The definitions of (a) low, (b) medium and (c) high risk probability are defined in Table D.I. of Planning Policy Statement 25 (PPS25) ‘Development and Flood Risk’, which Communities and Local Government published in December 2006.
The definitions are as follows:
(a) Low Probability (Flood Zone 1)
This zone comprises land assessed as having a less than one in 1,000 annual probability of river or sea flooding in any year (<0.1%)
(b) Medium probability (Flood Zone 2)
This zone comprises land assessed as having between a one in 100 and one in 1,000 annual probability of river flooding (1 per cent. to 0.1 per cent. or between a one in 200 and one in 1,000 annual probability of sea flooding (0.5 per cent. to 1 per cent.) in any year.
(c) High probability (Flood Zone 3a)
This zone comprises land assessed as having a one in 100 or greater annual probability of river flooding (>1%) or a one in 200 or greater annual probability of flooding from the sea (>0.5%) in any year.
These flood zones refer to the probability of river and sea flooding, ignoring the presence of defences as these can be breached, overtopped and may not be in existence for the lifetime of the development.
PPS 25 is available at: