Build Over Agreements: The CCTV Drain Surveys You Need Before and After

If you are building an extension over or close to a public sewer in London, you will normally need a build over agreement from Thames Water, and the process often involves a CCTV survey of the sewer before work starts and another once it is finished. The surveys are evidence: the first shows the condition of the pipe before your builder touched it, the second shows your foundations have not damaged it.

What is a build over agreement?

A build over agreement is the water company's permission to build over, or within three metres of, a public sewer. It exists because the company owns the pipe and needs to know it can still be maintained, and because the Building Regulations require building work near sewers to protect them.

The trigger is not your own drain. A pipe carrying only your property's waste, inside your boundary, is yours, and building over it is a matter for building control rather than the water company. The trigger is a sewer, which in London usually means a pipe shared with neighbours. Since the 2011 transfer of private sewers, that includes a lot of pipework that runs through back gardens behind terraces and was never on anybody's plans. It is common for a homeowner to discover only at design stage that a "drain" in the garden is actually a public sewer.

Why the water company asks for a CCTV survey

The survey protects both sides. For the water company, it confirms the sewer's route, depth, size and condition, which decides whether building over it is acceptable and what protection it needs. For you, it records the condition of the pipe before work begins, so a defect that was already there cannot later be blamed on your extension.

Whether a pre-construction survey is required depends on the pipe and the proposal, and the water company will tell you as part of the application. Even where it is not required, it is worth having. Old clay sewers in London often already show cracked pipes, displaced joints and root ingress. Without a dated record of that, the post-construction survey will show those defects under your new floor, and you will be arguing about when they appeared.

A post-construction survey is commonly asked for once the work is complete, to show the sewer has not been damaged by excavation, piling or the load of the new foundations. Some agreements also ask for a survey at an intermediate stage, for example after the foundations are poured but before the slab goes on.

What a survey report needs to show

A build over survey has to be comparable with the one that comes after it. That means the same run, surveyed from the same chamber, in the same direction, with defects recorded against distance from that chamber.

• A plan of the surveyed run with chamber references and the direction of the survey.
• Pipe diameter, material and depth at each chamber.
• Chainage-referenced observations, so a defect at 4.2 metres before the build can be matched to the same position after it.
• Coded defects and condition grades using a recognised classification, rather than descriptions in plain words that nobody else can compare.
• The footprint of the proposed extension marked against the run, so it is clear which length sits beneath the new building.
• Dated, time-stamped footage and still images at each significant observation.

Footage on its own is not enough. If you need help reading the grades and codes in a report, our guide to reading a CCTV drain survey report translates them.

When the survey finds a problem

If the pre-construction survey finds a structural defect in the length you plan to build over, deal with it before the build, not after. Repairing a pipe with no extension on top of it is far easier and cheaper than repairing one under a new kitchen floor.

What happens next depends on whose pipe it is. A defect in a public sewer is the water company's to repair, and the survey report is the evidence you give them. The timing of that repair can affect your build programme, so raise it early. A defect in your own private drain is yours to fix, and the options range from patch repairs and lining to excavation. Our drain repair and excavation page covers the situations where lining is not enough.

Manholes are a separate issue. Building over an inspection chamber on a public sewer is generally not accepted, because it removes access. If a chamber sits where the extension will go, expect to need it moved or the sewer diverted, which is a larger and slower process than a standard build over agreement. Find this out before planning drawings are finalised.

Selling a house with an extension built without an agreement

Buyers' solicitors increasingly spot extensions sitting over public sewers with no build over agreement on file. The usual route is to apply for a retrospective agreement, and a CCTV survey showing the sewer beneath the extension is in acceptable condition is typically central to that application.

If you are the seller, commissioning that survey early avoids a late delay to exchange. If you are the buyer, the survey tells you whether you are inheriting a sound pipe under the extension or a defect that could mean lifting a floor. The wider pre-purchase picture is covered in buying a house: should you get a drain survey?

What a survey cannot settle

A CCTV survey records the condition of the pipe on the day. It does not approve your design, it does not replace the water company's own assessment, and it cannot see below standing water or past a blockage, so a run may need cleaning before it can be surveyed properly.

It also cannot tell you whether a sewer is public or private on its own. That depends on what connects into it and where the boundary falls, which sometimes needs tracing as well as a camera. Establish ownership first, because it decides whether you need a build over agreement at all.