Can I Write My Own Flood Risk Assessment?

Every year, homeowners and small developers across the UK face the same question when preparing a planning application: do I really need to pay for a flood risk assessment, or can I write one myself?

It is a fair question. Planning applications are already expensive. Architects, structural engineers, planning consultants — the professional fees stack up quickly. If there is any way to reduce costs without cutting corners, most people want to know about it.

The honest answer is that writing your own flood risk assessment is sometimes possible, particularly for very straightforward minor works. But “possible” and “advisable” are two different things. This article explains the legal position, what a proper FRA actually needs to contain, the data and skills required to produce one, and — crucially — what can go wrong if you get it wrong.

Is There a Legal Requirement to Use a Qualified Professional?

Let us start with the good news. There is no statutory requirement in England and Wales for a flood risk assessment submitted with a planning application to be produced by a qualified professional. Unlike, say, a structural calculation or a building regulations application, the planning system does not impose a mandatory accreditation requirement on who can write an FRA.

The National Planning Policy Framework (NPPF) and the accompanying Planning Practice Guidance (PPG) set out what an FRA must demonstrate — but they do not specify who must produce it.

In practice, however, local planning authorities (LPAs) and the Environment Agency do look at the quality and credibility of the assessment. An FRA that is clearly superficial, uses incorrect data, or fails to address the relevant flood sources is likely to result in either an objection from the Environment Agency or an outright refusal from the LPA.

So whilst you are not legally prohibited from writing your own FRA, you are entirely responsible for its quality — and the consequences if it falls short.

When Does a Planning Application Need an FRA?

Before asking whether you can write your own FRA, it is worth checking whether you actually need one.

Under the NPPF and the Flood Risk and Coastal Change chapter of the Planning Practice Guidance, a site-specific flood risk assessment is required when a planning application involves:

• Any development in Flood Zone 2 or Flood Zone 3
• Development of 1 hectare or more in Flood Zone 1 (the lowest risk zone)
• Any development in an area that has been identified by the Lead Local Flood Authority as having critical drainage problems
• Certain changes of use that would increase the vulnerability of the site to flooding

For the vast majority of homeowners, the trigger is likely to be the first point — the property is in Flood Zone 2 or 3, and they want to build an extension, convert a garage, or add an outbuilding.

You can check your property’s flood zone on the Environment Agency’s “Check the long-term flood risk for an area” service at GOV.UK. This is free and takes less than a minute.

What Does a Flood Risk Assessment Actually Need to Include?

This is where the gap between “anyone can write one” and “it is worth doing yourself” becomes apparent. A proper FRA for a planning application is not simply a letter saying “we checked the maps and we think flooding is not too bad”. It needs to address a specific set of technical matters.

The Planning Practice Guidance sets out the requirements in detail. In summary, a site-specific FRA must:

Identify All Sources of Flood Risk

Flood risk does not only come from rivers and the sea. A comprehensive FRA must consider:

• Fluvial flooding — flooding from watercourses, including ordinary watercourses and main rivers
• Tidal and coastal flooding — relevant for coastal and estuarine sites
• Surface water flooding — rainfall that does not drain quickly enough, causing ponding and overland flow
• Groundwater flooding — rising water tables, particularly in areas with chalk or limestone geology
• Sewer flooding — capacity issues in the public sewer network
• Reservoir and artificial source flooding — where relevant to the site

Many homeowners focus entirely on the Environment Agency’s flood zone maps and miss other significant sources. A property in Flood Zone 1 with low river flood risk might still have a significant surface water flood risk — something a simple check of the EA maps will not reveal.

Demonstrate the Sequential and Exception Tests Have Been Considered

The NPPF requires that new development is directed away from areas at highest flood risk. This is achieved through the Sequential Test and, where necessary, the Exception Test.

For householder applications (extensions to existing dwellings), these tests generally do not apply in the same way as they do for new-build developments. However, for change of use applications or anything that increases the number of dwellings, the tests may be relevant — and the FRA needs to demonstrate that the LPA and/or Environment Agency’s requirements in this regard have been met.

