How to Do a Flood Risk Assessment for Planning

A flood risk assessment (FRA) is a technical report that identifies how a site could flood, now and over the lifetime of a development, and shows a planning authority that the proposal will be safe and will not make flooding worse elsewhere. Knowing how to do a flood risk assessment means working through seven linked stages, from screening the national data to modelling the flood mechanisms and demonstrating that the scheme complies with policy. Each stage has to be done to a standard the Environment Agency and the local planning authority will accept.

This guide sets out the whole method: the data and standards behind each step, what the finished report has to contain, and the points where a judgement call decides whether an application is approved or refused. It covers England, where the National Planning Policy Framework (NPPF) governs flood risk. Wales works to a separate system and requires a Flood Consequence Assessment instead.

The Environment Agency's latest National Flood Risk Assessment puts 6.3 million properties in England in areas at risk of flooding today, a figure that could rise to around 8 million, or one in four properties, by the middle of the century once climate change is accounted for.

What is a flood risk assessment, and when do you need one?

A flood risk assessment is a site-specific study of the probability and consequences of flooding from every source that could affect a development, together with the measures needed to manage that risk. It is not the same as a postcode flood check or a conveyancing search: those are screening tools, while an FRA is the evidence a planning decision is built on. You can see where your site sits using our free flood risk map, but that is only ever a starting point.

Under government guidance, an assessment is expected in defined circumstances. The gov.uk guidance on flood risk assessments for planning applications is explicit that you should complete one for all development, including minor development and changes of use, proposed in these situations:

• Flood Zone 2 or 3: any site shown in the medium or high probability zones on the Flood Map for Planning, including the functional floodplain (Zone 3b).
• Over one hectare in Flood Zone 1: larger low-probability sites still need an assessment, mainly to deal with surface water.
• Critical drainage or surface water risk: sites in an area with known surface water flooding or a critical drainage problem, even in Zone 1.
• A change that increases vulnerability: for example converting commercial space to housing, which raises the vulnerability classification.

The detail of when the trigger applies is worth reading in full, and we cover it in when do you need a flood risk assessment and explain the mapping behind it in flood zones explained.

How to do a flood risk assessment: the seven steps

A planning-ready flood risk assessment moves through seven stages, and each one builds on the last. The first two are broadly screening-level work. From the third stage onwards, the method depends on survey data, modelling and policy interpretation, and that is work for a competent specialist. The list below shows where that line falls.

1. Identify the flood sources (screening): confirm the trigger and list every source that could reach the site: rivers, sea, surface water, groundwater, sewers and reservoirs.
2. Screen the datasets (screening): review the Flood Map for Planning, Environment Agency data and the local Strategic Flood Risk Assessment to set the baseline.
3. Establish the site levels (specialist): use a topographic survey and LiDAR to fix ground levels to Ordnance Datum, because small errors here move the whole result.
4. Model the flood mechanisms (specialist): interpret or build hydraulic and GIS modelling to derive design flood levels, depths, velocities and hazard for the site.
5. Assess the proposal and design mitigation (specialist): set finished floor levels, safe access, floodplain compensation and a surface water drainage strategy around the design flood level.
6. Demonstrate NPPF compliance (specialist): apply the Sequential and Exception Tests, vulnerability classification and climate change allowances, and show the proposal passes.
7. Compile the report and residual risk (specialist): draw the evidence into a clear, policy-referenced report, addressing residual risk if a defence were breached or overtopped.

The steps that follow explain what each stage involves and why the later ones so rarely survive a shortcut.

Identifying flood sources and interrogating the data

Every assessment starts by asking which types of flooding could reach the site. Rivers and the sea are the sources the Flood Map for Planning shows, but they are not the only ones: surface water, groundwater, sewers and reservoirs all have to be considered, and the national map does not show them. Government guidance is unambiguous on this point.

Getting the sources right means reading the correct datasets and knowing what they do not show. The Environment Agency's standing advice makes that clear, and we set out how far it will actually take you in planning with EA flood risk standing advice. The Flood Map for Planning is the starting dataset, and it moves. The 28 May 2026 update added surface water climate change extents and banded depth layers, and put some sites inside a mapped surface water risk area for the first time. The Flood Map is not interchangeable with the public long-term flood risk map either, as we explain in why you can't use the long term flood risk map for planning. Detailed work often draws on Environment Agency flood data products, and sources the map misses, such as groundwater flooding, frequently decide a site. Our overview of the types of flooding sets out all six sources an assessment must weigh.

Why site levels and modelling decide the outcome

Once the sources are known, the assessment becomes a question of levels. Flood risk is a comparison between the estimated flood level and the level of the ground and the proposed floors, so the accuracy of the survey underpins everything after it. A topographic survey, often supported by LiDAR data, ties the site to Ordnance Datum; a difference of a few hundred millimetres can change flood extents, flow routes and the storage volume the design has to replace.

The Environment Agency's standing advice expects finished floor levels to be set at least 600 millimetres above the estimated flood level, reduced to 300 millimetres only where there is high confidence in the flood estimate.

Where the mapped data is too coarse, which is most sites in Flood Zones 2 and 3, the assessment relies on hydraulic modelling to derive design flood levels, depths and velocities. That work is built on GIS analysis and the flood modelling software the industry uses, and it has to meet the Environment Agency's river modelling standards. Interpreting a model, or commissioning one, is where a general planning consultant usually reaches the limit of what they can do, and where a dedicated flood modelling capability earns its place.

