When you need a build-over agreement
A Thames Water build-over (or build-near) agreement is required whenever foundations, structural slabs or load-bearing walls are placed within 3 metres of a public sewer (the council's drainage map shows the route) or within 1 metre of a private lateral drain shared with neighbours. Common triggers: side-return extensions over the back-of-house combined sewer, rear extensions extending past the existing drainage line, wraparound extensions, basement excavations, and any new dwelling on a plot crossed by a public sewer. The agreement is separate from planning and Building Regulations approval and must be in place before the extension is signed off on completion.
Cost breakdown
Standard self-certified application (sewer ≤225mm diameter, ≤3m deep, simple foundation): £318. Full structural review (larger sewer, deeper invert, complex foundation): £1,420. Pre-build CCTV survey to confirm sewer condition and exact route: £400–£1,500 depending on access. Engineering drawings showing concrete-encased lintels or reinforced raft over the sewer: typically £600–£1,500 from a structural engineer. Manhole repositioning or replacement (if access cover falls within the new extension footprint): £1,800–£4,500. Total typical cost on a London side-return extension: £1,400–£3,500.
How to avoid build-over costs and delays
Survey early: commission a £400–£800 CCTV drainage survey at design stage to confirm the sewer route before architectural drawings are finalised. Many extensions can be redesigned to avoid building over altogether by shifting foundations 200–300mm or rotating the extension. Use lintels not piles: a concrete-encased lintel spanning over the sewer is cheaper and faster to approve than piling near it. Reposition manholes before extension build: doing it as part of the foundation works is cheaper than retrofitting after. Apply early: submit the build-over application alongside the Building Notice or Full Plans, not later — Thames Water determination runs 6–10 weeks and will hold up completion sign-off if not in place.
