Flood Risk Assessments: The Complete Guide

What Is a Flood Risk Assessment?

A flood risk assessment (FRA) is a technical study that evaluates all sources of flood risk to and from a proposed development. It considers fluvial (river), tidal, surface water, groundwater, and sewer flooding, and sets out how those risks will be managed over the lifetime of the development.

The FRA is a formal requirement of the National Planning Policy Framework (NPPF) and forms part of a planning application. Its purpose is twofold: to demonstrate that the development will be safe for its users, and to ensure that development does not increase flood risk elsewhere.

For most developers, the FRA is one of the first technical reports commissioned in the pre-application phase. Getting it right early avoids costly redesigns and delays during the planning process. Getting it wrong — or submitting a substandard report — is one of the most common reasons for planning objections from the Environment Agency.

When Do You Need a Flood Risk Assessment?

The triggers for requiring an FRA are set out in the NPPF and the accompanying Planning Practice Guidance (PPG). You will need an FRA if any of the following apply:

• Your site is in Flood Zone 2 or 3. These zones represent areas with a medium to high probability of fluvial or tidal flooding. Any development in these zones requires an FRA, regardless of size.
• Your site is larger than 1 hectare and in Flood Zone 1. Even in low-risk areas, larger sites must be assessed because of the potential for surface water runoff impacts.
• Your site is in a critical drainage area designated by the Environment Agency or the lead local flood authority (LLFA).
• The local planning authority (LPA) requests one. Many LPAs apply their own local policies that require FRAs for smaller sites or sites with known surface water or groundwater issues.
• Your site is near a main river, ordinary watercourse, or the coast. Proximity to water features may trigger an FRA requirement even if the Environment Agency flood map does not show the site in a higher flood zone.
• There is a history of flooding in the area. Local flood records, community concerns, or insurance claims can prompt an LPA to request an assessment.

If you are unsure whether you need an FRA, check the Environment Agency flood map as a starting point. For a definitive answer, request a pre-application screening from a specialist consultant or your local planning authority.

Types of Flood Risk Assessment

Not all FRAs are created equal. The level of detail required depends on the flood zone, site complexity, and the nature of the proposed development.

Screening or Scoping FRA

A lightweight review that confirms whether a full FRA is needed. This is useful for sites on the boundary of flood zones or where risk is ambiguous. It typically involves a review of Environment Agency data and a brief desktop analysis.

Desktop FRA (Level 1)

The most common type. A desktop FRA is prepared using existing published data — EA flood maps, historical flood records, LiDAR data, geology maps, and sewer records. It is suitable for straightforward sites in Flood Zone 1 (over 1 hectare) or low-complexity sites in Flood Zone 2.

A well-prepared desktop FRA typically includes:

• Site location and description
• Review of all flood sources (fluvial, tidal, surface water, groundwater, sewers, artificial)
• Climate change assessment
• Proposed mitigation measures
• Surface water drainage strategy overview
• Finished floor level and safe access recommendations

Detailed FRA (Level 2/3)

Required for sites in Flood Zone 3, sites requiring the Sequential Test or Exception Test, or sites where existing flood data is insufficient. A detailed FRA may include:

• Bespoke hydraulic flood modelling to establish flood levels and depths
• Detailed topographic survey analysis
• Floodplain compensation calculations
• Breach and overtopping assessments for defences
• Detailed climate change sensitivity testing using current EA allowances
• Emergency flood plan

The FRA Process: Step by Step

1. Data Collection

The consultant gathers existing flood data from the Environment Agency, the LLFA, water companies, and other sources. This typically includes:

• EA Flood Map for Planning
• Product 4 (detailed flood data from EA, where available)
• Risk of Flooding from Surface Water (RoFSW) maps
• Historical flood records
• LiDAR data and topographic surveys
• Geology and groundwater data
• Sewer flood records from the water company

2. Site Assessment

The consultant reviews the site in the context of all available data. For some projects, a site visit is needed to verify ground levels, identify watercourse features, check existing drainage infrastructure, and understand the local flood context.

3. Risk Analysis

All sources of flood risk are assessed. For each source, the consultant determines the probability and severity of flooding, taking account of existing defences, land levels, and climate change projections.

4. Mitigation Design

Based on the risk analysis, the consultant proposes appropriate mitigation measures. These might include:

• Raising finished floor levels above the design flood level plus freeboard
• Setting development back from watercourses
• Providing floodplain compensation storage (level for level, volume for volume)
• Designing safe access and egress routes
• Incorporating flood-resilient construction techniques
• Integrating surface water management through SuDS

5. Report Writing

The FRA is documented as a formal technical report structured to address EA and LPA requirements. A clear, well-structured FRA significantly reduces the risk of planning objections.

6. Submission and Consultation

The FRA is submitted as part of the planning application. The Environment Agency is a statutory consultee for developments in Flood Zones 2 and 3, and the LLFA comments on surface water drainage. The consultant should be prepared to respond to queries and, if necessary, provide supplementary information.

