Flood zones are areas classified by their probability of flooding, used in the planning system to guide what kind of development is appropriate where. The Planning System and Flood Risk Management Guidelines define three zones, A, B and C, based on the likelihood of river or coastal flooding in any given year.
The national guidelines set out a sequential approach: avoid development in areas at risk of flooding where possible, substitute a less vulnerable use where avoidance is not possible, and only then consider mitigation. Flood zones exist to make that sequence operable, by classifying land according to flood probability rather than by whether it has flooded recently.
The zones deal with fluvial (river) and coastal flooding. Other sources — pluvial or surface water flooding, groundwater flooding and drainage failures — are considered separately and can affect a site that is not within Zone A or B.