Flood Risk Assessment for Change of Use Applications

Change of use applications present a unique set of challenges when it comes to flood risk. Unlike new-build development, where the developer has full control over the design, finished floor levels, and flood mitigation, change of use applications involve adapting an existing building — often one that was constructed long before modern flood risk standards were introduced.

The planning framework treats change of use applications differently from new development in several important respects, and understanding these differences is essential for preparing a successful application.

When Is an FRA Required for Change of Use?

The requirement for a Flood Risk Assessment on a change of use application follows the same broad principles as for any other planning application:

Flood Zones 2 and 3. If the site is in Flood Zone 2 or 3, a site-specific FRA is required for any change of use that requires planning permission.

Flood Zone 1 with a site area of 1 hectare or more. This threshold is more commonly relevant for change of use applications than for householder extensions, as commercial and industrial premises can be large.

Known flood risk from other sources. The LPA may require an FRA in Flood Zone 1 if there is known surface water, groundwater, or sewer flood risk.

The critical additional factor for change of use applications is the vulnerability classification. This determines whether the proposed new use is appropriate in the relevant flood zone, and whether the Sequential Test and Exception Test need to be applied.

Vulnerability Classification

The NPPF classifies all land uses into flood risk vulnerability categories. These categories determine what development is appropriate in each flood zone:

Essential Infrastructure

Power stations, water treatment works, electricity substations, transport infrastructure. This category has its own rules and is rarely relevant to standard change of use applications.

Highly Vulnerable

Police and ambulance stations, basement dwellings, caravans and mobile homes for permanent residential use, installations requiring hazardous substances consent.

More Vulnerable

Hospitals, residential institutions, dwelling houses, drinking establishments, nightclubs, hotels, hostels, buildings used for education, landfill and waste management facilities.

Less Vulnerable

Shops, offices, restaurants, general industry, storage and distribution, car parks, mineral working and processing.

Water-Compatible

Flood control infrastructure, water transmission infrastructure, sewage transmission infrastructure, sand and gravel working, essential utility infrastructure, navigation facilities, outdoor sport and recreation.

The Compatibility Matrix

The NPPF includes a flood risk vulnerability and flood zone compatibility table that determines which combinations are appropriate:

Vulnerability	Zone 1	Zone 2	Zone 3a	Zone 3b
Essential Infrastructure	Yes	Yes	Exception Test	Exception Test
Highly Vulnerable	Yes	Exception Test	No	No
More Vulnerable	Yes	Yes	Exception Test	No
Less Vulnerable	Yes	Yes	Yes	No
Water-Compatible	Yes	Yes	Yes	Yes

“No” means the development should not be permitted. “Exception Test” means the development must pass the Exception Test to be permitted.

Why Change of Use Is Different

The critical issue for change of use applications is that the vulnerability classification may change even though the physical building does not.

Example: Office to Residential

Converting an office building (Use Class E — “less vulnerable”) to residential use (Use Class C3 — “more vulnerable”) in Flood Zone 3a triggers the Exception Test. The building itself is unchanged, but the new use introduces a higher vulnerability because:

• Residential occupants are present 24 hours a day, including overnight when flood warnings may not be received
• Residential properties contain personal possessions that are difficult to protect
• The occupants may include vulnerable individuals (elderly, disabled, children)
• There is a higher expectation of safety and amenity for residential occupants

Example: Shop to Restaurant

Converting a shop (Use Class E — “less vulnerable”) to a restaurant (also Use Class E — “less vulnerable”) in Flood Zone 3a does not change the vulnerability classification, so the Exception Test is not required. However, an FRA is still needed to demonstrate that the development is safe and does not increase flood risk elsewhere.

Example: Warehouse to Nightclub

Converting a warehouse (Use Class B8 — “less vulnerable”) to a nightclub (Sui Generis — “more vulnerable”) in Flood Zone 2 does not trigger the Exception Test (more vulnerable is appropriate in Zone 2), but the FRA should address the higher occupancy and the potential for occupants to be impaired by alcohol, which affects their ability to respond to a flood warning.

The Sequential Test for Change of Use

The Sequential Test requires the LPA to consider whether the proposed development could be directed to a site at lower flood risk. For change of use applications, this creates an interesting tension:

The applicant is proposing to use a specific existing building. They cannot physically move that building to a lower-risk location. Does the Sequential Test still apply?

The answer, according to the PPG, is nuanced. Paragraph 023 of the PPG on Flood Risk states that for change of use applications, the Sequential Test “can be applied by the local planning authority to the development type proposed, with the area of search determined by local circumstances.”

In practice, this means:

Where the change of use involves works that constitute development (not just a change of use class), the LPA may apply the Sequential Test in the conventional way — asking whether the same type of development could be located elsewhere in a lower-risk area.

Where the change of use is purely a change of use class (no physical works), many LPAs take a more pragmatic approach. The reasoning is that the building already exists, and refusing the change of use does not reduce flood risk — it simply leaves the building in its current use or potentially vacant.

However, this is not a universal approach. Some LPAs apply the Sequential Test strictly to all change of use applications, and case law supports their right to do so. The safe approach is to address the Sequential Test in the FRA, even if you believe it should not apply.

