The Environment Agency divides England into flood zones that describe the probability of flooding from rivers and the sea. Understanding which flood zone your site falls in — and the Sequential and Exception Tests that go with it — is the starting point for almost every planning flood risk question.
The flood zones, defined
- Flood Zone 1 (low probability): less than a 1 in 1,000 annual chance of river or sea flooding (below 0.1%). Most development is acceptable in principle.
- Flood Zone 2 (medium probability): between a 1 in 100 and 1 in 1,000 annual chance from rivers (1% to 0.1%), or 1 in 200 to 1 in 1,000 from the sea.
- Flood Zone 3a (high probability): a 1 in 100 or greater annual chance from rivers (above 1%), or 1 in 200 or greater from the sea.
- Flood Zone 3b (the functional floodplain): land where water has to flow or be stored in times of flood. Development here is very restricted.
Importantly, these zones ignore the presence of flood defences — they show the natural, undefended risk — and they do not cover surface water, groundwater or sewer flooding, which are assessed separately.
Why the zone matters: vulnerability classification
Planning policy pairs each flood zone with the "vulnerability" of the proposed use — from water-compatible and less vulnerable, through more vulnerable (such as housing), to highly vulnerable (such as basement dwellings). Some combinations are acceptable, some require the Exception Test, and some should not be permitted at all. A block of flats in Flood Zone 3a is treated very differently from a warehouse in the same zone.
The Sequential Test
The Sequential Test is the first hurdle: it steers development towards areas at the lowest probability of flooding. Applicants may need to show that there are no reasonably available sites in a lower flood zone that could accommodate the proposed development. If a lower-risk site is available, the authority can refuse the application on that basis alone, so the Sequential Test is often addressed early.
The Exception Test
Where the Sequential Test is passed but the use is still vulnerable for its zone, the Exception Test may apply. It requires two things to be demonstrated: that the development provides wider sustainability benefits to the community, and that it will be safe for its lifetime without increasing flood risk elsewhere — the second part being the job of the site-specific FRA.
Read next: What goes into a site-specific FRA, or get a fixed-fee quote.
FAQs
My site has flood defences — does that change the zone?
The published flood zones show undefended risk, so defences do not change your mapped zone. A site-specific FRA can, however, take the standard and condition of defences into account when assessing actual risk.
Who carries out the Sequential Test — me or the council?
The local planning authority is responsible for applying it, but applicants are frequently asked to provide a Sequential Test assessment to support it, particularly for windfall and smaller sites.
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