What Is a Build-Over Agreement?
A build-over agreement is a formal consent obtained from a sewerage undertaker (typically Thames Water in London, or the relevant water company elsewhere) that allows you to build over or close to a public sewer. Public sewers are sewers that have been adopted by the sewerage undertaker and are maintained at public expense. They run under many properties in the UK, including gardens and back plots of residential houses.
The sewerage undertaker has legal rights to access and maintain public sewers and the land above them. Building a structure over or in close proximity to a public sewer without consent can obstruct this access, damage the sewer, and create significant legal and practical problems when the issue is discovered — which may be years later when you come to sell the property or when the sewer requires maintenance or repair.
When Is a Build-Over Agreement Required?
You need a build-over agreement if you intend to build within three metres of a public sewer with a diameter up to 160mm, or within six metres of a public sewer with a diameter greater than 160mm. Within these zones, the weight and vibration of foundations and structures can damage the sewer, and access for maintenance and repair can be obstructed.
The most common scenario where this arises on residential projects is a rear extension or outbuilding that will be built over or adjacent to the public sewer that typically runs along the backs of terraced properties. In many Victorian terraces across London, a shared public sewer runs along the boundary of back gardens, right in the path of a proposed rear extension.
How to Find Out If a Public Sewer Runs Under Your Site
You can check the position of public sewers using the sewerage undertaker's sewer maps, which are available for search through their drainage search services. Drainage searches are typically carried out as part of the conveyancing process when a property is bought, but you can commission a drainage search separately if you are planning construction and want to identify sewer constraints before design.
Be aware that sewer maps are not always perfectly accurate — the recorded position of a sewer may differ from its actual position by a metre or more. For critical decisions about foundation placement, a CCTV drain survey to locate the actual sewer position is often worthwhile.
The Application Process
Applications for build-over agreements are made to the relevant sewerage undertaker, typically via their online portal. For straightforward proposals, a standard build-over agreement is granted relatively quickly. For more complex proposals — larger structures, deeper foundations, or sewers of larger diameter — a more detailed technical review is required.
The agreement typically sets out conditions on how the foundations are to be designed and constructed to avoid loading the sewer, how inspection chambers must remain accessible, and what restrictions apply to the use of the space directly above the sewer. Some agreements require the developer to commission a CCTV survey of the sewer before and after construction, and to remedy any defects caused by the works.
What If There Is No Agreement and the Sewer Is Discovered Later?
If a build-over agreement was not obtained and a public sewer under the building is discovered — which often happens when buyers commission drainage surveys as part of purchasing the property — the sewerage undertaker has the right to require access for maintenance and repair, which could mean excavating through your new extension. This is an extreme outcome, but it demonstrates the importance of resolving the build-over position before construction starts rather than hoping the issue will not be discovered.
Indemnity insurance policies are available for build-over situations where works were carried out without an agreement, but these do not provide the same level of protection as a properly obtained agreement and are not always accepted by mortgage lenders.
How RCB Handles Build-Over Checks
At RCB Design and Build, checking for public sewers is part of our standard pre-design review for any extension project. We commission drainage searches where needed, identify build-over constraints early, and coordinate the application for a build-over agreement before the planning application or construction programme is locked in. Contact us to discuss how we handle drainage and sewer constraints on your project.