Rainwater drainage carries rainfall from roofs or other surfaces towards a subsequent destination. Outlets, downpipes and downstream connections belong to that route. One visible pipe does not therefore constitute a fully assessed drainage arrangement.
How this term relates to your project
An extension changes where water collects and may make existing connections inaccessible. Draw the route from the roof to its final treatment or discharge. Identify existing pipes requiring investigation and components to be altered. Consider inspection and maintenance access before paving or finishes are installed.
Assign coordination of the boundary between roofing and groundworks. Separate quotations may otherwise leave that connection unnamed. Keep an updated plan with identifiable photographs of concealed routes. Future work can then locate drainage using current information rather than relying entirely on old drawings or memory of construction that is no longer visible.