Freeboard is a safety margin — typically expressed in millimetres — added on top of the design flood level when determining the minimum finished floor level for a development. It accounts for uncertainties inherent in flood level prediction, including limitations in hydraulic modelling, localised wave action, debris accumulation, and variability in ground levels.

There is no single nationally prescribed freeboard value in England. The required freeboard is determined through negotiation with the Environment Agency and local planning authority, informed by the level of confidence in the flood level predictions. Common values include:

300mm: Frequently applied where robust hydraulic modelling underpins the design flood level
600mm: Applied where flood level estimates are less certain, or where the consequences of flooding are particularly severe
Greater than 600mm: Occasionally required in high-consequence situations, such as development for vulnerable populations

The freeboard sits above the design flood level, which itself already includes climate change allowances. So the finished floor level calculation is:

Finished floor level = design flood level + climate change uplift + freeboard

In practice, achieving the required finished floor level may involve raising ground levels, constructing raised platforms, or designing split-level buildings. The chosen approach must not increase flood risk elsewhere by displacing floodwater — a consideration addressed in the Flood Risk Assessment.

Aegaea’s flood risk assessment reports clearly set out the freeboard justification and resulting finished floor levels for each development.