How Long Does a Flood Risk Assessment Take?

If you are buying a property in a flood zone, applying for planning permission, or developing a site near a watercourse, you will almost certainly need a flood risk assessment (FRA). One of the first questions people ask us is: how long will it take?

The honest answer is that it depends on the type of FRA your site requires. A straightforward desktop assessment for a small development in Flood Zone 2 is a very different undertaking from a detailed FRA with hydraulic modelling for a major residential scheme in Flood Zone 3. This article explains the realistic timelines for each, what factors affect how long your FRA will take, and what you can do to keep things moving.

The Two Main Types of Flood Risk Assessment

Before getting into timelines, it helps to understand that not all flood risk assessments are the same. The depth of work required depends on your flood zone, the type and scale of development, and what the local planning authority (LPA) or the Environment Agency (EA) will expect to see.

Desktop Flood Risk Assessment

A desktop FRA is a desk-based study that draws on existing data — the EA’s Flood Map for Planning, strategic flood risk assessments (SFRAs), historical flood records, indicative flood levels, and similar sources. No site survey or new hydraulic modelling is involved.

Desktop FRAs are typically appropriate for:

• Householder extensions and conversions in Flood Zone 2 or Flood Zone 3 (where the change of use is not to a more vulnerable category)
• Small commercial developments where the EA’s indicative flood levels are sufficient
• Change of use applications where the new use is of the same or lower vulnerability category
• Pre-application screening to understand the flood risk constraints before committing to a full assessment

Typical timeline: 5 to 10 working days from the point at which we receive all the information we need.

This assumes the relevant data is readily available — which it usually is for sites within the EA’s flood warning and indicative flood mapping coverage. We order data from the EA’s data services, review the SFRA, assess the site against NPPF policy requirements, and produce a report that sets out the flood risk, the policy context, and any required mitigation measures.

Detailed Flood Risk Assessment

A detailed FRA involves a higher level of analysis. It may include new hydraulic modelling, topographic survey data, a Sequential Test, an Exception Test, and a mitigation strategy designed specifically for the site. This is the type of FRA required for major planning applications in Flood Zone 3, or for any site where the existing EA modelling is inadequate or does not cover the relevant flood mechanism.

Detailed FRAs are typically required for:

• New residential or commercial development in Flood Zone 3
• Sites where the EA has flagged that their indicative modelling does not apply
• Applications that require the Sequential Test or Exception Test to be addressed formally
• Developments that involve a change to the floodplain — for example, where compensatory storage calculations are required
• Sites at risk from multiple sources of flooding (fluvial, tidal, surface water, groundwater)

Typical timeline: 3 to 6 weeks for a straightforward detailed FRA. More complex projects involving bespoke hydraulic modelling can take 2 to 3 months or longer.

The wide range reflects the variability in project complexity, data availability, and third-party dependencies — particularly the EA’s response times.

What Affects the Timeline?

Several factors can extend or compress how long your flood risk assessment takes. Understanding these in advance means fewer surprises.

Data Availability

The EA’s data products — detailed flood models, flood level datasets, historic flood outlines — are ordered through the EA’s Data Services. For straightforward sites, this data is usually returned within a few days. For sites in areas where the EA’s modelling is more limited, or where bespoke flood model data has been commissioned previously, the process can be more complex. In some cases, we need to request a Pre-Application Flood Risk Activity permit, or seek specialist data from internal EA teams rather than the standard data portal.

EA Pre-Application Consultation Response Times

For major applications in Flood Zone 3, we often recommend engaging the EA through their Pre-Application service before submitting planning. This allows us to discuss the proposed scope of the FRA with the EA’s flood risk team and get early agreement on the methodology. The EA’s pre-application service currently operates with response times of 21 to 28 working days for a standard response, and longer for complex sites. This is outside our control but worth factoring into your programme from the outset.

Site Complexity

A site at risk from a single source of flooding — for example, a straightforward fluvial floodplain site — is simpler to assess than a site affected by river flooding, tidal influences, surface water, and overland flow from adjacent high ground. Multi-source flood risk assessments require more data, more analysis, and more careful mitigation design.

Similarly, sites where the proposed development modifies the floodplain — through raising ground levels, constructing flood walls, or redirecting drainage — require more detailed modelling to demonstrate that the development does not increase flood risk elsewhere.

Topographic Survey

For detailed FRAs, we often require a topographic survey of the site and the surrounding area. This gives us accurate ground levels to use in the flood level assessment and the freeboard calculations. Commissioning and completing a topographic survey typically adds 1 to 2 weeks to the programme, depending on the size of the site and the availability of a survey contractor.

