Drains under a garage conversion: sewers and build-over checks

Before converting a garage over or near a drain, identify whether the pipe is a private drain, lateral drain or public sewer, where inspection chambers sit and what access must remain. A new floor, bathroom, foundation or wall can affect the pipe. Planning permission and building regulations do not automatically give permission to build over a public sewer.

Locate the route before fixing the floor

Start with drainage plans, visible chambers, rodding points and the direction of existing pipes. A drainage specialist can trace uncertain routes and inspect condition with a camera. Record levels as well as the line, because a new WC or shower needs a workable fall or a properly specified pumped arrangement.

Private drains and public sewers differ

Responsibility changes at different points in the network and is not always obvious from ownership of the ground. The relevant water and sewerage company determines its process for public assets in its region. Water UK guidance is a useful starting point, but do not assume one company's distance, fee or agreement applies across the whole country.

Build-over and access questions

• Will the proposed floor or foundation sit over the pipe?
• Can the sewer still be inspected, repaired and accessed?
• Is an internal manhole being retained, moved or removed?
• Will a water-company survey, agreement, CCTV report or protection detail be needed?
• Does the building-control submission show the pipe and protective construction?

Contact the relevant provider before construction if its guidance indicates an agreement is needed. Keep the approval, survey and as-built information with the completion file. Read the plumbing and drainage guide for room-service design, then use this page for the underground constraint.

Adding a bathroom changes the drainage brief

A new WC, shower or utility may need a new connection, a pump, a deeper floor or a revised route to the existing drain. Confirm invert levels and access before choosing the room layout. If an inspection chamber sits inside the proposed room, ask the sewerage provider and building-control body how it should be treated; do not simply cover it with a new floor.