Flood Zone 3b
FZ3bThe functional floodplain -- land where water must flow or be stored in times of flood, typically defined as the 5% (1 in 20) annual probability flood extent.
Flood Zone 3b represents the functional floodplain, defined by Planning Practice Guidance as land where water has to flow or be stored during a flood event. It is typically delineated using the 5% annual probability (1 in 20 year) flood extent, or as defined in the local Strategic Flood Risk Assessment.
Development in Flood Zone 3b is subject to the most restrictive planning controls. Only “water-compatible” development and “essential infrastructure” (subject to the Exception Test) are permitted. Residential, commercial, and most other development types are not appropriate in the functional floodplain.
Water-compatible development includes:
- Flood control infrastructure
- Water and sewage treatment works
- Navigation facilities
- Outdoor sports and recreation (excluding buildings)
- Essential transport and utilities infrastructure that must cross the floodplain
Where a site partially falls within Flood Zone 3b, the Flood Risk Assessment should demonstrate that built development is directed to the areas of lowest risk through sequential design. The 3b extent may differ from the Environment Agency’s Flood Map for Planning, which does not distinguish between 3a and 3b — the local SFRA or site-specific modelling is required to establish the boundary.
If you are considering development on land that may include functional floodplain, an early-stage assessment through Aegaea’s FloodMetric platform or a scoping discussion with our flood risk team can help establish the constraints before significant investment is committed.