Flood Risk Feasibility Study

A flood risk feasibility study answers the question that comes before the planning application: is this site worth the money, and what will flood risk force you to change? It is an early-stage piece of work, commissioned when you are appraising land, weighing up an option agreement or preparing a pre-application enquiry, and its job is to surface the flood constraints while you can still act on them.

By the time you are writing a Flood Risk Assessment for planning, the layout is usually fixed and the site is usually bought. A feasibility study runs earlier, and it can end with a recommendation not to proceed.

The cheapest flood constraint is the one you find before you exchange.

What is a flood risk feasibility study?

A flood risk feasibility study is a pre-application assessment of the flood risk affecting a site and what it means for a proposed development. It draws on Environment Agency mapping and detailed hydraulic model data, the local Strategic Flood Risk Assessment and site topography, then sets out the constraints, the likely mitigation and the planning position before a scheme is designed or land is bought.

It is not a lighter version of an FRA. The technical work is the same standard, applied to a different question. An FRA argues that a defined scheme is safe and acceptable. A feasibility study asks what scheme, if any, could be made safe and acceptable here, and tells you what that would cost you in layout, floor levels and buildable area.

The output is written so it can go straight to the Environment Agency or the local planning authority as part of a pre-application enquiry. Where the answer is positive, most of the technical work carries forward into the eventual FRA rather than being repeated.

Two levels: a data review, or a full feasibility study

Not every site needs a bound report. Some questions are settled by getting the Environment Agency's modelled data, interrogating it properly and writing up what it means. Others need the full study. We scope to whichever answers your question.

A data review often becomes a feasibility study once the data comes back and the site turns out to be more interesting than the published mapping suggested. That is a normal progression, not a failure of scoping.

Is this the right flood risk assessment for you?

Flood risk work goes by a lot of names and the wrong one wastes weeks. If your situation is on this list, the linked page is the one you want:

• You have a scheme and you are submitting a planning application. You need a Flood Risk Assessment for planning, not a feasibility study.
• Your site is in Wales. The equivalent is a Flood Consequence Assessment under TAN15, which works to different flood map definitions.
• The council has asked you to justify the site's location. That is a Sequential and Exception Test report.
• You already own the building and want to understand its exposure. A due diligence flood risk assessment covers an existing asset, including a site visit and resilience advice.
• You are buying a house. A homebuyer flood risk survey is written for the buyer and their conveyancer.
• An insurer or broker has queried the risk. That calls for an insurance flood risk assessment.
• The published mapping is too coarse to answer your question. We build and run our own models: see flood modelling services.
• You need a Pol 03 credit. That is a BREEAM flood risk assessment.
• Groundwater is the concern. See groundwater flood risk assessment.

If none of those fit, or two of them do, call us and we will tell you which piece of work you actually need. We would rather scope it correctly than sell you the wrong report.

When a feasibility study earns its keep

The pattern is consistent: the study pays for itself when a decision is still reversible. Once contracts are exchanged or a layout is fixed with the architect, the same findings become expensive.

• Before buying land. The Flood Map for Planning, published on the government's service, shows Flood Zones 2 and 3 and nothing else. It will not tell you whether part of the site is functional floodplain, how deep the water gets, or how much developable area survives once climate change allowances are applied.
• Ahead of a pre-application enquiry. Officers and Environment Agency staff respond far better to a specific technical position than to a question. A study gives them something to agree with.
• Option and promotion agreements. Where your return depends on achieving a consented scheme of a certain size, developable area is the number that matters, and flood risk is one of the constraints that sets it.
• Site selection across a portfolio. Comparing three or four candidate sites on a like-for-like flood basis is quick, and it stops the wrong site absorbing a year of planning effort.
• Change of use and conversions. Changes of use are exempt from the Sequential Test in most cases, but that does not make the flood risk go away, and floor levels in an existing building are far harder to move than in a new build.
• Where the mapping and the ground disagree. Sites that look constrained on the national mapping but sit on higher ground, and sites that look clear but have a drain along the boundary, are both common. Both are resolved with data, not opinion.

Around 6.3 million properties in England are at risk of flooding from rivers, the sea or surface water, and about 4.6 million from surface water alone.

Those NaFRA2 figures are why surface water now derails as many schemes as rivers do, and why a study that only looks at Flood Zones is not worth commissioning.

What Environment Agency flood data tells you, and what it does not

Most feasibility work starts with a request to the Environment Agency for detailed flood data, usually Product 4, and sometimes the model report and raw hydraulic outputs alongside it. The Environment Agency sets out what it will supply and how to ask for it in its flood risk assessment data guidance. Our guide to Environment Agency flood data products 4, 5, 6 and 7 explains what each one contains and how to request it.

