Start with the sewerage provider
These procedures apply where Thames Water is responsible for the sewerage network. Confirm the provider for the address rather than assuming the company supplying drinking water necessarily answers every sewer question. Thames Water distinguishes its public sewers and lateral drains from private drains for which the landowner is responsible. [2]
Obtain the relevant asset information through the published map or property-search service. Keep the search date and plan with the site survey so the designer can see what was checked. [1] [3]
Do not treat a blank map as proof of no sewer
Thames Water's guidance explicitly addresses sewers that are not mapped. Where the proposed footprint is close to existing drainage, investigate rather than assuming the extension can be positioned solely from the map. [2]
A useful survey brief asks for the pipe route, internal diameter, flow direction, connections and accessible chamber levels needed for the design. The eventual build-over application asks for existing and proposed drainage layouts and a foundation section showing the sewer relationship. [4] [5]
Where CCTV fits in
A drainage survey can help establish the layout and condition of pipes before the design is fixed. Thames Water also requires its own pre- and post-construction survey arrangements for relevant build-over classes. Do not assume that a privately commissioned report automatically replaces the company's required survey or technical review. [5]
Ask the surveyor to provide a marked plan as well as any footage or report. The design team needs to understand where a finding sits relative to the proposed wall and foundation, not merely that a defect or connection exists somewhere in the recording.
What the result can change
Where work is within three metres of a public sewer or one metre of a public lateral drain, Thames Water identifies a build-over approval requirement for relevant work to existing properties. Its assessment can affect the foundation detail, sewer access and the location of manholes. [6] [2]
New buildings and certain sewer types need different consideration; do not assume an ordinary extension build-over route covers every development. Raise the issue before ordering the final structural design. [2]
Coordinate the approvals
Planning permission does not authorise building over a sewer. Building regulations also address drainage and construction over sewers, while the water company's agreement is a separate process. Use the verified survey to prepare a consistent design for both. [2] [7]
Keep the search, survey, approved drainage drawing and any agreement in the completion record so future owners can understand what was built.