Sequential test for flood risk: an illustrative example

The flood risk sequential test asks one question: are there reasonably available sites appropriate for the proposed development in areas at a lower risk of flooding? If there are, permission should be refused, whatever the merits of the scheme. A sequential test worked example is therefore an exercise in evidence rather than argument, and the evidence has to be assembled in a particular order.

Planning Practice Guidance on flood risk and coastal change sets the method. The applicant carries out the test, the local planning authority decides whether it is satisfied, and the Environment Agency will often comment on the flood data while leaving the site comparison to the council. What follows sets out the sequence and then a clearly labelled illustrative comparison.

What the test actually asks

The test compares the application site against alternatives. It does not ask whether the proposed development can be made safe, which is the job of the Flood Risk Assessment and, where the test is passed, of the Exception Test.

Three exclusions save a great deal of wasted work. The test is not required for development proposals in Flood Zone 1 unless the strategic flood risk assessment identifies other sources of flooding. It is not required for minor development, which in this context means householder extensions, small non-residential extensions and alterations with no increase in floorspace. And it is not required for changes of use, other than a change of use to a caravan, camping or chalet site, or to a mobile home or park home site.

Where the test does apply, it applies to the whole of the development. Splitting a scheme so that the vulnerable parts sit in the lower zone is a sequential approach within the site, which is a separate and additional requirement.

Fixing the search area before you start

Agree the geography with the case officer in writing before you search. This is the single step that decides whether the test survives scrutiny, because a search area that looks self-serving will be rejected however careful the rest of the work.

The area should reflect the catchment the development serves. A local convenience store draws from a walkable neighbourhood. A district office relocation draws from a travel-to-work area. A local authority boundary is the usual default for housing, because that is the area over which the council plans for supply, but a smaller area can be justified where the need is local in fact, such as replacement premises for an established business with a named workforce.

Record the reasoning. Two sentences explaining why the search covers the parish rather than the district, supported by the operational need, is worth more than a page of assertion.

Which sites count as reasonably available

A site is reasonably available if it is in a suitable location for the development and there is a reasonable prospect that it will be available within a relevant timescale. Ownership by a third party does not by itself make a site unavailable, which surprises applicants. A site marketed for sale, or allocated in the development plan, is likely to be treated as available unless you can show otherwise.

Search these sources and say that you did:

1. Site allocations and any safeguarded land in the adopted development plan and its policies map.
2. The strategic housing and economic land availability assessment.
3. The brownfield land register.
4. Sites with an extant permission that has not been implemented.
5. Commercial property listings over a stated period, with the search date recorded.
6. Land in the applicant's own ownership or control.

Then explain, site by site, why each one is or is not appropriate. The honest discounting reasons are size, an incompatible allocation, a physical constraint such as a ransom strip, a designation that would prevent the use, or contamination cost that a viability appraisal shows is prohibitive. The weak reasons are commercial preference and the fact that the applicant already owns the application site.

Comparing flood zones without flattening the detail

Flood Zone 1 covers land with less than a 0.1 per cent annual probability of river or sea flooding. Zone 2 covers between 0.1 and 1 per cent for rivers and between 0.1 and 0.5 per cent for the sea. Zone 3a is 1 per cent or greater for rivers and 0.5 per cent or greater for the sea, and Zone 3b is the functional floodplain defined in the strategic flood risk assessment.

Two points matter when you tabulate them. First, the published Flood Map for Planning shows zones without defences, so a defended site still sits in Zone 3 on the map, and the comparison must be made on that basis. Second, surface water and groundwater risk sit outside the zones, so a Zone 1 alternative on a critical drainage area is not automatically the lower-risk option. Say so where it applies.

An illustrative worked comparison

The following comparison is illustrative only. It uses invented sites to show the shape of the table, not real land.

A scheme of eight dwellings is proposed on a 0.35 hectare site in Flood Zone 3a within a market town. The search area agreed with the officer is the town's settlement boundary plus 2 kilometres.

On these illustrative facts the test fails, because Site B is both available and in a lower zone. The correct advice to the client at that point is to look at Site B, not to reach for the exception test.

Had Site B carried a live permission for a school, or been shown by a signed contract to be committed elsewhere, the column would read differently and the conclusion would change. That is why the availability column needs a source and a date against every entry.

Writing the conclusion the officer can adopt

Officers copy sequential test conclusions into committee reports and delegated notes. Make that easy. State the search area and why, the date of the search, the sites considered, the discounting reason for each, and a single closing sentence that the test is passed because no reasonably available site exists in a lower zone.

Keep the flood modelling in the flood risk assessment itself and the surface water design in the Drainage Strategy, rather than repeating either in the test. Our note on drainage strategy for a minor residential scheme covers what the drainage side should contain on a scheme of this size.

Where the test is passed and the development is in Zone 2 or 3, the exception test may follow. The Sequential Test (Flood Risk) page sets out what PlanScope's draft covers and where a reviewer needs to add local evidence.

When the test is failed, and what follows

A failed test is not a drafting problem. Rewording it to disqualify the inconvenient site invites a refusal that an inspector will uphold, and it damages the applicant's credibility on the rest of the application. Better to say that the test is not passed, set out the exceptional circumstances the client wishes to rely on, and let the authority weigh them openly.

There is one honest area of uncertainty. Judgements about what is reasonably available in a rising or falling land market differ between officers, and the same site list can produce different conclusions in two neighbouring authorities. Fix the search date, keep the marketing evidence, and expect to defend the availability column rather than the flood data.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.