Start with the gullies and channel drains — a five-minute clean out in autumn prevents most rain-related flooding. Downpipe shoes and back-inlet gullies silt up quickly under trees.
If the gullies are clear and it still floods, the pipe capacity is the issue. A partially collapsed or root-restricted run behaves fine in dry weather and fails the moment rainfall load is added.
Misconnections — a downpipe plumbed into the foul drain, or a bathroom waste plumbed into the surface water drain — are common in extended properties and worth checking with a survey.