You may have got planning permission for an extension. But that doesn’t mean you can go ahead straight away and start digging foundations. If it involves building over drains, you’ll need special permission, called a build-over agreement. The water company needs to be able to access public sewers at any time for maintenance and repair work. (A Public Sewer is one that connects more than one property to the main sewer system.)
Firstly you’ll need to contact your local water company with plans from your architect or structural engineer. These should show the existing pipes and manholes, and any proposed ones, together with details of how the building work is going to bridge these pipes, or if you’re planning to divert the sewer so it’s not affected by the building work. You’ll need to do this even if the new building is close to the sewers rather than directly over them. You’ll usually find you won’t be able to build at all if it means building over a manhole or inspection point on a public sewer.
Often you can submit your application online, uploading all the documents needed, and make any payment due. The payment amount may depend on the size of the sewer pipes affected.
If your application is approved, the water company may need to carry out a pre-construction CCTV survey to see if any repair work is needed first. And then once your building work has finished, there may need to be a post-construction CCTV survey to ensure that the work didn’t cause any damage. Only then will you receive the final build-over agreement, which you’ll need to keep safe as it’ll be needed if you later sell your home.
So check before you get the builders in! Retrospective applications may not always get approval and in the worst case scenario, the water company could make you demolish your lovely new extension!