Drainage and Sewer Build-Over in London: What You Need to Know

A build-over occurs when a proposed extension or structure is built over, or within 3m of, a public sewer. Under the Water Industry Act 1991, building over a public sewer without a Build-Over Agreement is prohibited — Thames Water has the right to access its sewers for maintenance and repair, and structures over them obstruct this access. A public sewer is one that is adopted by Thames Water — the distinction between private (your responsibility) and public (Thames Water's responsibility) drains is important. Drains within a single property's boundary are typically private. At the point where a drain first serves more than one property, or at the boundary of a property, it may become a public sewer. Since 2011, many private sewers that previously lay outside the boundary (in shared access routes, shared gardens, or along rear boundaries between properties) were transferred to Thames Water ownership under the Private Sewer Transfer. This significantly increased the number of public sewers in London. Always check whether a sewer is public or private by requesting a Thames Water sewer map (available through their Build-Over team or through a drainage consultant).

The Build-Over Agreement process with Thames Water for London extensions: Step 1 — Sewer map check: obtain a Thames Water sewer map showing the position of public sewers relative to your proposed extension. Request this through Thames Water's website or through a drainage consultant. Step 2 — Determine if a Build-Over Agreement is required: if the extension footprint is within 3m of a public sewer centreline, a Build-Over Agreement is required. If the sewer is between 3-6m away, the design must ensure the extension foundation does not affect the sewer — advice from a structural engineer is required. Step 3 — CCTV survey of the existing sewer: before agreeing to a build-over, Thames Water requires a CCTV survey of the existing sewer to establish its pre-works condition. This is carried out by an approved drainage contractor. Cost: £300-£700 for a standard residential CCTV survey. Step 4 — Design review: Thames Water reviews the extension design to confirm it does not adversely affect the sewer (structural loading, access for future maintenance). They may specify design requirements — minimum cover to the sewer pipe, foundation type (piled rather than strip foundations to avoid loading the sewer), and access chamber requirements. Step 5 — Agreement and fee: Thames Water issues a Build-Over Agreement on payment of their fee. Current fee (2025): approximately £350-£500 for a standard residential build-over. Step 6 — Post-construction CCTV survey: after construction, a second CCTV survey of the sewer confirms it has not been damaged. This is typically provided to Thames Water to discharge the build-over agreement conditions.

In some cases, Thames Water will not approve a build-over — typically where: the sewer is a large-diameter or critical strategic sewer that serves many properties and cannot safely support a structure above it; the proposed extension would make the sewer completely inaccessible without dismantling the extension; or the existing sewer is in poor condition and requires replacement rather than maintenance-access provision. If a Build-Over Agreement cannot be obtained, the options are: Redesign the extension to avoid the sewer (change the footprint, reduce the depth of the foundation, or use a suspended floor that allows sewer access); Divert the public sewer: Thames Water will divert a public sewer at the building owner's expense — this is a significant cost (£10,000-£50,000+ depending on the length and depth of the diversion) and a complex process. However, it permanently moves the sewer out of conflict with the extension. If a private sewer (serving only your property) is in conflict with the extension, the situation is different — you own the private drain and can divert it yourself with a drainage contractor at much lower cost (£2,000-£8,000 for a standard private drain diversion).

Beyond the build-over issue, drainage design for a London extension involves: Surface water disposal: the new extension roof area creates additional surface water that must be drained. In London, surface water is ideally discharged to a soakaway (a rubble or modular plastic soakaway pit in the garden, allowing water to soak into the ground). Where the ground conditions do not permit soakaways (London clay does not allow soakaway drainage in most areas — the clay is impermeable), the surface water must connect to the combined or surface water sewer. For extensions in London, the most common solution is connecting the new rainwater downpipe to the existing surface water drainage system. Check with your drainage contractor whether the existing system has capacity for the additional flow. Foul drainage for an additional WC or bathroom in an extension: the new drainage must connect to the existing foul drainage system. Running new foul drainage pipes within the extension structure or along the external wall to the existing inspection chamber requires careful planning — gradient (fall of approximately 1:40-1:80 for 100mm pipe), accessibility, and connection at the inspection chamber. Rodding access: Building Regulations require rodding access to all drainage runs — access points must be provided at every change of direction and at defined maximum intervals so that blockages can be cleared.