Flood Zone B covers land with a moderate probability of river or coastal flooding — lower than Zone A but higher than Zone C. Less vulnerable development may be acceptable in Zone B, while highly vulnerable development such as housing generally requires the Justification Test and a site-specific flood risk assessment.
Zone B sits between the high probability Zone A and the low probability Zone C. Commercial and industrial uses, classed as less vulnerable, can often be accommodated there subject to appropriate design and mitigation. Housing, being highly vulnerable, is treated more restrictively.
In practice, a planning application on a Zone B site will usually need a site-specific flood risk assessment demonstrating the flood levels for relevant events, how finished floor levels and access have been designed, and that the development does not increase flood risk elsewhere.