Northumbrian Water (NWL) assumed responsibility for an estimated 13,500km of private drains and sewers when the law changed to transfer ownership from property owners to local water companies. However, only 5% of the inherited network was mapped. 1Spatial proposed an approach built on its 1Integrate solution. Working with subject-matter experts at Northumbrian Water, the team developed a series of rules to infer the missing information, based on limited records available and the expertise of staff.