Flood Risk Assessment for Permitted Development

One of the most common misconceptions in the planning system is that permitted development rights exempt a development from flood risk requirements. They do not. While permitted development (PD) removes the need for a full planning application, many PD rights include a prior approval process that requires the local planning authority to consider flood risk — and to refuse prior approval where the flood risk is unacceptable.

This article explains how flood risk interacts with permitted development rights, when a Flood Risk Assessment is needed, and how to navigate the prior approval process.

What Is Permitted Development?

Permitted development rights are rights to carry out certain types of development without needing to apply for planning permission. They are granted by the Town and Country Planning (General Permitted Development) (England) Order 2015 (as amended), commonly known as the GPDO.

Permitted development rights cover a wide range of development types, from small householder extensions to the conversion of commercial buildings to residential use. Some PD rights are automatic (no application to the LPA required), while others require a “prior approval” application.

Prior Approval

Prior approval is a lighter-touch application process compared to a full planning application. The LPA can only consider the specific matters listed in the GPDO for each PD class. These matters vary between classes but often include:

• Transport and highways impacts
• Contamination risks
• Flooding risks
• Noise impacts
• Natural light
• External appearance

The critical point for our purposes is that “flooding risks” is a specified prior approval matter for many of the most commonly used PD rights, including the conversion of commercial buildings to residential use.

Key Permitted Development Classes and Flood Risk

Class MA — Commercial, Business and Service to Dwelling Houses

Class MA is one of the most widely used PD rights and allows the change of use from Use Class E (shops, offices, restaurants, light industry, gyms, health centres, creches, and other commercial uses) to Use Class C3 (dwelling houses).

Flood risk is a prior approval matter for Class MA. The LPA must consider “the flood risk on the site” before granting prior approval.

This means:

• If the site is in Flood Zone 2 or 3, you 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 change in vulnerability classification
• The LPA can refuse prior approval on flood risk grounds

The vulnerability classification issue is particularly important for Class MA conversions. A change from Use Class E to Use Class C3 is a change from “less vulnerable” to “more vulnerable” in flood risk terms. In Flood Zone 3a, this change would normally trigger the Exception Test under the NPPF.

Whether the Exception Test applies formally to prior approval applications is a matter of some debate and has been considered in appeal decisions. The safest approach is to prepare the FRA as if the Exception Test applies — demonstrating both wider sustainability benefits and safety — as this provides the LPA with the strongest basis for approval.

Class A — Enlargement of a Dwelling House

Class A permits the enlargement, improvement, or other alteration of a dwelling house, subject to various size and design limitations. This is the PD right that covers most single-storey rear extensions and some two-storey extensions.

Flood risk is not a specified prior approval matter for Class A extensions that comply with the size limits. This means that for a standard rear extension within PD limits, you do not need to submit an FRA to the LPA.

However, this does not mean flood risk is irrelevant:

• Building Regulations still apply. The extension must comply with Approved Document H (drainage) and may need to address flood risk as part of the structural design.
• You still have a moral and practical obligation to build in a way that does not increase flood risk to your property or to your neighbours.
• If the extension later proves to be unsafe or increases flood risk elsewhere, you may face enforcement action or civil liability.

For extensions in Flood Zone 2 or 3, it is good practice to commission an FRA even if it is not a formal planning requirement, to ensure the design is appropriate.

Class AA — Enlargement of a Dwelling House by Construction of Additional Storeys

Class AA permits the construction of up to two additional storeys on a dwelling house, subject to prior approval. The prior approval matters include “the impact of the development on the amenity of any adjoining premises” but do not specifically mention flood risk.

However, if the LPA considers that the additional storeys create a material change in flood risk — for example, by introducing additional bedrooms below the flood level — they may raise flood risk concerns through the amenity assessment or through other considerations.

Class Q — Agricultural to Dwelling Houses

Class Q permits the change of use of agricultural buildings to dwelling houses, subject to prior approval. Flood risk is a prior approval matter for Class Q.

Agricultural buildings are often located in rural floodplains, making flood risk a common issue for Class Q conversions. The FRA for a Class Q application should address:

• The flood zone designation and actual flood risk from all sources
• The change in vulnerability classification (from agricultural to residential)
• The safety of occupants, including flood warning, access, and egress
• The impact on flood storage (rural floodplains often play an important role in attenuating floodwater)

Class R — Agricultural to Commercial

Class R permits the change of use of agricultural buildings to flexible commercial use (shops, offices, restaurants, light industry, storage). Flood risk is a prior approval matter for Class R.

The change from agricultural to commercial use generally does not change the vulnerability classification (both are “less vulnerable”), but the FRA should still address the safety of occupants and the impact on flood storage.

Class S — Agricultural to State-Funded School

Class S permits the change of use of agricultural buildings to state-funded schools. Flood risk is a prior approval matter for Class S. Educational buildings are classified as “more vulnerable,” so this change represents an increase in vulnerability.

Class PA — Light Industrial to Dwelling Houses

Class PA permits the change of use from Use Class B1(c) (light industrial) to Use Class C3 (dwelling houses). This class has largely been superseded by Class MA for applications made after 1 August 2021. Flood risk is a prior approval matter for Class PA.