The Exception Test for Change of Use

Where the change of use results in a more vulnerable classification in Flood Zone 3a, or a highly vulnerable classification in Flood Zone 2, the Exception Test must be passed. The Exception Test has two parts:

Part 1: Wider Sustainability Benefits

The development must provide wider sustainability benefits to the community that outweigh the flood risk. For change of use applications, this is often relatively straightforward to demonstrate:

• Bringing a vacant building back into productive use
• Providing housing in an area with identified housing need
• Regenerating a neglected area
• Reducing the need to develop greenfield sites
• Supporting the local economy through job creation or service provision

Part 2: Safe Development

The development must be safe for its lifetime, taking account of the vulnerability of its users, without increasing flood risk elsewhere, and where possible reducing flood risk overall. This is where the FRA is critical.

For change of use applications, demonstrating safety can be more challenging than for new-build development because:

• The finished floor level is fixed. Unlike new development, you cannot raise the floor above the flood level (or at least, doing so is expensive and may not be practical).
• The building’s construction may not be flood-resilient. Older buildings may have materials and services that are vulnerable to flood damage.
• Safe access and egress may be compromised. The building’s location and surroundings are fixed, and there may not be a dry route out of the building during a flood event.

What the FRA Should Address

For a change of use application, the FRA should cover:

Flood Risk Assessment

• Current flood risk from all sources (fluvial, tidal, surface water, groundwater, sewer, artificial)
• Design flood levels with climate change allowances
• Residual risk from flood defences (including breach and overtopping scenarios where relevant)

Vulnerability Classification

• Clear identification of the existing and proposed use class
• Clear identification of the existing and proposed vulnerability classification
• Determination of whether the change triggers the Sequential Test, Exception Test, or both

Sequential and Exception Tests

• Where required, evidence to address the Sequential Test (area of search, available alternative sites)
• Where required, evidence to address both parts of the Exception Test

Safety of Occupants

• Finished floor levels relative to the design flood level
• Depth and duration of flooding at the site
• Flood warning lead time and arrangements
• Safe access and egress routes
• Emergency evacuation plan (particularly for residential conversions and buildings with sleeping accommodation)

Flood Resistance and Resilience

• Assessment of the existing building’s vulnerability to flood damage
• Recommendations for flood resistance measures (to keep water out)
• Recommendations for flood resilience measures (to reduce damage if water enters)
• Cost-benefit analysis of different options

Surface Water Drainage

• Assessment of any change to surface water drainage resulting from the change of use
• Where the change of use involves external works (new hard standing, car parking), a drainage strategy

Flood Risk Elsewhere

• Demonstration that the change of use does not increase flood risk to surrounding properties
• Assessment of any changes to flood storage or conveyance resulting from the change of use

Permitted Development Rights and Change of Use

Some changes of use can be carried out under permitted development rights, without the need for planning permission. The most relevant are:

Class MA — Commercial to residential. The Town and Country Planning (General Permitted Development) (England) Order 2015 (as amended) allows the change of use from Use Class E (commercial, business, and service) to Use Class C3 (dwelling houses) without planning permission, subject to prior approval.

The prior approval process includes consideration of flood risk. The LPA must consider “the flood risk on the site” before granting prior approval. In practice, this means:

• If the site is in Flood Zone 2 or 3, the applicant should submit a Flood Risk Assessment with the prior approval application
• The LPA will assess whether the proposed residential use is safe, taking account of the vulnerability classification
• The LPA can refuse prior approval on flood risk grounds

Class O — Office to residential. This has now been subsumed into Class MA but the same principles apply.

The critical point is that permitted development rights do not exempt the development from flood risk considerations. The prior approval process requires the LPA to consider flood risk, and the applicant should provide an FRA to demonstrate that the development is safe.

Common Challenges

Fixed Floor Levels

The most common challenge is that the existing building’s floor level is below the design flood level. For new development, the solution is to raise the floor level — but for change of use, this may not be practical or cost-effective.

Alternative approaches include:

• Flood barriers and resistance measures to keep water out of the building up to the design flood level
• Resilient design to reduce damage if water does enter
• Flood warning and evacuation plans to ensure occupants can reach safety
• Locating sleeping accommodation above the flood level (e.g., on the first floor or above)

Safe Access and Egress

Demonstrating safe access and egress can be challenging for buildings in areas where the surrounding roads and paths are also below the flood level. Where a dry route cannot be maintained during the design flood event, the FRA may need to demonstrate that:

• Flood warnings provide sufficient lead time for evacuation before the route becomes unsafe
• The building has a safe internal refuge above the flood level
• Emergency services can access the building during a flood event

Insurance and Mortgage Implications

A change of use to residential in a flood zone can have implications for insurance and mortgage availability. Properties that were previously commercial may not automatically qualify for Flood Re (which only covers residential properties built before 2009). If the building was constructed after 2009, or if the residential use is a new conversion, Flood Re eligibility should be checked.

Cost and Timescale

FRAs for change of use applications typically cost between GBP 750 and GBP 2,500, depending on the complexity of the site, the flood zone, and whether the Exception Test needs to be addressed. The timescale is usually 3-5 weeks, including data procurement.

Where the change of use involves a large or complex site, or where detailed hydraulic modelling is required, costs and timescales may be higher.

How Aegaea Can Help

Aegaea prepares Flood Risk Assessments for change of use applications across England, Scotland, and Wales. We have extensive experience of navigating the vulnerability classification system and addressing the Sequential and Exception Tests for conversion projects.

Our FRAs are prepared by Chartered engineers and are designed to provide the evidence the LPA needs to approve the application. We work closely with architects and developers to ensure that flood risk is addressed early in the design process, avoiding costly delays at the application stage.

If you are planning a change of use and need a Flood Risk Assessment, contact us for a no-obligation discussion and quote.