What happened

An extension's surface water was discharged to a new soakaway sized on assumed ground conditions. The actual soil percolated far slower than assumed.

How it was discovered

The first heavy rainfall after completion flooded the rear lawn and seeped into the neighbouring garden.

The consequences

Redesign as an attenuation tank with a controlled discharge to the surface water sewer. Excavation, reinstatement, and a slightly awkward conversation with the neighbour.

What homeowners can learn

Soakaway design needs a real percolation test, not an assumption. Tests are cheap. Floods are not.