Can I build an extension over a drain or sewer?
Often yes, but building over or within 3 m of a public sewer in England or Wales needs the water company's consent, usually through a build-over agreement, before the foundations go in. Private drains can usually be diverted or protected under building regulations. Manholes inside an extension need careful detailing or relocation.
Public or private?
Drains that serve only your house are usually private. Since 2011, most shared drains and sewers in England and Wales have been public and belong to the water company, even when they run under gardens. The sewer map from your water company shows what is where.
Build-over agreements
The water company checks that the extension will not damage the sewer and that it can still be reached for maintenance. It may require a CCTV survey before and after the work, bridging over the pipe or a diversion.
Manholes and inspection chambers
A manhole inside an extension needs a sealed, double cover or, more often, relocation outside. Building regulations also require access for rodding.
Scotland and Northern Ireland
In Scotland and Northern Ireland, building near public sewers needs the agreement of Scottish Water or NI Water, and we check this during design.
This guide is general information, not legal or professional advice for your property. Rules can change; we check the current position for your address at the survey.