Why the agreement is needed
The process protects the pipe from the building work and preserves access for maintenance. Planning permission is not permission to build over a sewer, and Thames Water advises against starting the relevant construction before the agreement or required consent to commence is in place. [3]
For a new building, a strategic sewer or a pressurised sewer, different restrictions can apply. Discuss the actual asset and development rather than assuming the standard house-extension route will be accepted. [3]
What the application needs
Thames Water asks for a location plan, existing and proposed building and drainage layouts, and a section showing the foundations in relation to the sewer. The sewer diameter and relevant levels need to be established; piling or raft proposals can require additional information. [4] [2]
Coordinate this material with the structural and building regulations drawings. If the extension or foundation changes after submission, identify whether the sewer agreement also needs revisiting.
Cost: obtain the applicable current charge
Thames Water publishes developer-service charging arrangements for 2026–27, effective from 1 April 2026. The charging document directs readers to its website for build-over fees. Obtain the current amount for your build-over class directly rather than using an older charge as today's price. [5][6]
Budget separately for the application, the drawings and investigations needed to support it, and any construction changes or legal arrangements required. Thames Water explains that more complex applications can involve engineering surveys and a deed of easement. [1] [4]
Timescale: distinguish review from completion
The company states that its technical team will review the application within 21 days after submission and payment. Missing information can delay that review. For relevant larger-sewer classes, the process then includes pre-construction surveys, permission to proceed and post-construction checks before the final agreement. [2]
For complex class 3 cases, Thames Water's developer guidance gives an application-to-construction period of three to nine months or longer, depending on complexity. This is an indicative range; obtain a project-specific programme from Thames Water rather than applying it to every domestic extension. [1]
Before the builder starts
Confirm the approved drawing revision, any inspection or survey obligations, and the document authorising the relevant work to proceed. Keep the completed agreement with the building control certificate rather than assuming either document replaces the other. [2] [3]