If your new extension would sit over or close to a public sewer, contact the company responsible for that sewer while the design is still flexible. The water company will tell you whether your proposal needs a formal build over agreement, qualifies for another route or needs to change. Its answer can affect the footprint, foundations and access to the drains.
Start this check before agreeing a fixed construction price. A change to a foundation line, an inspection chamber or a proposed diversion can change the builder’s scope.
Find Out What Runs Under the Plot
Look for drainage information in the property documents and ask the sewerage company how to obtain its asset plan. Tell your designer where you can see inspection covers, downpipes or gullies, and share any previous survey information. Many shared drains and sewers changed ownership in 2011. A pipe running through a private garden can still be a public asset, and the mapping may not show every relevant pipe.
If the route or depth is uncertain, ask the designer and water company what evidence is needed. A specialist survey may be necessary. An inspection cover alone does not establish the direction of the pipe, who maintains it, or whether it can be built over.
Match the Drainage Information to the Proposed Drawings
The designer needs the same information the water company will use to assess the proposal:
- The location, type and status of the known drain or sewer.
- The position of the existing house, inspection points and proposed extension.
- The distance between the pipe and the planned foundations, with depths and other details where the company asks for them.
- How the asset will remain accessible and which details the company wants to check before and after work.
For example, Severn Trent has routes for both eligible self-certification and a formal agreement. The circumstances of the asset and the design determine the route. If an asset cannot be built over or near as proposed, discuss a revised layout or any options the water company will consider before moving forward. Check that your extension designer and any drainage specialist are working from the same information.
Example: A homeowner wants to extend across the full rear wall. The designer overlays the sewer information and sees a shared line close to the new footprint. The homeowner checks the line’s status with the water company and commissions the evidence it asks for. With that information, the team can compare a shorter extension with a design that needs more drainage work before choosing which to price.
Track the Sewer Agreement Alongside Building Control
Ask who will submit each application and who will deal with queries. Keep the water company’s written response and referenced drawings alongside the building control documents. Tell both teams about changes that move the footprint or alter the relevant foundation or drainage detail. Confirm any required inspections before work covers the pipe or surrounding details.
If you know there is a drain or manhole near the proposed extension, mention it when you request a design scope.