01 What an FRA is, and when it is required
A flood risk assessment (FRA) is a site-specific study of how likely a site is to flood, now and over the lifetime of the development, and how that risk can be managed. It sits underneath the national planning policy on flood risk and supports the planning application - it is not the same thing as the Environment Agency's online flood map.
In England, a site-specific FRA is required for development in Flood Zones 2 or 3, for any site larger than one hectare even in Flood Zone 1, and for land in a critical drainage area or with another known source of flooding. That one-hectare trigger catches most logistics platforms, so on a big-shed site you should expect an FRA in the data room even when the river zone looks benign.
02 The flood zones, and why the headline zone is not the whole story
The flood zones describe the probability of flooding from rivers and the sea, ignoring defences. Zone 1 is low probability (less than a 1 in 1,000 annual chance). Zone 2 is medium (between 1 in 1,000 and 1 in 100 from rivers). Zone 3a is high (a 1 in 100 or greater annual chance from rivers, 1 in 200 from the sea). Zone 3b is the functional floodplain - land that has to flood or store water - where built development is generally not acceptable.
The zone is a planning gate, not a verdict. It comes from a national model, it excludes surface-water and groundwater risk, and it ignores defences and ground levels. A site can sit in Zone 1 on the map and still carry a real surface-water problem. The FRA is where that site-specific picture is resolved.
03 Which sources of flooding the FRA covers
A competent FRA assesses every relevant source, not just the river. Expect it to address fluvial (rivers), tidal or coastal, pluvial or surface water (rain overwhelming the ground and drains), groundwater, and flooding from sewers and artificial sources such as reservoirs and canals.
On flat industrial land, surface water is often the binding constraint. A large, impermeable roof and yard generate a lot of run-off, so the drainage strategy and the surface-water modelling can matter more to the deliverable scheme than the fluvial zone. If the FRA is thin on surface water, treat that as a gap to price, not a clean bill of health.
04 What climate-change allowance was used
An FRA models flood levels over the development's lifetime - typically 100 years for commercial buildings - by applying Environment Agency climate-change allowances that uplift peak river flows and sea levels. These allowances are banded by river basin, time period (epoch) and a central or upper percentile, and they are revised over time.
This is the single number most worth checking. If the assessment uses an out-of-date or lower allowance, it understates future flood levels, and the finished floor levels and storage volumes designed off the back of it will be too low. Confirm which epoch and percentile were used and whether they match the current allowances for that catchment - an FRA written a few years ago may need revisiting.
05 The mitigations that move a bid
What actually determines cost and deliverability is the mitigation package. Look for: the sequential test (has development been steered to lower-risk land), finished floor levels set above the modelled flood level plus freeboard (commonly 300mm), compensatory flood storage where any floodplain is built on, a SuDS-led drainage strategy that restricts run-off to greenfield rates, and demonstrably safe access and egress in a flood.
Each of these has a price and a land take. Compensatory storage and attenuation ponds consume developable area; raised platforms mean imported fill. A site that "passes" only with substantial off-site works or a large attenuation basin is developable, but at a margin you need to model before you commit to a number.
06 Red flags to catch before you bid
Some findings should change your bid, not just your notes. Watch for: a meaningful extent of functional floodplain (Zone 3b), which is largely undevelopable; reliance on existing flood defences whose condition and maintenance are uncertain; surface-water hotspots shown on the ground the building wants to sit on; a requirement for off-site storage or third-party land to make the numbers work; and an FRA written against superseded climate-change allowances.
None of these is automatically fatal, but each converts a clean site into a conditional one. The job before a bid is to turn every caveat in the report into either a cost, a programme risk, or a reason to walk - and to make sure none of them are buried in an appendix.
07 Frequently asked questions
When is a flood risk assessment required?
For any development in Flood Zones 2 or 3, for development larger than one hectare in Flood Zone 1, and for sites in a critical drainage area or with another known source of flooding such as surface water. The trigger is not only the river or sea zone - size and surface-water risk can require an FRA on a Zone 1 site.
What is the difference between Flood Zone 2 and Flood Zone 3?
Zone 2 is medium probability - between a 1 in 1,000 and a 1 in 100 annual chance from rivers (0.1% to 1%), or 1 in 1,000 to 1 in 200 from the sea. Zone 3 is high probability - a 1 in 100 or greater annual chance from rivers, or 1 in 200 from the sea. Zone 3b, the functional floodplain, is land that must flow or store flood water, where most development is not acceptable.
Can you build a warehouse in Flood Zone 3?
Often in Zone 3a, because industrial, storage and distribution are classed as less vulnerable uses. It usually needs the sequential test applied, raised finished floor levels with freeboard, compensatory storage for any lost floodplain, and safe access and egress. The functional floodplain (Zone 3b) is generally not appropriate for built development.
Why does the climate-change allowance matter?
Because the scheme is designed to the flood levels the FRA models, and those levels depend on the Environment Agency allowance applied. An out-of-date or lower allowance understates future flood levels, so floor levels and storage can be set too low. Check the epoch and percentile used against the current figures for the catchment.
How much does a flood risk assessment cost?
It varies with the site and the level of assessment. A straightforward site-specific FRA in a lower-risk zone is a modest fixed fee; a Flood Zone 3 site that needs hydraulic modelling, climate-change analysis and a drainage strategy costs considerably more. For a bid the fee matters far less than the conclusion - the mitigation the report requires is the number that moves the appraisal.
Who can prepare a flood risk assessment?
An FRA supporting a planning application should be produced by a competent person, usually a flood-risk or drainage consultant or a chartered engineer with hydrology experience. It is not a do-it-yourself exercise for anything beyond the simplest site, because the planning authority and the Environment Agency expect modelling, the right climate-change allowances and a drainage strategy to a professional standard.
Turn the FRA into a position, in hours
Plumb reads the flood risk assessment in a deal's data room alongside the planning, title, drainage and ground reports, pulls out the zone, the allowances, the floor levels and the mitigation package, and flags where the report leaves risk unquantified - every point cited back to the page it came from, so your read is easy to defend in committee.