Sewer Build-Over Agreements in London: What They Are, When You Need One, and How to Get Approved

Key Takeaways

• ✓Public sewers and lateral drains: the Water Industry Act 1991 and the Water Industry (Schemes for Adoption of Private Sewers) Regulations 2011 (the "October 2011 transfer") define public sewers as those adopted and maintained by the sewerage undertaker (Thames Water in London). Before October 2011, only sewers serving more than one property were adopted — private drains serving a single property (connecting the property's internal drainage to the public sewer in the street) were the owner's responsibility. After October 2011, the private sewers and lateral drains between a property boundary and the public sewer main were transferred to Thames Water (the sewerage undertaker) and are now public sewers, maintained by Thames Water. The practical consequence is that the network of public sewers in London is now much larger than it was before 2011 — many pipes running through domestic gardens that were previously private drains are now classified as public sewers and are subject to Build Over Agreement requirements. An important implication for rear extension projects: the lateral drain running from the house's internal drainage system to the public sewer in the street typically runs through the rear garden (or under the house) — and this lateral drain is now a public sewer. If the proposed extension is to be built over or within the exclusion zone of this lateral drain, a Build Over Agreement is required from Thames Water. The distinction between a "drain" (serving a single property, privately owned) and a "sewer" (serving more than one property, adopted by Thames Water) remains relevant for drainage that has not been transferred — for example, the underground drainage serving a single property entirely within that property's boundaries, running from the soil stack to the boundary, may still be privately owned and therefore not subject to a Build Over Agreement — but the determination of adoption status must be confirmed by a Thames Water records search before this assumption is relied upon
• ✓Thames Water's Build Over Agreement requirements: Thames Water's Build Over Policy (published on their website and updated periodically) sets out the conditions under which they will permit construction over or near public sewers. The key requirements as of 2025 are: a pre-application CCTV survey of the public sewer must be carried out by a contractor accredited by the Water Research Centre (WRcN) or similar accredited body, to establish the existing condition of the sewer before construction begins. Thames Water requires the CCTV survey results to confirm that the sewer is in a reasonable state of repair before the build-over agreement is granted — if the sewer is in poor condition, Thames Water may require the sewer to be repaired before construction proceeds; the new structure must not: be within 1 metre of the pipe centreline for pipes up to 160mm diameter and within 3 metres for pipes of 225mm diameter and above (this is a simplified summary of Thames Water's exclusion zone requirements — the actual minimum distances depend on the pipe diameter, depth, material, and construction method, and are set out in the Thames Water Build Over Agreement guidance); significantly increase the loading on the sewer (construction materials and structural loads that exceed the existing ground loading above the sewer will require structural support to be designed to bridge over the sewer, transferring the structural loads to foundations on either side of the sewer exclusion zone); allow access to the sewer for maintenance and inspection (Thames Water requires that inspection access to manholes in the path of the development is either maintained or relocated — a new inspection chamber may need to be constructed at a location that allows access despite the new structure above); the proposed structure must be designed and built so that the sewer can be accessed for emergency repairs without demolishing significant parts of the building above. Post-construction CCTV survey: Thames Water typically requires a post-construction CCTV survey of the sewer after the works are complete (usually 3 months after completion, when any settlement has occurred) to confirm that the construction has not caused damage to the sewer. The results of the post-construction CCTV survey are compared with the pre-application survey to identify any changes to the sewer's condition
• ✓How to identify whether a public sewer runs through your London garden: the starting point is a Thames Water records search — an online sewer records search, available for free through the Thames Water website, showing the location of recorded public sewers (within approximately 1:1250 scale accuracy). The Thames Water sewer records show: the location and diameter of recorded public sewers (including both the adopted main network sewers and the October 2011 transferred lateral drains); the location of manholes and inspection chambers; the direction of flow in the sewer (useful for understanding the drainage catchment). Important caveats on Thames Water sewer records: the records show the recorded position of sewers, which may not always correspond exactly with the actual position of the pipe in the ground — particularly for older Victorian sewers in London, where records may be inaccurate by up to 500mm or more in position and depth; some sewers in London, particularly older combined sewers and smaller lateral drains, may be unrecorded in Thames Water's database — a drainage engineer who encounters a previously unrecorded sewer during ground investigation must notify Thames Water so the sewer can be added to the records; the sewer records show the horizontal position of the pipe but do not typically show the depth, which is important for structural design. A drainage CCTV survey (carried out before design begins) provides more reliable information about the position, depth, condition, and construction material of any sewer within the proposed development area than the Thames Water records alone. For any rear extension or basement project in London, a pre-design drainage survey (including Thames Water records check, visual inspection of any accessible manholes or inspection chambers, and CCTV survey of any sewers within or close to the proposed works area) is strongly recommended as part of the early project feasibility stage
• ✓What happens if you build over a public sewer without a Build Over Agreement: Thames Water has a legal right under the Water Industry Act 1991 to access public sewers for maintenance, repair, and improvement. If a building is constructed over a public sewer without a Build Over Agreement, Thames Water can: require the building owner to demolish or alter the structure at the building owner's cost to allow access to the sewer; carry out works to the sewer in an emergency and charge the costs (including any resulting structural damage to the building above) to the building owner; refuse to issue retrospective Build Over Agreement approval, requiring the building to be modified or relocated. Additionally, building over a public sewer without a Build Over Agreement may: prevent you from obtaining Building Regulations completion certificate (Local Authority Building Control and private approved inspectors are required to check drainage and sewer compliance as part of the Part H inspection); create complications when selling the property (the CON29DW drainage search carried out by the buyer's conveyancer will reveal whether a Build Over Agreement was obtained, and an absence of a Build Over Agreement for a building over or near a sewer will be flagged as a risk item requiring investigation and potential remediation before the transaction can proceed). The cost of a retrospective Build Over Agreement application (where one was required but not obtained) is typically higher than a pre-application agreement — Thames Water may require additional information, reinstatement of access chambers, or structural works to the building — and the outcome is not certain (Thames Water may require structural modifications as a condition of retrospective approval)