Flood-risk assessments

What a flood-risk assessment is

A flood-risk assessment, often shortened to FRA, is a report that evaluates the risk of flooding to and from a site. Where a property flood survey focuses on how water would enter one building, an FRA looks at the wider picture: the sources of flooding that could affect the land, how likely and severe flooding might be, how that risk could change with climate change, and whether a proposed development would increase flood risk elsewhere.

FRAs are most commonly required as part of a planning application, but they are also commissioned by buyers, lenders, landlords and businesses who want a clear, professional view of the risk before committing to a property or site.

When you need one

In England, planning policy requires a site-specific FRA for most developments in areas at medium or high risk of river or sea flooding, for larger sites, and in areas with critical drainage problems identified by the local authority. Wales, Scotland and Northern Ireland have their own planning policies and guidance, and requirements differ between them. Your local planning authority can confirm whether an FRA is required for your proposal.

Outside planning, an FRA can support a purchase decision, inform the design of resilience measures for a commercial site, or provide evidence to an insurer. For existing homes, a property flood survey is usually more useful than a planning-style FRA, but the two are sometimes combined.

Sources of flooding considered

A thorough assessment considers every relevant source. River flooding happens when watercourses overtop their banks. Coastal flooding comes from high tides, storm surges and wave action. Surface water flooding occurs when heavy rain cannot drain away fast enough, and it is now the most widespread flood risk in many parts of the UK. Groundwater flooding happens when water tables rise, often slowly and for long periods. Sewer flooding occurs when drainage systems are overwhelmed. Reservoir and canal failures are rarer but may also be relevant.

Each source behaves differently. Groundwater may persist for weeks, making barriers ineffective, while surface water can arrive within minutes with little warning. An FRA explains which sources matter for the site and what that implies for design and mitigation.

What the report contains

A typical FRA includes a site description, a review of official flood maps and historical records, consultation with the relevant environment agency or authority where required, and an assessment of flood levels for design events including an allowance for climate change. For development proposals it will address the sequential and exception tests where they apply, the proposed finished floor levels, safe access and escape, and a surface water drainage strategy.

For more complex sites, the assessment may include hydraulic modelling. This is more expensive but gives more reliable estimates of depth, velocity and duration, which can make the difference between a scheme being approved or refused.

Costs

Fees vary widely. A simple desk-based assessment for a small site in a well-mapped area may be modest, while an assessment requiring topographic survey, modelling and detailed drainage design will cost considerably more. Ask for a fee proposal that states exactly what is included, whether consultation with the agency is included, and what happens if the planning authority requests further information.

Choosing a consultant

Look for consultants with experience of your type of site and your local authority. Chartered engineers, hydrologists and experienced flood consultants are common in this field. Ask for examples of similar assessments and whether they were accepted without significant revision. Check professional indemnity insurance, which is important for any report that will be relied upon in planning or investment decisions.

Flood Protection Finder lists consultants with their stated qualifications and memberships. Verified profiles show which of these we have checked with the issuing body, and when. We do not assess the technical quality of reports, so review sample work and ask for references.

Using the results

An FRA is not only a planning document. The flood levels and sources it identifies should feed directly into building design, the choice of resilience measures, emergency plans and insurance discussions. If you are building new, setting floor levels and services correctly from the outset is far cheaper than retrofitting protection later.