How the LPA Assesses Flood Risk for Prior Approval

When assessing flood risk for a prior approval application, the LPA’s powers are more constrained than for a full planning application:

What the LPA Can Consider

The LPA can only consider the specific matters listed in the GPDO for the relevant PD class. For flood risk, this is typically described as “the flood risk on the site.” The LPA will assess:

• Whether the site is in a flood zone
• The level of flood risk from all sources
• Whether the proposed use is appropriate for the level of risk
• Whether the development can be made safe for its lifetime

What the LPA Cannot Do

Under prior approval, the LPA cannot:

• Apply policies from the Local Plan that are not relevant to the specified prior approval matters
• Impose conditions unrelated to the specified matters
• Refuse prior approval on grounds not listed in the GPDO

However, the LPA can refuse prior approval if it considers that the flood risk makes the development unacceptable. And case law has established that the LPA can have regard to the NPPF and PPG when assessing flood risk, including the Sequential Test and Exception Test where relevant.

The Environment Agency’s Role

For prior approval applications, the Environment Agency is not a statutory consultee in the same way as for full planning applications. However, many LPAs do consult the EA on prior approval applications in flood zones, and the EA’s views will carry weight in the LPA’s assessment.

If the EA objects to the application on flood risk grounds, the LPA is likely to refuse prior approval unless the applicant can provide compelling evidence to address the EA’s concerns.

What Should the FRA Cover?

The scope of the FRA for a prior approval application should be proportionate to the flood risk and the nature of the development. As a minimum, it should cover:

Flood Risk from All Sources

A comprehensive assessment of flood risk from rivers, the sea, surface water, groundwater, sewers, and artificial sources. The assessment should use Environment Agency flood zone maps, surface water flood risk maps, and any available detailed modelling data.

Vulnerability Classification

A clear statement of the existing and proposed vulnerability classifications, and an assessment of whether the change in classification is compatible with the flood zone.

Design Flood Levels

Identification of the design flood level (1% annual probability for rivers, 0.5% for tidal, with appropriate climate change allowances) and comparison with the existing floor levels.

Safety Measures

Recommendations for ensuring the safety of occupants, including:

• Flood resistance and resilience measures for the building
• Flood warning arrangements
• Safe access and egress (or, where this is not achievable, a safe internal refuge)
• An emergency flood plan

Drainage

An assessment of any changes to surface water drainage resulting from the change of use, and recommendations for managing surface water sustainably.

Flood Risk Elsewhere

A demonstration that the change of use does not increase flood risk to surrounding properties or communities.

Appeal Decisions and Case Law

Several important appeal decisions have shaped the approach to flood risk in prior approval applications:

Flood Risk as a Reason for Refusal

Inspectors have consistently upheld LPA decisions to refuse prior approval on flood risk grounds where the FRA is inadequate or where the development cannot be demonstrated to be safe. The key principle is that the LPA’s obligation to consider “the flood risk on the site” encompasses a full assessment of whether the development is safe for its lifetime.

The Exception Test in Prior Approval

The application of the Exception Test to prior approval applications has been considered in several appeals. While the GPDO does not explicitly require the Exception Test, inspectors have applied the principles of the NPPF and PPG when assessing flood risk. The practical effect is that applicants should address the Exception Test where the proposed change increases the vulnerability classification to a level that would normally trigger the Exception Test.

Conditions

Inspectors have confirmed that LPAs can impose conditions on prior approval that relate to flood risk, provided they are relevant to the flood risk on the site. Common conditions include:

• Implementation of flood resilience measures before occupation
• Submission and approval of a flood warning and evacuation plan
• Implementation of a surface water drainage scheme
• Minimum finished floor levels for habitable accommodation

Practical Tips

Engage Early

Before submitting a prior approval application, check the flood zone and surface water flood risk for the site. If the site is in a flood zone, commission an FRA before submitting the application. Submitting without an FRA in a flood zone is likely to result in refusal.

Address the Vulnerability Change

If the change of use increases the vulnerability classification, address this explicitly in the FRA. Explain why the development is safe despite the increased vulnerability, and provide evidence to support this conclusion.

Consider the Building’s Suitability

Not every building is suitable for conversion to residential use in a flood zone. If the building has a low floor level, no dry access route, and no practical means of providing safe refuge above the flood level, the conversion may not be approvable on flood risk grounds. It is better to identify this early than to invest in a prior approval application that is refused.

Design in Resilience

Where possible, design the conversion to incorporate flood resilience measures. This may include raising the finished floor level of habitable rooms, locating bedrooms above the ground floor, installing flood barriers and non-return valves, and using flood-resilient materials and finishes. These measures strengthen the FRA and demonstrate that the development can be safe.

Cost and Timescale

FRAs for prior approval applications typically cost between GBP 600 and GBP 2,000, depending on the complexity of the site and the flood risk. The timescale is usually 2-4 weeks, including data procurement from the Environment Agency.

How Aegaea Can Help

Aegaea prepares Flood Risk Assessments for prior approval and permitted development applications across England, Scotland, and Wales. We understand the specific requirements of the prior approval process and how flood risk interacts with different PD classes.

Our FRAs are prepared by Chartered engineers and are designed to provide the LPA with the evidence they need to approve the application. We can also advise at the pre-application stage on whether a site is suitable for conversion in flood risk terms, potentially saving you the cost of pursuing an unviable scheme.

Contact us for a no-obligation discussion about your project.