Flood Risk Assessment for Change of Use

The NPPF sets out a hierarchy of flood vulnerability classifications. Highly vulnerable uses include basement dwellings and certain forms of essential infrastructure, as defined in national planning guidance.

More vulnerable uses include houses, residential institutions, student halls and caravan parks. Less vulnerable uses include commercial, retail, offices and industrial.

Water-compatible uses include flood control infrastructure and amenity open space. A change from any lower classification to a higher one is a material flood risk consideration in planning terms, regardless of flood zone.

In Flood Zone 1, an FRA is not automatically required for a change of use unless the development is over one hectare or the site is at risk of surface water flooding or other flood risks. However, where the proposed use is more vulnerable and the site has any documented flood risk from any source, the planning authority will expect flood risk to be considered.

Many change of use applications in Flood Zone 1 are refused or conditioned because surface water flooding, or groundwater flooding was not addressed.

In Flood Zone 2, all changes of use to more vulnerable classifications require an FRA. The Sequential Test and Exception Test do not apply to changes of use under the NPPF, but the FRA must demonstrate that the proposed use is appropriate in flood risk terms and can be made safe for its lifetime.

In Flood Zone 3a, the same applies: an FRA is required and must show the development will be safe, but the formal Exception Test is not triggered by a change of use alone. In Flood Zone 3b functional floodplain, changes of use to more vulnerable, less vulnerable or highly vulnerable uses are not permitted. The only acceptable uses in Zone 3b are water-compatible development and, where the Exception Test is passed, essential infrastructure.

Understanding which zone applies, and which classification change is proposed, is the essential first step in scoping any change of use FRA correctly.

• Change of use to a more vulnerable classification in any flood zone, including Flood Zone 1 where surface water or other flood risk is documented.
• Change from commercial, retail or office use to residential in Flood Zone 2 or 3.
• Agricultural or rural buildings being converted to dwellings in any flood zone.
• Change of use that increases the number of people sleeping on a site in Flood Zone 2 or 3.
• Change to a more vulnerable use on a site identified in the relevant SFRA as being at flood risk from any source.
• Any change of use where the local planning authority or Environment Agency has indicated flood risk is a material consideration.

Change of use applications are where flood risk is most often misunderstood. The policy position is not always obvious, and small shifts in vulnerability or occupancy can materially change what is required.

We don’t approach these as a tick-box exercise. We assess the proposed use properly, how the building will actually be occupied, how risk changes in practice, and what needs to be demonstrated to satisfy the local planning authority.
That includes:
• correctly identifying vulnerability classifications and how they change with the proposed use
• understanding when an FRA is genuinely required, and what level of assessment is proportionate
• addressing all sources of flood risk, not just Flood Zone mapping
• considering residual risk, safe access, and how the building would function during a flood event
• Many change of use applications fail not because they are unacceptable, but because they are poorly scoped or unsupported at submission. Early instruction avoids that, it is the difference between an application that proceeds and one that stalls at validation or is refused on grounds that could have been addressed from the outset.
• We work across residential conversions, agricultural to residential, commercial to residential, and barn conversions across England and Wales, providing clear, defensible advice from the start.