Build Over Agreements for London Sewer Extensions: A Developer Guide
A build over agreement is required by Thames Water when a building, extension, or structure is proposed within a specified distance of a public sewer in London. The requirement applies to a very large number of London development projects, from simple domestic extensions to major residential and commercial developments, and failing to obtain a build over agreement before commencing work can result in the requirement to demolish the completed structure. Understanding how the build over agreement process works is essential for London developers and householders.
What Is a Build Over Agreement and When Is It Required in London
A build over agreement is a formal consent granted by Thames Water that allows a building, extension, or structure to be constructed over or near a public sewer that is maintained by Thames Water. The requirement for a build over agreement applies to any development within three metres of the centreline of a public sewer up to 160mm in diameter, and within six metres of the centreline of a public sewer larger than 160mm in diameter. Given the density of the public sewer network across London, where public sewers run through the rear gardens and under the buildings of a very high proportion of residential properties, these thresholds are triggered by a large number of London extension and development projects that would appear straightforward to the developer or homeowner.
The build over agreement process exists to protect the public sewer from damage during and after construction, to ensure that Thames Water retains the right to access the sewer for maintenance and repair, and to prevent the construction of foundations or structures that would compromise the structural integrity of the sewer or prevent future access for maintenance. Without a build over agreement, Thames Water would have no record of the construction work over their sewer asset, and if the sewer subsequently required repair or replacement, Thames Water would have the right to demolish any structure built over it without any obligation to compensate the owner. Obtaining a build over agreement protects the developer and the future owner of the property against this risk.
The Build Over Agreement Application Process with Thames Water in London
The first step in the build over agreement process is to obtain a Thames Water sewer record for the site, which shows the location, depth, diameter, and direction of public sewers in and around the plot. Thames Water provides sewer records through their developer services portal, and the records should be obtained and checked before the design is finalised so that the foundation and structural design can be developed to accommodate the sewer position. Where the sewer record shows a public sewer within the proximity thresholds, the structural engineer should design the foundations and structure to avoid loading the sewer and to provide access for Thames Water to maintain the sewer without disrupting the building above.
The formal build over agreement application is submitted to Thames Water Developer Services with drawings showing the proposed development in relation to the sewer, structural details of the proposed foundations, and details of any access arrangements for the sewer within or beneath the development. Thames Water will assess the application against their technical standards and will issue either a build over agreement, a build near agreement for structures within the proximity zone but not directly over the sewer, or a technical refusal if the proposed development would unacceptably compromise the sewer. In some cases, Thames Water may require a CCTV survey of the sewer before and after construction to establish its condition and to confirm that the construction works have not caused any damage. Prestige Engineers can carry out pre-application sewer surveys, advise on foundation design implications, and manage the Thames Water build over agreement application process for London development projects of all scales.