Assess the Design Flood Level

This is one of the most technically demanding elements of an FRA. You need to establish the actual water level at your site during a flood event of a specified probability — typically the 1% annual exceedance probability (AEP) event for river flooding, or the 0.5% AEP event for tidal flooding.

This is not simply a matter of reading a number off a map. It requires accessing the Environment Agency’s detailed flood modelling data (where it exists), understanding how flood levels vary across a site, and interpreting hydraulic modelling outputs. Where detailed modelling data is not available, you may need to commission new hydraulic modelling — a specialist task that requires specific software and expertise.

Apply Climate Change Allowances

This is an area where DIY FRAs frequently fall down. Government guidance requires that FRAs account for the impacts of climate change on future flood risk over the lifetime of the development.

The Environment Agency publishes climate change allowances for different river basins and coastal areas, updated periodically as climate projections improve. For a residential extension with a 75-year design life, you typically need to apply the “Higher Central” climate change allowance for the relevant river basin, which increases the design flood level by a percentage that varies by region and time horizon.

Getting this wrong — or ignoring it entirely — is one of the most common reasons FRAs are rejected or require significant revision.

Demonstrate the Development Is Safe and Does Not Increase Flood Risk Elsewhere

The FRA must show that:

• The finished floor level of any habitable accommodation is set at or above the design flood level (including climate change)
• Safe access and egress is available during a design flood event
• The development does not increase flood risk to third parties, either by increasing runoff or by reducing flood storage capacity
• Where relevant, appropriate sustainable drainage measures are incorporated

Be Supported by Appropriate Evidence

A credible FRA is not just a narrative — it needs to be backed by data. This typically includes:

• Environment Agency flood zone maps and detailed modelling data
• Flood history records
• Ordnance Survey topographic data
• Evidence of finished floor levels relative to modelled flood levels
• Any relevant drainage calculations

The Skills and Data Required

To write an FRA yourself, you would need to be comfortable:

• Navigating the Environment Agency’s data services, including the Long-Term Flood Risk Service, the National Flood and Coastal Erosion Risk Management Strategy evidence base, and the EA’s request process for detailed modelling data
• Understanding hydraulic modelling outputs and flood level data, including the difference between modelled flood levels and observed flood levels
• Applying the correct climate change allowances from the EA’s guidance tables, cross-referencing your river basin, flood source, and development design life
• Interpreting the NPPF and Planning Practice Guidance requirements correctly
• Communicating the findings in a format that a planning officer and the Environment Agency’s planning liaison team will find credible

None of this is impossible. If you have a technical background in engineering, environmental science, or a related field, you may find it manageable. But for most homeowners, this is genuinely specialist territory — not because it is impenetrable, but because the details matter enormously and the margin for error is small.

What Are the Risks of Getting It Wrong?

The consequences of submitting an inadequate FRA range from mildly frustrating to genuinely expensive.

Planning Refusal

The most straightforward consequence is that the LPA refuses planning permission, citing an inadequate FRA. You then either need to appeal the decision or submit a new application with an improved FRA — both of which take time and money.

Delays While the Application Is Put On Hold

Even if the LPA does not immediately refuse, they may hold the application pending further information, or the Environment Agency may object and require the FRA to be revised before they lift their objection. Delays of weeks or months are not uncommon. For householders who are waiting to start a project, this can be deeply frustrating.

The Cost of Revision

If your FRA is rejected or objected to, you may need to commission a professional to rewrite it anyway — but now against the backdrop of a failed application, and possibly with a deadline imposed by the LPA or the Environment Agency. Commissioning a professional to fix a problem is invariably more expensive than commissioning one to do the job correctly in the first place.

Liability for Future Issues

This is a less commonly considered risk, but it matters. If your FRA contains significant errors — for example, it underestimates the design flood level or fails to account properly for climate change — and the property subsequently floods, this could have implications for insurance claims, future sales, and in extreme cases, legal liability if the development has increased flood risk to neighbours.

