What happened
A rear extension landed on top of a private drain serving two neighbouring properties. The drain had to be diverted around the new footprint.
How it was discovered
Excavation. The drain was not on any drawing. A CCTV survey at design stage would have shown it.
The consequences
New chambers, new pipework, neighbour agreements, water authority involvement. Cost: low five figures. Programme: four weeks.
What homeowners can learn
A CCTV survey costs hundreds. A surprise diversion costs thousands.