The basic rule
Drains can pass under foundations in tightly defined circumstances, with concrete encasement, lintels over the drain, or other approved details. Approved Document H gives the standard cases.
Build-over agreements
Where the drain is a public sewer, you need a build-over (or build-near) agreement with the water authority — separate from Building Control approval.
Common failure points
Missing agreements, drain runs not marked on drawings, manholes covered by paving, and inadequate protection where drains and foundations cross.
Frequently asked
Legally the owner, though designers and builders often manage it. Confirm in writing who is doing it on your project.