Client Response Times

This is one factor that is entirely within your control. Our FRAs require certain information from the client at the outset — site plans, proposed floor levels, drainage strategy, proposed finished floor levels, and so on. Where this information is available promptly, we can proceed without interruption. Where we are waiting for drawings or decisions about the proposed scheme, the FRA pauses.

Planning Authority Requirements

Some LPAs have specific requirements that go beyond the standard NPPF framework — for example, particular freeboard requirements, specific climate change allowances, or a requirement to address a particular source of flooding in detail. Where we are not familiar with the LPA’s local policy (uncommon, but it happens), we may need to review their SFRA and seek clarification before finalising the FRA.

A Note on “48-Hour” FRAs

Some companies advertise flood risk assessments with very fast turnarounds — 24 or 48 hours is not uncommon. We understand the appeal. Planning is time-sensitive, and nobody wants to hold up an application unnecessarily.

But we would encourage some scepticism about these claims.

A genuine flood risk assessment requires the assessor to obtain EA data, review the relevant SFRA, apply the Sequential Test where required, assess the proposed development against the flood risk, and design appropriate mitigation. It requires professional judgement, not just a data pull. Doing this properly, for most sites, takes longer than 48 hours.

What is often delivered in 48 hours is a templated report with site-specific data inserted — adequate for the simplest of householder cases, but potentially inadequate for anything more complex. An FRA that fails to address the EA’s concerns, or that the LPA’s flood risk consultant identifies as superficial, will trigger either a planning refusal or a request for further information. That costs more time and money than getting it right the first time.

We are not the fastest. We aim to be the most thorough, and to give our clients something that will actually hold up under scrutiny.

What You Can Do to Speed Things Up

There are several steps you can take to minimise delays on your side of the process.

Instruct Early

The biggest cause of programme delays is late instruction. If your planning application is being prepared and you know a flood risk assessment is required, instruct the FRA at the same time as the rest of your technical consultants — not after everything else is done. A flood risk assessment is rarely the fastest item on the programme, and it is often the last item commissioned.

Have Your Information Ready

Before instructing us, gather the following:

• A site location plan and boundary plan
• Proposed site layout or scheme drawings, including proposed finished floor levels if known
• Any drainage strategy or surface water management plan
• Any previous flood risk assessments or hydraulic modelling carried out on the site
• Contact details for the LPA case officer if a pre-application discussion has already taken place

The more complete this information is at the point of instruction, the faster we can get started.

Confirm Floor Levels Early

Finished floor level decisions are fundamental to a flood risk assessment. The FRA must demonstrate that the internal floor levels are above the design flood level (with an appropriate freeboard). If floor levels are not yet decided, we can provide the minimum floor levels required — but if the scheme changes after the FRA is issued, revisions may be needed.

Engage the EA Early for Complex Sites

If your site is in Flood Zone 3 and involves a significant development, early engagement with the EA through their pre-application service is almost always worthwhile, despite the lead times. It avoids the situation where a planning application is submitted, the EA requests further information, and the application clock is paused while additional work is carried out.

At Aegaea, we can manage the pre-application engagement with the EA on your behalf, agree the scope of work upfront, and ensure the FRA addresses exactly what is required.

Typical Programme Summary

To give a clearer overview:

Type of FRA	Typical Timeline	Key Dependencies
Desktop FRA (householder / minor development)	5-10 working days	EA data (usually 2-3 days), client information
Desktop FRA with Sequential Test	10-15 working days	EA data, SFRA review, LPA clarification if needed
Detailed FRA (no bespoke modelling)	3-6 weeks	EA data, survey if required, EA pre-app response
Detailed FRA with bespoke hydraulic modelling	2-4 months	Model build, calibration, EA engagement, survey

These timelines run from the point of instruction with all required information. They do not include the LPA’s determination period or any subsequent planning conditions.

Getting Started

If you are not sure what type of flood risk assessment your site needs, or you want a clear timeline before committing to an instruction, we are happy to give you a no-obligation scoping opinion. We will look at the site, assess the flood zone, identify any obvious complexities, and give you an honest estimate of the work involved and how long it will take.

We work with homeowners, architects, developers, and planning consultants across England and Wales. Whether you need a straightforward desktop study or a complex multi-source assessment with hydraulic modelling, we can help.

Get in touch with the Aegaea team to discuss your site.