The Environment Agency works to a published service standard of up to 20 working days for detailed data products. That is a genuine problem when you are inside a due diligence window. We license and maintain our own copies of a large proportion of the Environment Agency's hydraulic models, so on many sites we can begin the technical work immediately and treat the formal data request as confirmation rather than a dependency.

Once the data is in, the interpretation is where the value sits. Some of what we routinely find:

• The model does not contain the event the policy needs. The NPPF defines functional floodplain, Flood Zone 3b, as the 1 in 30 year event. Plenty of older models were never run for it, so the 1 in 20 and 1 in 50 year outputs have to be interrogated to bracket the answer.
• The SFRA and the model disagree. A Strategic Flood Risk Assessment may show a site inside 3b at a scale that cannot be resolved on a single plot, while the underlying model, compared against surveyed levels, puts part of that land above the flood level entirely.
• Flood levels only mean something against site levels. A modelled level of, say, 15.49 metres above ordnance datum is a number until you set it against topography. The difference between the two is the depth, and the depth is what drives finished floor levels and whether a scheme is viable.
• The mapping and the modelling cover different things. Flood Zones cover rivers and the sea. Surface water, groundwater, sewers, reservoirs and canals sit in separate datasets and have to be assessed separately.
• Defended and undefended extents are not the same site. On defended sites the residual risk of breach or overtopping is a planning issue in its own right, and it is usually the undefended case that sets floor levels.

Where the modelling is too coarse, too old, or simply absent, we say so and price the modelling that would settle it, rather than dressing up an assumption. Our guide to flood modelling software and techniques covers what that involves.

What we assess

A feasibility study covers every source of flooding that could reasonably affect the site, because the planning system now expects a risk-based approach to all of them and not just to rivers.

• Fluvial. Flood Zones from the Flood Map for Planning, modelled extents and levels for the design events, functional floodplain, flood storage areas, defences and residual risk.
• Tidal and coastal. Present-day and climate change still-water levels, defended and undefended cases, and the long design horizons that coastal sites now have to work to.
• Surface water. The Environment Agency's surface water mapping including the climate change layers added in 2026, extents, depths and flow routes across and around the site.
• Groundwater. Superficial and bedrock geology, recorded groundwater emergence, and whether infiltration drainage is plausible at all.
• Sewers and reservoirs. Sewer flooding records where they exist, and reservoir inundation extents where the site falls within one.
• Historic flooding. Recorded flood outlines, Section 19 investigations, local records and, where the vendor or neighbours have it, photographic evidence of events the Environment Agency never captured.
• Topography and ground levels. LiDAR as a minimum, and a topographical survey where the margins are tight. LiDAR data is free and good, but it is not a substitute for surveyed floor levels on a conversion.
• Policy context. The NPPF and its planning practice guidance, the current climate change allowances for your river basin district, the local SFRA and any local plan policy that bites on the site.

What you receive

The full study is a standalone report, written to be read by a planning officer or an Environment Agency officer as easily as by your board. It follows the structure of an FRA, which is deliberate: it makes the eventual planning submission cheaper and it means the Environment Agency recognises the format.

1. Key facts. The flood risk posed and the recommended management measures, on the first page, in plain terms. Most readers get what they need here.
2. The existing site. Current use, area, topography, ground conditions and geology, watercourses and drainage features on or near the boundary.
3. The development proposal. What you are contemplating, its vulnerability classification, and how that classification interacts with the flood zone.
4. Assessment of flood risk. Source by source, with annotated figures: mapped extents, modelled extents and levels, depths against site levels, historic events, defences and residual risk.
5. Flood risk management. Finished floor levels, the sequential approach within the site, resistance and resilience measures, safe access and escape, off-site impact and floodplain compensation, and the surface water drainage position.
6. Discussion and conclusions. Whether the scheme is deliverable, what has to change for it to be, and where the residual planning risk sits.

A data review is shorter and is issued as a written technical summary with annotated figures, but the analytical work behind it is the same.

How the findings shape the scheme

A study that only describes risk is not much use. The point is the design consequences, and they are usually specific enough for an architect to work with straight away.

• Finished floor levels. Set above the design flood level with freeboard. On new build this is a levels exercise. On a conversion, where floor levels cannot easily move, it may mean putting habitable accommodation upstairs and confining non-habitable uses to the ground floor.
• Developable area. Applying the sequential approach within the site pushes built form to the lowest-risk land. That number, not the site area, is what your appraisal should use.
• Layout and access. Safe access and egress during the design event has to be demonstrated. Where it cannot be, internal access to a higher floor within each unit becomes the fallback, and the Environment Agency will expect to see it.
• Floodplain compensation. No net loss of floodplain storage means no built form or land raising below the design flood extent, which can be the constraint that decides the scheme.
• Drainage. Almost every scheme will need a surface water drainage strategy, and knowing early whether infiltration is plausible affects both the layout and the cost.
• Building form. On tightly constrained sites, floodable voids, stilts and undercrofts are sometimes the answer, and sometimes an expensive way to avoid admitting a site does not work.

