Can I write a Flood Risk Assessment myself?
If you have been told your planning application requires a Flood Risk Assessment (FRA), you might be wondering: can I write a Flood Risk Assessment myself?
While it may be possible for very minor, low-risk projects, the vast majority of developments require the technical expertise of a professional flood risk consultant. Even the very minor can be rejected on intricate planning policy requirements which to a non-specialist could be overlooked.
For UK homeowners, developers and architects, managing project costs is important. However, attempting to prepare a Flood Risk Assessment without the necessary technical input can lead to delays, additional costs, or planning refusal.
Ensuring that the right level of information is submitted at the outset is becoming increasingly important, particularly in light of recent changes to the planning appeals process, which place greater emphasis on the quality and completeness of submissions.
In short: you can sometimes write your own FRA – but only for the simplest of projects.
If you are unsure whether you need an FRA at all, see our guide: Why do I need a Flood Risk Assessment?
When can you write your own FRA?
In very limited circumstances, it may be possible to prepare a basic Flood Risk Assessment yourself – typically for minor developments such as small extensions.
The Environment Agency defines a minor extension as a householder or non-domestic extension with a floor space of no more than 250 square metres. In these cases, applicants may be able to rely on standing advice to support their planning application.
However, even for relatively simple developments, preparing a compliant Flood Risk Assessment is rarely straightforward.
A basic FRA must still include:
- a clear description of the site and proposed development
- estimated flood levels, including climate change allowances
- proposed finished floor levels (typically set above predicted flood levels)
- appropriate flood resistance and resilience measures
In addition, all levels must be accurately referenced to Ordnance Datum, which typically requires a professional topographical survey.
In practice, even where a simplified approach may appear possible, meeting planning authority expectations without technical input can be challenging.
Why DIY FRAs are rarely suitable
In practice, local planning authorities increasingly expect clear and robust flood risk information to be submitted at the outset of an application, even for relatively small developments.
In practice, local planning authorities This reflects national policy set out in the National Planning Policy Framework (NPPF) and Planning Practice Guidance (PPG), which require a comprehensive assessment of all sources of flooding and clear evidence that development will remain safe for its lifetime without increasing flood risk elsewhere.
As a result, even where a simplified or DIY approach may appear technically acceptable, it may not meet the level of detail and clarity typically expected by planning authorities, particularly in areas affected by surface water or drainage constraints.
When do you need a professional Flood Risk Consultant?
For most planning applications (including new builds, changes of use, larger developments, or sites with environmental constraints), you will require input from a professional flood risk consultant.
This is because a compliant Flood Risk Assessment must be:
- Accurate (based on reliable data and appropriate modelling)
- Proportionate (aligned with the level of risk)
- Policy-compliant (meeting NPPF and local authority requirements)
A professional consultant is typically required because:
- Accurate flood level data is needed
A standard FRA must assess flood depths for key events (e.g. 1 in 100-year river flooding), including climate change allowances. While Environment Agency datasets provide a starting point, national guidance makes clear that these are not always sufficient or site-specific, and may need to be refined or supplemented through detailed analysis. - Modelling may be required
More complex sites often require hydraulic or drainage modelling to demonstrate that development will not increase flood risk elsewhere, particularly where existing data is limited or uncertain. - Planning policy tests must be met
For sites in higher-risk areas, you may need to pass the Sequential and Exception Tests, both of which require robust and well-justified technical evidence. - Design safety must be demonstrated
Flood resistance measures are not always appropriate. For example, attempting to prevent water ingress beyond certain depths can introduce structural risks, meaning expert input is required to ensure safe and proportionate design.
The Risks of the DIY Approach
Attempting to prepare your own FRA beyond a simple minor extension can create significant risks:
- Planning refusal
If the FRA is not robust or does not use appropriate datasets, the application may be refused. - Validation delays
Missing information, such as topographical data or climate change allowances, can prevent your application from being validated. - Missed design opportunities
A professional FRA often identifies practical mitigation strategies that improve site viability and reduce long-term costs. - Underestimating site-specific risk
National flood mapping does not always capture local features such as walls, culverts, or ground levels, which can significantly affect flood behaviour.
When a professional FRA adds value
A professionally prepared FRA does more than satisfy planning requirements, it helps shape your development.
In many cases, early input from a consultant can:
- Identify constraints before design is fixed
- Avoid unnecessary planning delays
- Provide practical, cost-effective mitigation strategies
- Demonstrate that a site is viable when it may initially appear constrained
Conclusion
While it may be possible to prepare your own Flood Risk Assessment for a simple extension, most developments require a professional, planning-ready report.
Starting with the right level of expertise can help avoid delays, reduce risk, and ensure your application meets the expectations of the planning authority.
At Aegaea, we specialise in delivering clear, compliant Flood Risk Assessments tailored to the needs of each project.
For more detail, read our related guides, or speak with our team to understand how flood risk requirements may affect your development.
FAQs: DIY Flood Risk Assessments
In very limited cases, it may be possible for simple, low-risk developments such as small extensions. However, most planning applications require a professional consultant to ensure the assessment is accurate, proportionate, and meets planning policy requirements.
Indicative flood level information may be available from Environment Agency datasets. However, national guidance makes clear that this data is not always site-specific or sufficiently accurate for planning purposes.
For most developments, flood levels need to be derived or refined using specialist tools, modelling, and additional environmental datasets. This is typically undertaken by experienced consultants to ensure the results are robust and suitable for planning submission.
A topographical survey is often required to accurately define site levels and reference them to Ordnance Datum.
In some cases, preliminary assessments can be informed using existing datasets such as LiDAR. However, these sources have limitations and may introduce uncertainty.
Given current planning expectations, a topographical survey is typically required prior to submission to provide accurate ground levels and support mitigation design, reducing the risk of errors or challenges during the planning process.
Standing advice is guidance provided by the Environment Agency for minor developments in flood risk areas. It outlines standard requirements that can be followed without the need for detailed modelling, although it still requires accurate and appropriate supporting information.
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