and that becomes a real problem the day you try to sell.
Build near or build over: two different routes
Wessex Water splits this into two cases, and the difference matters for your budget.
Building near the pipe (within 3m, but not over it). If it's a domestic extension over a minor gravity sewer, you're using strip or trench-fill foundations, and the footings sit at least 500mm clear of the pipe, you can self-certify online in a few minutes. No fee and no waiting. We do this for you at design stage.
Building over the pipe, or near it with piled or raft foundations, or where you don't meet the standard criteria: a formal application is needed. Wessex Water review the drawings, run a CCTV survey of their sewer and issue an approval letter, often with conditions attached.
The one thing they won't allow is a manhole inside the new building. If there's an inspection chamber where your extension is going, it has to be relocated outside the footprint or the sewer diverted, and that's the single biggest cost driver we see on these jobs.
What it costs in Dorset in 2026
We researched current rates for this post, and these figures include our margin, so they're what you'd actually pay through us rather than raw trade cost.
Wessex Water application, handled and submitted by us: around £390. Their optional pre-application site visit is roughly £90, which comes off the application fee if you go ahead.
Plan and sectional drawings showing the sewer position and foundation depths: from around £600 if we produce them.
Our own CCTV drain survey with sonde tracing to pinpoint the pipe before we design anything: £180 to £420. Wessex Water survey their sewer as part of the application, but knowing exactly where it runs beforehand avoids a redesign later.
Taking trench-fill foundations down below the bottom of the sewer, which is required within 2m of the pipe: £160 to £195 per linear metre for the affected run, against £115 to £140 for standard-depth strip footings.
Bridging the pipe with pre-stressed concrete lintels where walls cross it: a modest extra, usually a pair of lintels and a day's labour per crossing.
Relocating a manhole outside the footprint: £1,200 to £1,800.
Diverting the sewer around the extension: £1,800 to £9,000, with most domestic diversions landing between £5,400 and £7,200.
On a typical 4m rear extension in Poole where the sewer clips a corner of the footprint, we'd budget £1,500 to £3,000 for the sewer-related extras on top of the build itself. Where a manhole has to move, add a couple of thousand. Where a diversion looks unavoidable, it's worth talking to us about tweaking the footprint instead. Shifting a wall by 600mm has saved clients £5,000 more than once.
How long it takes
Self-certification is instant. For a formal application we tell clients to allow six to eight weeks from submission to approval letter, and longer if Wessex Water's CCTV finds a defect that has to be repaired first. Work must not start on site until that letter is in hand, and building control will ask to see it, which is why we deal with it at design stage rather than the week before we dig.
If you've already got an extension that was built years ago without an agreement, a retrospective one is usually possible, backed by a CCTV survey showing the pipe is sound. We've sorted a few of those for sellers in Christchurch who only found out at the conveyancing stage.
How we handle it on your project
We survey before we quote, every time. That means pulling the Wessex Water asset map, lifting the manhole lids and tracing the run so we know where the sewer sits before a price goes on paper. If it's within 3m we tell you at quote stage, price the extras honestly, and either self-certify or submit the application with our own drawings.
From there we build the foundations to Wessex Water's specification, bridge the pipe with lintels to their standard detail, and carry out any conditioned drainage work through our own drainage team, so there's no waiting on a third party. It's the same joined-up approach we take on all our single-storey rear extensions across Dorset and Hampshire: one point of contact from survey to completion certificate.
Related reading
Planning an extension near a sewer?
Send us your address and a rough idea of what you'd like to build, and we'll check the sewer records before we even visit. Book a free site survey with Complete Builds and we'll tell you straight whether a build-over agreement applies and exactly what it adds to the job.
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