Summary
Flood Zones are planning policy bands for river and sea flooding. They help decide where development is appropriate, but they are not the same as address level long term flood risk. Flood Zone 3b, the functional floodplain, is the land where water has to flow or be stored during floods and is usually identified through the local Strategic Flood Risk Assessment rather than being separately shown on the national Flood Map for Planning.
Definition
Flood Zones for planning classify land by the annual probability of river or sea flooding. In England, Zone 1 is low probability, Zone 2 is medium probability, Zone 3a is high probability and Zone 3b is the functional floodplain. The functional floodplain normally includes land with a 3.3% or greater annual probability of flooding with infrastructure operating effectively, or land designed to flood, although local planning authorities must define it in Strategic Flood Risk Assessments in agreement with the Environment Agency.