Flood risk assessment for a change of use application

A change of use flood risk assessment catches people out because nothing is being built. You are taking a shop and making flats above it, or turning an office into a nursery, and the walls stay exactly where they are. The flood risk has not changed at all. What has changed is who will be inside the building when the water arrives, and that is what the policy tests measure.

The classification system is the whole of it. Move up a vulnerability class in Flood Zone 2 or 3 and you need an assessment, whatever the building works amount to.

How a change of use raises flood risk without building anything

National flood risk policy sorts uses by how vulnerable the occupants are and how serious the consequences of flooding would be. A shop with staff who can leave in ten minutes is a different proposition from a flat with a family asleep upstairs at three in the morning.

Three factors drive the classification. Whether people sleep on the premises. Whether the occupants can evacuate themselves without assistance. And whether the use is one the community depends on during a flood, such as emergency services.

So a change that adds sleeping accommodation almost always moves up a class, and a change that adds occupants who cannot self-evacuate, such as a care setting or a nursery, does the same. The building envelope is irrelevant to that judgement.

Vulnerability classification and moving up a class

Planning Practice Guidance sets out the classification in a table, running from water-compatible development through less vulnerable and more vulnerable to highly vulnerable, with essential infrastructure treated separately.

The move from less vulnerable to more vulnerable is the one most commercial occupiers meet. A shop to flats conversion, an office to a hotel, a unit becoming a day nursery.

Two details matter. A basement flat is highly vulnerable rather than more vulnerable, which changes the answer completely in Flood Zone 3a, and basement development and flood risk is worth reading before drawing a lower ground floor unit. And the classification applies to the part of the building in the relevant use, so ground floor commercial with flats above is assessed for what is where.

Where the sequential and exception tests bite

The sequential test is generally not applied to change of use applications, with a stated exception for changes of use to caravan, camping, chalet, mobile home and park home sites. That exception exists precisely because those uses combine high vulnerability with no building to shelter in.

The Exception Test is different and can apply. Where a more vulnerable use is proposed in Flood Zone 3a, the compatibility table requires the Exception Test to be passed, which means showing wider sustainability benefits to the community that outweigh the flood risk, and that the development will be safe for its lifetime without increasing flood risk elsewhere. An Exception Test submission has to address both limbs, and a document that only argues safety fails the first.

The tests have been reworded between editions of the guidance, so check the current Planning Practice Guidance wording rather than relying on a precedent document from a previous scheme. A Sequential Test that was correct three years ago may cite a test that no longer reads the same way.

Prior approval routes and the flooding matter

Some conversions run through permitted development with prior approval rather than a full application, and flooding is usually one of the matters the council can consider. The commercial to residential route under Part 3 of Schedule 2 to the General Permitted Development Order includes flooding risks in relation to the building among the prior approval matters.

That is narrower than a full flood risk assessment but it is not a formality. The council can refuse prior approval on flooding grounds, and where the site is in Flood Zone 2 or 3 it will expect an assessment with the submission rather than a statement that the applicant is aware of the risk.

The permitted development classes for changes of use have been amended repeatedly, including changes to floorspace limits and qualifying periods. Read the current text on legislation.gov.uk before advising anyone, because guidance written even a year ago may describe conditions that no longer apply.

Mitigation when the building cannot change

This is the practical problem. In a new build you raise the floor level. In a conversion you usually cannot, because the floor is where it is and the ceiling height above it is fixed.

What is normally available:

1. Put the sleeping accommodation above the design flood level, which often means the conversion works only if the ground floor stays commercial or becomes circulation and storage.
2. Flood resistance measures at openings, including demountable barriers, non-return valves on drainage and sealed service entries.
3. Flood resilience inside, with solid floors, closed-cell insulation, raised electrical sockets and consumer units, and finishes that can be dried out.
4. A flood warning and evacuation plan, with occupants signed up to the Environment Agency warning service and a named route to a place of safety.
5. Safe refuge within the building at a level above the design flood level with external access for rescue.

Be realistic about barriers. They depend on somebody being present and able to deploy them, which is a weak assumption for a flat occupied by people who may be away or asleep. Permanent measures are worth more in the assessment than temporary ones.

What the council and the Environment Agency will want

A Flood Risk Assessment for a change of use is shorter than one for a new development but it has to answer specific questions. The flood zone from the Flood Map for Planning, the source and mechanism of flooding, the design flood level and how it was derived, the climate change allowance applied, the existing and proposed use with their classifications, the mitigation, and the residual risk.

Surface water and groundwater deserve their own paragraph. Many town centre conversions sit in Flood Zone 1 for river flooding but on a surface water flow path, and the council's own surface water mapping and the lead local flood authority response will pick that up even where the Environment Agency has no interest.

The Environment Agency is a statutory consultee for development in Flood Zones 2 and 3 and publishes standing advice for smaller proposals. For a straightforward conversion, the standing advice route is often what applies, and following it closely is faster than waiting for a bespoke response.

Conditions to expect

Expect a condition requiring a flood evacuation plan to be submitted and approved before occupation, one requiring the resistance and resilience measures shown in the assessment to be installed and retained, and one preventing sleeping accommodation below a stated level.

The one to read carefully is any condition setting a finished floor level. In a conversion that can be undeliverable without rebuilding the floor, so if the assessment has not proposed it, check the wording before the decision is issued rather than applying to vary it afterwards.

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.

Enter your postcode on PlanScope to see the flood zone for your site and which reports a change of use is likely to need.