Sequential test for a windfall housing site

If your windfall housing site sits in Flood Zone 2 or 3, the sequential test is the first thing the case officer looks for and the most common reason small residential applications in flood risk areas are refused. The test asks whether there are reasonably available sites in an area at lower flood risk where the development could go instead. It is not about your site being well designed, and it is not answered by a flood risk assessment. A windfall site is unallocated by definition, so nobody has applied the test for you at plan-making stage, and the burden sits with the applicant.

What the test asks

The flood risk and coastal change chapter of the NPPF (August 2026 edition) applies a sequential, risk-based approach: steer new development to areas with the lowest probability of flooding, from any source. The Planning Practice Guidance chapter on flood risk and coastal change sets out how to apply it at application stage.

Three points decide most cases.

First, the test covers flooding from every source, with rivers and the sea only part of it. Surface water, groundwater, sewers and reservoirs all count, which is why a site shown as Zone 1 on the Environment Agency flood map for planning can still need a sequential argument where the council's surface water mapping shows a flow path across it.

Second, the test is comparative. You are not proving your site is safe, that comes later. You are showing that lower-risk alternatives are not reasonably available.

Third, the vulnerability classification matters. Housing is "more vulnerable" development, which is not appropriate in Flood Zone 3b, the functional floodplain, whatever the sequential test says. In Zone 3a it needs the Exception Test as well, and in Zone 2 the sequential test alone is usually the hurdle.

Defining the area of search

The area of search is the single most contested part of a small-site sequential test, and the guidance is clear that it should be agreed with the local planning authority before you do the work. Send an email, get a reply, keep it.

The default is the local planning authority's administrative area. It can properly be narrower where the development has a functional or policy tie to a place:

• A rural exception site meeting a need evidenced for a named parish.
• Affordable housing secured for a particular settlement through a local connection condition.
• A replacement dwelling on the footprint of an existing one.
• Development serving an existing business or agricultural holding at that location.

It can also be wider than you would like. A market housing scheme with no locational tie has a weak case for narrowing the search to one village, and inspectors have consistently rejected search areas drawn to the applicant's landholding. Wanting to build where you own land is not a locational constraint in planning terms.

Write the area of search down with a boundary, a stated reason, and the date the authority agreed it.

Finding reasonably available sites

"Reasonably available" means a site in a suitable location for this type of development, with a reasonable prospect of being available and developable within a reasonable period. That is an objective question about the market and the plan, not about your own options.

Search these in order, and record what you found and when:

1. Housing allocations in the adopted local plan that have not been delivered, with their flood zone checked individually.
2. The council's brownfield land register, which lists sites the authority considers suitable for housing.
3. The latest housing and economic land availability assessment, where one is published.
4. Sites with an extant permission for housing that are on the market.
5. Land currently advertised for residential development by agents in the search area.

For each candidate, record the reference, the site area, its flood zone, and the reason it is or is not a reasonable alternative. Suitable reasons include being in the same or a higher flood zone, being too small for the proposal, being under construction, or having a policy constraint that rules the proposal out. Unsuitable reasons include "the owner would not sell to me" and "the price was too high", both of which inspectors treat as commercial preference rather than availability.

Give the search a cut-off date and state it. A test done against a register from three years ago is worthless.

Evidence for a single plot or small windfall site

Proportionality is real here, and it cuts both ways. Nobody expects a 12-page appraisal for one dwelling. They do expect a table.

For a single plot, a sequential test of about a page and a half usually does the job: the site, its flood zone and the source of risk, the agreed area of search, a table of candidate sites with the reason each was discounted, a cut-off date, and the conclusion. A Sequential Test (Flood Risk) document does not need to repeat the modelling from the flood risk assessment.

Two honest observations about small sites. A single self-build plot inside a settlement boundary very often passes, because the realistic comparators are large allocations that cannot be substituted for one house. And a single plot in open countryside in Zone 3a usually fails, because the sequential test is then doing the same work as the countryside policy and both point the same way.

Where the test does not apply

The guidance exempts some development from the sequential test at application stage. Householder development, such as an extension to an existing house, is outside it. Minor development, including small non-residential extensions and alterations that do not increase the footprint, is outside it. Changes of use are generally outside it, apart from changes to a caravan, camping or chalet site.

Sites allocated in the development plan where the test was applied during plan-making do not need it reapplied, provided nothing material has changed, such as new modelling or an updated climate change allowance.

A windfall site gets none of these. New-build housing on an unallocated site in Zone 2 or 3 needs the test, even where the site is a garden plot in the middle of a built-up street. What changes with a small site is the depth of evidence, not whether the test applies.

Why officers reject a sequential test

The refusal reasons repeat. No area of search stated at all, so the exercise cannot be assessed. A search area drawn around the applicant's own land. No named candidate sites, only a statement that no alternatives exist. Candidate sites discounted on price or ownership. Surface water risk ignored because the site is in Zone 1 for river flooding. No cut-off date. And the most frequent one: a document titled "sequential test" that is actually a flood risk assessment, describing finished floor levels and escape routes without ever comparing sites.

Officers also object where the test conflicts with the application's own case. If your planning statement argues the site is uniquely suited to the locality while your sequential test says the area of search is the whole district, one of the two is wrong.

Drafting a test an officer can sign off

Put it in this order, keep it short, and keep the modelling separate in the Flood Risk Assessment.

1. Proposal and vulnerability classification.
2. Flood risk at the site, all sources, with the map references and date of checking.
3. Area of search, boundary, justification, and the date the authority agreed it.
4. Sources searched, with dates.
5. Candidate sites table with the discount reason for each.
6. Conclusion on whether reasonably available lower-risk sites exist.
7. If Zone 3a, a cross-reference to the exception test and the two limbs it needs.

Send the draft area of search to the case officer before you write the rest. Ten minutes of their time removes the single largest risk in the document. Drainage evidence runs alongside this and is a separate exercise, set out in the note on drainage evidence for a single new dwelling.

Enter your postcode on PlanScope to see whether your site falls in Flood Zone 2 or 3 and which flood documents an application there is likely to need.

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.