Using a feasibility study in a pre-application enquiry

Pre-application advice is worth having, and it is worth having with evidence attached. A feasibility study lets you put a specific position to the local planning authority and the Environment Agency: this is the design flood level, this is where we propose to build, these are the floor levels, this is the safe escape route. Agreement at that stage is not binding, but disagreement at that stage is very cheap to fix.

It also matters for the Sequential and Exception Tests. The Gladman judgment and the Yatton appeal have both sharpened how the Sequential Test is applied, and the September 2025 update to the government's planning practice guidance on flood risk and coastal change confirmed the risk-based approach applies to every flood source. Finding out at pre-app that your site will struggle is uncomfortable. Finding out at committee is worse.

Since April 2026 most planning appeals are determined by written representations by default, which means the evidence base is effectively fixed at submission. Our note on why flood risk and drainage have to be right first time sets out what that changes. Weak flood evidence is also one of the more common reasons a planning application is invalidated before it is even registered.

What does a flood risk feasibility study cost?

We price each study to the site rather than publishing a flat fee, because the work varies by an order of magnitude between a Flood Zone 1 site with a surface water flow path and a defended tidal site needing a model interrogated. The drivers are:

• How many flood sources are in play. A site with fluvial, tidal and surface water risk is three assessments, not one.
• Whether usable model data exists. An up-to-date model that covers the design events is quick to work with. An old model missing the events the policy needs, or no model at all, means more work or a modelling exercise.
• Site size and complexity. A single plot is quicker than a multi-parcel site with different constraints on each parcel.
• Whether a walkover or topographic survey is needed. Often it is not. On conversions and tight-margin sites it usually is.
• What you need it to do. An internal appraisal note is a different document from something going to the Environment Agency with a pre-application enquiry.

Our article on what drives flood risk assessment cost explains the pricing logic in more detail and why fixed-fee reports tend to be thin ones. Send us the site and we will scope it properly.

Why look at flood risk before you commit

The financial argument has strengthened considerably. Research indicates that flood exposure can reduce sale prices by around a quarter for affected homes, and the wider picture is set out in our analysis of whether flood risk affects house prices and the financial threat rising flood risk poses to the housing market. A site bought at full value on the assumption that flood risk is a detail is a site bought badly.

The planning argument is blunter. Development on floodplains is not prohibited, but it is heavily constrained: More Vulnerable and Less Vulnerable uses are not permitted in functional floodplain at all, and Flood Zone 3a schemes have to clear the Sequential Test and often the Exception Test as well. Knowing which of those applies to your site, and to which part of it, changes what you should pay for the land.

Every constraint a feasibility study finds is a constraint the Environment Agency was going to find anyway. The only variable is when, and how much it costs you by then.

How the process works

1. Tell us about the site. A postcode or red-line plan and a sentence on what you are considering is enough for us to scope and price the work.
2. We check what data exists. Published mapping, the local SFRA, and whether a hydraulic model covers the site. Where we already hold the model under licence, we can start straight away.
3. We request the Environment Agency data. Where a formal request is needed, we handle it, and we tell you honestly whether the 20 working day service standard sits on your critical path.
4. We assess the site. Every relevant flood source, modelled levels compared against site topography, historic events, defences and residual risk, with a walkover where the site warrants it.
5. We work out what it means. Floor levels, developable area, sequential approach within the site, safe escape, compensation and drainage, discussed with your architect where that helps.
6. You get the report. Written to go to the Environment Agency or the local planning authority as it stands, and we stay available to talk it through with them.

Why Unda

We are a chartered flood risk and drainage consultancy and we have produced more than 5,000 flood risk assessments since 2014. Our consultants are chartered through the Chartered Institution of Water and Environmental Management, and the same people who write the feasibility study are the ones who would write the FRA and negotiate with the Environment Agency afterwards.

Two things matter more than credentials on this kind of work. The first is model access: holding licensed copies of a large proportion of the Environment Agency's hydraulic models means we can give you a technical answer in days rather than after a data request. The second is local knowledge, which is why we maintain guides by area, by river and by local planning authority, so we know what a particular council's SFRA says and how its officers apply it before we start.

We are also willing to tell you a site does not work. That is occasionally an unpopular thing to put in writing, and it is the main reason to commission the study early rather than late. You can read more about Unda, or check a site yourself on our free flood risk map postcode checker and live river levels map.

Frequently asked questions

Talk to us about your site

If you are appraising land, preparing a pre-application enquiry, or simply want to know whether a flood constraint on the mapping is real, our experienced consultants will scope the right level of work and tell you what it will cost before you commit. Call +44 (0) 1293 214444 or email enquiries@unda.co.uk, and we will come back to you quickly with a clear view of what the site needs.