What Makes a Good FRA

Having reviewed thousands of FRAs — both as authors and as peer reviewers — we know what separates effective assessments from those that stall in planning. A good FRA:

• Addresses all flood sources, not just the obvious ones. Surface water and groundwater are frequently overlooked but often drive EA objections.
• Uses up-to-date data. Relying on outdated flood maps or ignoring recent modelling studies is a common reason for rejection.
• Applies the correct climate change allowances. The EA updates these periodically, and using the wrong figures is an immediate red flag. See our climate change allowances guide for the latest.
• Proposes proportionate mitigation. Over-engineering mitigation wastes money; under-engineering it invites objections. The right approach balances safety with commercial reality.
• Is clearly written and well-structured. The EA and LLFA review hundreds of FRAs each year. Reports that are easy to navigate and clearly address policy requirements are resolved faster.
• Integrates with the wider design. The FRA should not exist in isolation — it must work with the drainage strategy, the site layout, and the architectural design. Disconnected reports lead to contradictory submissions.

Common Mistakes That Delay Planning

In our experience, the following issues cause the most delays:

1. Submitting the FRA too late. An FRA should inform the site layout and levels strategy, not be retrofitted after the design is fixed.
2. Ignoring surface water risk. A site in Flood Zone 1 can still have severe surface water flood risk. The flood zones only capture fluvial and tidal risk.
3. Using generic climate change allowances. Allowances vary by river basin district and must be correctly applied. Using a national average or an outdated figure is a common error.
4. Failing to engage with the EA early. For complex sites, a pre-application discussion with the EA can identify issues before they become objections.
5. Not providing a drainage strategy. The FRA and the drainage strategy are closely linked. Submitting one without the other, or with conflicting assumptions, is a recipe for delay.
6. Incomplete Sequential Test evidence. For sites in Flood Zones 2 and 3, the Sequential Test must be passed. Leaving this to the LPA without providing supporting evidence is risky.

How FRAs Interact with Planning Applications

The FRA is submitted as a supporting document with the planning application. For major applications in flood risk areas, the following sequence typically applies:

1. The LPA validates the application and consults the EA and LLFA.
2. The EA reviews the FRA against national policy and their own standing advice. For sites in Flood Zone 3 or where modelling is involved, this is a detailed technical review.
3. The LLFA reviews the surface water drainage aspects, often under their role as statutory consultee for major developments.
4. If the EA objects, the applicant must address their concerns — either through an amended FRA, additional modelling, or revised mitigation. Unresolved EA objections can lead to refusal.
5. Once the EA withdraws their objection (or has no objection from the outset), the flood risk element of the application is typically resolved.

Planning conditions related to flood risk and drainage are almost always applied, even where the EA has no objection. These conditions typically require the submission of a detailed drainage scheme and, in some cases, a flood emergency plan before development commences.

Working with the Environment Agency

The EA’s approach to FRA review has evolved significantly in recent years. Key points to understand:

• Standing advice applies to most Flood Zone 2 sites and simple Flood Zone 3 sites. The EA does not individually review these — the LPA applies their standing advice instead. This means the quality of your FRA is assessed by the LPA’s planning officer, not a flood risk specialist.
• Detailed review is reserved for larger or more complex sites, particularly those requiring the Exception Test, involving flood modelling, or near sensitive receptors.
• Pre-application advice is available from the EA on a charged basis. For complex sites, this is money well spent — it identifies requirements before the application is submitted.
• Product 4 data (detailed flood data) can be requested from the EA and typically takes 20 working days to receive. Order this early to avoid it becoming a critical path item.

Climate Change and Future Risk

Every FRA must account for how flood risk will change over the lifetime of the development. For residential development, this means looking ahead to the 2080s — a 100-year design horizon.

The Environment Agency publishes climate change allowances that specify percentage uplifts to peak river flows, peak rainfall intensity, sea level rise, and offshore wind speed and wave height. These allowances are differentiated by river basin district and time period.

Climate change assessment is not a box-ticking exercise. It fundamentally affects finished floor levels, floodplain compensation volumes, and the design capacity of drainage systems. An FRA that downplays climate change risk is likely to face objections.

Costs and Timelines

Typical Costs

FRA Type	Typical Cost	Suitable For
Desktop FRA	£750 - £2,000	Simple residential sites, Flood Zone 1 (>1ha), low-complexity Flood Zone 2
Standard FRA	£2,000 - £5,000	Medium-complexity sites, Flood Zone 2/3a with existing EA data
Detailed FRA with modelling	£5,000 - £15,000+	Complex sites, Flood Zone 3, sites requiring bespoke flood modelling

Costs depend on site size, data availability, the need for flood modelling, and the complexity of the flood risk environment. Sites near tidal-fluvial confluences, behind flood defences, or requiring floodplain compensation are typically at the higher end.

Typical Timelines

• Desktop FRA: 5-10 working days
• Standard FRA: 10-15 working days
• Detailed FRA with modelling: 3-6 weeks (plus 20 working days for EA Product 4 data if not already available)

How to Commission an FRA

When instructing a flood risk consultant, the following information will help them scope the work accurately:

• Site address and a red line plan showing the development boundary
• Proposed development — number of units, use class, and any available layout drawings
• Flood zone — check the EA flood map before instructing
• Any known flood history or drainage issues
• Planning timescales — when you need to submit the application

A good consultant will confirm the scope and fee before starting work, assign a named technical lead, and keep you informed throughout the process.

At Aegaea, we deliver flood risk assessments across England and Wales, from single-dwelling sites to strategic land allocations. Our team includes chartered engineers, hydrologists, and flood modellers who work across the full spectrum of flood risk complexity.

If you need an FRA or want to discuss your site before committing, get in touch — we will give you a straight answer on what is required and what it will cost.