The sequential test for a change of use application

A sequential test change of use application is usually not required, and that is the useful answer most people are looking for. National flood risk guidance provides that change of use applications do not generally need to be accompanied by a sequential test, with an exception for changes of use to caravan, camping, chalet, mobile home and park home sites. What you almost certainly do need instead is a site-specific flood risk assessment, and where the new use is more vulnerable to flooding than the old one in Flood Zone 3a, the exception test.

So the question to settle first is not whether to do a sequential test. It is whether the vulnerability of the use is going up.

When a change of use triggers the test at all

Three separate requirements get confused with each other.

The sequential test asks whether the development could be located on a site at lower flood risk somewhere else. It is a test about site selection, which is why it makes little sense for a proposal to reuse a building that already exists. Guidance reflects that, exempting change of use applications other than the residential caravan and chalet categories. Verify the current wording of the guidance before relying on it, because the flood risk chapter has been revised more than once.

The exception test asks two things about a development in a higher risk zone: whether it delivers wider sustainability benefits that outweigh the flood risk, and whether it will be safe for its lifetime without increasing flood risk elsewhere. It applies according to the flood zone and the vulnerability of the use, and it does apply to changes of use.

A site-specific flood risk assessment is required for development in Flood Zones 2 and 3, and for sites of one hectare or more in Zone 1. Change of use is development, so the assessment requirement is not avoided.

The vulnerability classifications

National guidance classifies uses into five categories by how badly flooding would affect the people and property involved. The categories that matter for most commercial changes of use are:

The category depends on the actual use rather than on the use class, which catches people out. Class E covers shops, offices, restaurants, nurseries and health centres, and those sit in different vulnerability categories. A movement entirely within Class E can therefore change the flood risk position even though it needs no planning permission for the change of use itself. That is a genuine oddity of the two systems sitting side by side.

Changes that increase vulnerability

These are the movements that create work:

• Shop or office to flats. Less vulnerable to more vulnerable, which in Flood Zone 3a brings in the exception test.
• Shop to nursery, health centre or place of worship with a community function. Often a move to more vulnerable.
• Office or shop to a hotel or aparthotel. More vulnerable.
• Restaurant to a drinking establishment or nightclub. Both sit in different categories in the guidance, and the later hours and higher occupancy matter for evacuation.
• Anything to a basement dwelling. Highly vulnerable, and not permitted in Flood Zone 3a.

Movements with no change in category are the easy ones. Shop to restaurant, office to light industry, retail to storage: same classification, same compatibility, and the assessment can be short.

Where the category does rise in Flood Zone 3a, the Exception Test has to be passed, and both limbs need evidence rather than assertion. The sustainability limb on a small commercial conversion is usually made out of town centre vitality, bringing a vacant unit back into use, and housing supply where flats are proposed. The safety limb is where applications fail, and the two limbs in practice deserve separate attention.

Where the test can be scoped out

Even where a sequential test is not required, some authorities ask for one anyway, either through a local plan policy or out of habit. Two responses work better than arguing in the abstract.

First, cite the guidance provision that exempts change of use applications and ask the officer to confirm which policy requires it. Often the requirement disappears at that point.

Second, apply a sequential approach within the site instead, which is what the underlying policy is actually driving at. Put the more vulnerable accommodation on the upper floors, keep the ground floor for the less vulnerable use, locate plant and electrical intake above the design flood level, and show it on the drawings. An officer who wanted reassurance about flood risk gets it, and the application is not delayed by a search of alternative sites that nobody expects you to relocate to.

Where the proposal is one of the caravan, chalet or park home categories that are not exempt, the sequential test does apply in full, and the Sequential Test has to be done properly with a defined area of search.

Defining the area of search when it does apply

Where a test is required, the area of search should be agreed with the local planning authority before the work is done, because a search over the wrong area is worthless. In practice authorities usually accept the administrative area, or a functional area such as the settlement or the catchment the business serves, and the reasoning has to be stated.

Then the search has to be evidenced rather than described. Sites identified from the authority's own land availability assessment, allocations and any brownfield register, with each one assessed for flood zone, availability and suitability for the specific use, and a reason recorded where a site is discounted. A table of a dozen sites with one line each is more persuasive than three paragraphs of narrative.

Mitigation inside an existing building

An existing building brings a fixed floor level, which removes the usual first line of mitigation. What remains is still substantial.

Locate the vulnerable activity above the design flood level, which for a shop to flats conversion means no bedrooms at ground floor and, in higher risk situations, no habitable rooms there at all. Provide a refuge above the design flood level with communication. Raise electrical consumer units, sockets and boilers. Specify flood resilient finishes at low level, meaning closed cell insulation, solid floor build-ups and plaster or render specifications that survive wetting. Fit non-return valves on drainage to stop backflow through the system.

Illustrative example: a vacant ground floor shop in Flood Zone 3a converted to two flats. The design flood level is 300mm above the shop floor. The accepted scheme puts both flats over the ground and first floors as maisonettes with all sleeping accommodation upstairs, raises the services, specifies resilient finishes to 600mm, provides a flood warning and evacuation plan, and keeps the ground floor entrance hall and a commercial storage area at street level. The floor level never changed.

Evidence for a small application

Proportionality is allowed and it is worth using. For a single unit change of use, a short assessment doing the following is usually enough:

1. The flood zone from the Flood Map for Planning, with the date of the check.
2. The design flood level where one is available, and a note of how it was obtained.
3. The existing and proposed vulnerability classifications, named from the guidance.
4. The compatibility conclusion and whether the exception test is engaged.
5. The mitigation, drawn on the plans rather than only described.
6. Safe access and egress, and the flood warning and evacuation arrangements.
7. Confirmation that flood risk elsewhere is not increased, which on a change of use inside an existing footprint is usually straightforward.

That can run to a handful of pages. A Flood Risk Assessment for a change of use does not need the modelling chapter of a greenfield major, and padding it does not help. What it does need is the vulnerability analysis stated explicitly, because that is the part the case officer has to rely on.

This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

PlanScope drafts a Flood Risk Assessment structured to UK planning policy from your site data and guided answers, and flags modelled levels as still to be confirmed rather than inventing them. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.