What happened

The trench had been excavated for a side extension in north London. The design assumed clean ground. On the pre-pour walk, an unrecorded clay drain was visible in the trench wall, running diagonally across the footprint.

How it was discovered

An independent inspection the afternoon before the pour, scheduled deliberately to walk the open trench. The drain was a shared sewer transferred to the water authority in 2011 — not on the original drainage drawing.

The consequences

Pour cancelled. Concrete lorry rescheduled. Engineer attended within twenty-four hours. A build-over agreement was applied for, the foundation redesigned with lintel protection, and the project resumed within three weeks.

What homeowners can learn

Independent eyes on an open trench cost a fraction of a remedial excavation. The single most cost-effective inspection on any extension is the one before the foundation pour.