Designing the development to be safe

With the flood levels established, the assessment turns to the proposal itself: showing that people and property will be safe, and that the development displaces no flood water onto neighbours. This is the constructive part of an FRA, where mitigation is designed rather than risk simply described.

• Finished floor levels: set above the design flood level, including the climate change allowance, to keep habitable space dry.
• Safe access and escape: a route that stays above the estimated flood level and connects the site to land clear of flood risk.
• Floodplain compensation: where a building takes up floodplain, equivalent storage is provided level-for-level so flood volumes are not pushed elsewhere.
• Surface water drainage: a strategy following the SuDS hierarchy so runoff is not increased, designed as part of the wider drainage strategy.

Surface water is the fastest-growing part of this work and the source most often underestimated, as we discuss in surface water flooding and planning. The drainage evidence an authority expects alongside the FRA is covered in what drainage is needed for planning permission.

Proving compliance with the NPPF

A technically sound assessment still has to be framed in planning policy. In England that means the National Planning Policy Framework and its supporting Planning Practice Guidance, which direct development away from the highest-risk land through the Sequential Test and, where that cannot be avoided, the Exception Test. Which tests and reports apply depends on the flood zone and the vulnerability of the use.

Each element here has its own detail worth understanding: how the NPPF governs flood risk, how the Sequential and Exception Tests work, and how the vulnerability classification in NPPF Annex 3 drives which test applies. The policy is also live: recent High Court rulings on the Sequential Test in Mead and on the Exception Test in the Yatton judgment have shifted how planning judgement is exercised, and every assessment now has to apply the current climate change allowances. Our NPPF guidance and Sequential and Exception Test services exist because this is the stage where good technical work is most often let down by weak policy framing.

What goes in a flood risk assessment report?

A flood risk assessment report works through the site and the proposal, then the risk from every flood source, then four things in order: how the layout avoids risk, how levels and flow routes control it, how the design mitigates what is left, and how the residual risk is managed once people are living there. The Environment Agency publishes a template that follows exactly that sequence, so a report structured any other way is making the case officer hunt for the answers.

The template is optional and aimed at non-major and major development, but its seven parts are what a case officer is looking for whether or not you use the form.

The Environment Agency's stated purpose for the template, set out on the Planning Portal, is "to help increase the chances that your FRA includes the right information first time". That is worth taking at face value. Refusals usually turn on a section that is thin or missing rather than on a wrong number: the officer has no way of telling whether a risk was considered and dismissed or never looked at. Whatever length the report runs to, it has to be proportionate to the degree of flood risk and appropriate to the scale, nature and location of what is proposed.

Why self-prepared and template FRAs get refused

It is legal to prepare your own flood risk assessment, and for a simple case, such as a small extension in Flood Zone 1 following the Environment Agency's standing advice, that can be enough. Beyond that narrow band, the reasons applications stall are consistent, and they cluster around the specialist steps. They include reliance on high-level mapping instead of site-specific modelling, incorrect or unsurveyed site levels, missing surface water or groundwater sources, and reports that describe risk without demonstrating policy compliance.

The most common flood-risk reasons an application is delayed or refused are not exotic: an assessment that leans on the national map, gets the site levels wrong, or fails to make the NPPF case, each of which a specialist is trained to avoid.

The consequences are real and increasingly expensive to unwind: an Environment Agency objection, an LLFA objection on drainage, or an application invalidated before it is even assessed. A cheap automated report compounds the problem, and we set out why in why conveyancing flood reports are not suitable for planning and can AI write a flood risk assessment?. The honest test of whether you can do it yourself is covered in full in can I do my own flood risk assessment?. The value of a chartered specialist is not the report itself but the judgement behind it: knowing which dataset governs, when a model is needed, and how to make the policy case first time. That is the mark of a CIWEM-chartered consultant.

How much does a flood risk assessment cost, and how long does it take?

There is no single price, because the work scales with the site. A low-risk Flood Zone 1 assessment is a modest desk study; a Flood Zone 3 site needing bespoke hydraulic modelling, floodplain compensation and an Exception Test is a substantially larger piece of work. Cost is driven by the flood zone, the sources in play, whether modelling is required, and the standard of survey data available, not by a fixed rate card. We break the drivers down in how much does a flood risk assessment cost?.

Timescales are shaped as much by consultation as by production: where the Environment Agency is a statutory consultee, its response period is part of the programme. Commissioning the assessment early, before the design is fixed, is the single most effective way to keep flood risk from becoming the item that holds up a decision.

Getting a flood risk assessment right first time

Done well, a flood risk assessment is not a hurdle but the evidence that unlocks a difficult site: it turns a mapped constraint into a designed, defensible proposal. That depends on the process above being followed properly: the right data, accurate levels, appropriate modelling and a clear policy case. It is exactly the work a specialist consultancy does day in, day out. Unda has prepared thousands of assessments across England and Wales, and our flood risk consultants handle the sites where getting it wrong is expensive.

If you have a site to assess, the quickest route is to talk to us: our flood risk assessment for planning service takes it from initial screening to a submitted, policy-compliant report.

Frequently asked questions