Checking a drafted flood risk assessment against the maps

Checking flood risk assessment data in a drafted report takes about twenty minutes and it is the difference between a document a council can validate and one that comes straight back. Three things go wrong most often. The flood zone is stated without the source and date of the check. The climate change allowance quoted is the one that applied when somebody last wrote a template. And the draft says Flood Zone 3 without saying whether that is 3a or 3b, which the published map does not distinguish.

Work through the official sources in order, record what you saw and when, and correct the draft rather than trusting it.

Finding the official flood zone for the site

The Flood Map for Planning is the source of record for Flood Zones 2 and 3 from rivers and the sea. It is the map the local planning authority and the Environment Agency will use, so it is the map the assessment has to cite.

Two properties of that map catch people out. It shows the undefended position, ignoring the presence of flood defences, because the zones express the probability of flooding in the absence of defences. A site behind a substantial embankment can therefore sit in Flood Zone 3 on the map while the actual present-day risk is much lower, and the assessment has to explain that rather than argue the map is wrong.

And it does not split Flood Zone 3 into 3a and 3b. The functional floodplain, Zone 3b, is defined in the authority's Strategic Flood Risk Assessment, so a draft that reports Flood Zone 3 has only done half the check. Find the Strategic Flood Risk Assessment, identify the 3b definition used, and say which zone applies.

Where the site straddles a boundary, report both zones and the proportion of the developable area in each. A Flood Risk Assessment that reports the lower zone because the access happens to sit in it will not survive consultation.

Surface water and reservoir risk layers

River and sea flooding is one source among several, and national policy requires all sources to be considered.

Check the long-term flood risk service for surface water risk at the site, which is reported as risk bands rather than as zones, and for reservoir flooding where a reservoir sits upstream. Check the lead local flood authority for any critical drainage area designation, since that can trigger a site-specific assessment even in Flood Zone 1. Check groundwater flooding, which is poorly mapped nationally and usually has to come from the Strategic Flood Risk Assessment, borehole records or local knowledge. And check sewer flooding records from the sewerage undertaker, which are obtained through a pre-development enquiry rather than from any public map.

A drafted assessment that addresses only fluvial risk on a site with a surface water risk band showing across the access road has missed the thing the officer will notice first. The surface water layer is read differently from the flood zones, and reading the surface water flood risk map for a site sets out how.

Climate change allowances and their revision

The allowances are published on GOV.UK by river basin district, with peak river flow allowances given as bands for different epochs and peak rainfall intensity allowances given separately. They are revised, and the revisions matter because the allowance drives the design flood level.

Never accept an allowance figure in a draft without checking it against the current page. This is the single most common outdated number in flood risk documents, and it is also the easiest for a consultee to spot, because the Environment Agency wrote the guidance.

State the development lifetime you have assumed and the reason for it. Residential development is normally assessed to a hundred year lifetime; a commercial or temporary use may be shorter, and the allowance band follows from that choice.

Sequential and exception test statements

A draft will often include a paragraph asserting that the sequential test is passed. Check whether it can be.

The sequential test is applied by the local planning authority, and the applicant supplies the evidence. That evidence has to include a search area agreed with the authority, the sites considered within it, and the reason each was discounted as not reasonably available. A statement that no alternative sites exist, with no search recorded, is not a sequential test.

Then check whether the exception test is engaged at all. That depends on the flood zone and the flood risk vulnerability classification of the proposed use, and the classification table in national guidance is the reference. Where the exception test applies, both limbs have to be addressed: wider sustainability benefits, and safety for the development's lifetime without increasing flood risk elsewhere.

Where the draft skips the classification step, add it explicitly. The reasoning is set out further in the guidance on the Sequential Test.

Finished floor levels and freeboard

Finished floor levels are where a drafted document is most likely to carry a placeholder, and PlanScope drafts insert a [TO BE CONFIRMED] flag rather than inventing a level. Treat every such flag as a task.

The level has to be derived, not chosen. Take the design flood level for the appropriate return period including the climate change allowance, from a modelled study where one exists or from Environment Agency product data where it does not, then add freeboard. A freeboard of 300 millimetres above the design flood level is commonly applied, and the figure should be confirmed with the Environment Agency or the lead local flood authority rather than assumed.

Check three things alongside it. That the level is expressed in metres above Ordnance Datum and tied to a stated survey, not to an assumed site datum. That the topographical survey covering the site is referenced by date and surveyor. And that access and egress have been assessed, since a dry floor level reached by a flooded drive is not safe access.

What the authority will verify

Validation officers and the Environment Agency check a short list, and knowing it lets you pre-empt the queries.

Whether a site-specific assessment was required at all, which it is for proposals in Flood Zones 2 and 3, for sites of one hectare or more in Flood Zone 1, for sites in a critical drainage area, and where other sources of risk apply. Whether the flood zone stated matches the published map. Whether the vulnerability classification is right for the proposed use. Whether the climate change allowance is current. Whether the sequential test evidence exists. And whether the drainage proposals discharge according to the hierarchy, with infiltration tested rather than assumed.

Getting these right is also the difference between an eight week determination and a holding objection, which is why a checklist for submissions run before the upload earns its time back.

Recording the data sources and dates

Every data point in a flood risk assessment should be traceable to a dated source, and the fastest way to do that is a table at the front of the report.

1. Flood Map for Planning extract, with the date accessed and the grid reference.
2. Long-term flood risk service results for surface water and reservoir risk, dated.
3. Strategic Flood Risk Assessment name, publishing authority and date, with the 3a and 3b definition used.
4. Climate change allowances guidance, with the version and date accessed.
5. Environment Agency product data reference and date, where modelled levels are used.
6. Topographical survey, with the surveyor, date and datum.
7. Sewerage undertaker pre-development enquiry reference and date.

Keep the extracts themselves as appendices with the dates visible on the images. Where the mapping is updated between submission and determination, that record is what lets you say precisely what was checked and when. The wider discipline of scanning a draft for factual errors to look for in a drafted planning document applies to every number in the report.

Draft a flood risk assessment on PlanScope by entering the postcode, then verify every zone, allowance and level against the published sources before submitting.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.