Sequential test for a single infill dwelling

A single new dwelling on an unallocated plot in Flood Zone 2 or 3 has to pass the sequential test, and the applicant has to do the work. The flood sequential test infill plot problem is that the test was designed for plan-making and for sites with options, while an infill application has exactly one site and an owner who does not own any others. That does not exempt you. It changes what the test looks like.

National policy steers development away from areas at highest flood risk, and the Planning Practice Guidance on flood risk and coastal change sets out how the test is applied. The guidance is revised periodically, so check the current page rather than quoting paragraph numbers from an older copy.

Why the test applies to an unallocated plot

The test applies to development in an area at risk of flooding unless an exemption applies. The exemptions that matter for a householder or self-builder are minor development and most changes of use, and a new dwelling is neither of those.

Two other routes out exist, and both are worth checking before you write anything. If the site is allocated in the development plan and the authority applied the sequential test at plan-making stage, the test does not need repeating for the same development. And if the site is in Flood Zone 1 with no other source of flood risk identified, there is nothing to sequentially test. Surface water, groundwater, reservoir and sewer flooding all count as other sources, so a Zone 1 site on a mapped surface water flow path is not automatically clear.

Defining the area of search

Everything turns on this. A search area covering the whole local authority will almost always contain a reasonably available site in a lower flood zone, which means the test fails. A search area of one plot is circular and will be rejected.

The guidance expects the area of search to be agreed with the local planning authority and to reflect the nature of the development. For a single open market dwelling with no particular locational need, the authority will usually start from the whole administrative area or the relevant settlement hierarchy tier. For development with a genuine locational requirement, the area can be narrower.

Arguments that have carried weight for a narrower search area:

• the scheme meets an identified local need recorded in a neighbourhood plan or housing needs survey for a named parish
• the development is a rural exception site or affordable housing tied to a local connection
• the proposal is the re-use of a previously developed plot within a defined settlement boundary where the plan supports infill
• an occupational need ties the occupier to a specific holding or business

Note: wanting to build on land you already own is not a locational need, and arguing it as one damages the rest of the submission.

Evidence of reasonably available sites

Once the area is agreed, the test asks whether there are reasonably available sites appropriate for the proposed development in areas at lower flood risk. Reasonably available means available to the market, suitable in principle for the development proposed, and deliverable.

The evidence sources a case officer accepts are public and finite:

1. The authority's brownfield land register and its strategic housing land availability assessment.
2. Site allocations in the adopted local plan and any emerging plan, with their flood zones checked.
3. Sites with extant permission for residential development that remain unimplemented.
4. Land actively marketed for sale with development potential, from commercial listings over a stated period.
5. The authority's five-year housing land supply schedule.

Then assess each candidate against the development proposed, and say why it is or is not a reasonable alternative. A table listing sites with a column stating flood zone, availability and suitability conclusion does the job. The common failure is to list the sites and stop, leaving the officer to draw the conclusion.

Where the test is passed by default

There are situations where the test is effectively satisfied without an extended exercise, and recognising them saves weeks.

Where the entire settlement sits within Flood Zone 2, no site in that settlement is in a lower zone, so the search within an agreed local area produces no alternatives. Where the plot is the only gap in an otherwise continuous frontage and the plan supports infill in that settlement, the number of candidate sites is small and the assessment is short. And where the development is a replacement dwelling on the footprint of an existing one, the policy question shifts to vulnerability and flood resilient design rather than to alternative sites.

Say which of these applies in the first paragraph of your sequential test document. An officer reading a clear statement of the position is far more likely to agree the approach than one working it out from an appendix.

Writing the test for a one plot application

Keep it short and structured. Four pages is usually enough for a single dwelling.

State the development proposed and its flood risk vulnerability classification. State the flood zone from the Environment Agency flood map for planning, with the date you checked it, and the other sources of flood risk from the long term flood risk service and the authority's surface water mapping. State the proposed area of search and the justification for it, with any pre-application correspondence attached. Set out the candidate sites, the sources searched and the dates. Conclude, in terms, that there are no reasonably available sites at lower risk, or identify any that exist and explain why they are not appropriate for this development.

Attach the flood map extract. Half of the disputes about these documents come from the applicant and the officer looking at different versions of the mapping.

Common reasons an officer rejects it

The rejections repeat, and all of them are avoidable:

• no area of search stated at all, or one defined as the site itself
• a search area agreed verbally and never recorded
• candidate sites listed without an availability or suitability conclusion
• the search limited to sites for sale, ignoring allocations and permissions
• no reference to other sources of flood risk beyond the river and sea zones
• a sequential test folded into a flood risk assessment as two paragraphs, when the authority expects a standalone document

That last point is worth taking seriously. The two documents answer different questions. The sequential test asks whether the development should be here. The flood risk assessment asks, given that it is here, how the risk is managed over the lifetime of the development.

When the exception test follows

For a more vulnerable use, which includes dwellings, the exception test is required in Flood Zone 3a. In Flood Zone 2 the sequential test applies but the exception test does not. In Flood Zone 3b, the functional floodplain, new dwellings are not appropriate.

The exception test has two limbs: that the development would provide wider sustainability benefits to the community that outweigh the flood risk, and that it will be safe for its lifetime without increasing flood risk elsewhere. For a single dwelling the first limb is the hard one, because one house rarely delivers wider community benefit. Be realistic about that before committing to a Zone 3a plot.

To see which flood and drainage documents your site is likely to need, enter the postcode on PlanScope and work through the guided questions.

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