Mortgage and Insurance Complications

A poorly evidenced FRA can create difficulties if you come to sell the property. Mortgage lenders and their valuers are increasingly scrutinising flood risk evidence, and an FRA that would not withstand professional scrutiny may raise questions about the basis on which planning permission was granted.

When Does It Make Sense to Write Your Own FRA?

To be balanced: there are limited circumstances where a self-prepared FRA might be reasonable.

If your development is very minor — a small outbuilding, a conservatory, a modest single-storey extension — and the site is at the lower end of Flood Zone 2 with no surface water, groundwater, or other complicating flood risk factors, a well-researched, clearly written statement of flood risk may be acceptable to your local authority.

This is more likely to be the case if:

• The proposal involves no habitable accommodation (so finished floor level requirements are less onerous)
• The site has never flooded historically
• The Environment Agency’s detailed flood level data is publicly available for your reach of river, and the modelled flood level is demonstrably below your ground level even with climate change applied
• Your LPA has a pre-application advice service and has indicated that a simple sequential test statement will suffice

Even in these cases, it is worth calling the LPA’s planning department or submitting a pre-application enquiry before you invest time in writing an FRA. Many LPAs will tell you what level of detail they expect for a given site and development type, which can save considerable effort.

When You Are Better Off Commissioning a Professional

For the majority of residential planning applications that trigger an FRA requirement, commissioning a specialist consultant is likely to be better value in the long run. This is especially true when:

• The site is in Flood Zone 3
• There is a history of flooding at or near the site
• The proposal involves habitable accommodation at or near ground floor level
• Surface water flooding is a significant issue in the area
• The development is more substantial (a full extension, a new building in the garden, a change of use)
• The Environment Agency has previously objected to development in the area
• You are working to a planning deadline or have already invested heavily in design fees

The cost of a professional FRA for a typical residential application typically ranges from around GBP 500 to GBP 1,500, depending on the complexity of the site and the flood risk. Against the total cost of a building project and the risk of planning delays or refusal, this is generally a proportionate investment.

A good flood risk consultant will not only produce an FRA that satisfies the LPA and the Environment Agency — they will also identify any genuine flood risk issues early, so you can factor appropriate mitigation into the design before it is too late to change course.

A Practical Checklist Before You Decide

If you are weighing up whether to write your own FRA, work through the following:

1. Check the flood zone at GOV.UK — what zone is the site in, and are there any additional sources of flood risk (surface water, groundwater, reservoir)?
2. Check the flood history — has the site or nearby properties ever flooded? Contact the Environment Agency and your Lead Local Flood Authority.
3. Contact the LPA — ask whether a pre-application flood risk statement would be acceptable for your development, or whether they expect a full FRA.
4. Access the EA’s detailed data — is modelled flood level data available for the watercourse affecting your site? If not, who would need to produce it?
5. Check the climate change requirements — can you confidently identify and apply the correct allowance for your river basin and development design life?
6. Be honest about your technical confidence — not whether you could read the guidance, but whether you would be confident defending the FRA if the EA or LPA asked probing questions about your methodology.

If you can answer all of these questions satisfactorily, you may be in a position to attempt a self-prepared FRA. If any of them give you pause, the safer and more cost-effective route is to commission a professional from the outset.

In Summary

Writing your own flood risk assessment is not prohibited, and for very straightforward minor works with uncomplicated flood risk it may be achievable. But the bar for an acceptable FRA is higher than many people expect — particularly the requirements around climate change allowances, design flood levels, and the full range of flood sources.

The risks of an inadequate FRA — planning refusal, delays, costs of revision, and potential future complications — typically outweigh the cost of commissioning a specialist.

If you are considering a residential development that requires an FRA and would like to understand what a professional assessment would involve for your specific site, take a look at our residential flood risk assessment services. We work with homeowners and small developers across England and Wales, and we are happy to have a straightforward conversation about what your project needs before you commit to anything.