Do I Need a Flood Risk Assessment? A Quick Decision Guide

It is one of the most common questions we receive from developers and planning consultants: does my project actually need a flood risk assessment? The answer is not always obvious, and getting it wrong can delay your planning application by weeks or months.

The National Planning Policy Framework (NPPF) and its accompanying Planning Practice Guidance (PPG) set out clear triggers for when a site-specific flood risk assessment (FRA) is required. Understanding these triggers early in the project lifecycle saves time, avoids unnecessary cost, and prevents unwelcome surprises at validation stage.

The Four Triggers

There are four main circumstances in which a flood risk assessment is required to accompany a planning application in England. If any one of these applies to your site, you will need an FRA.

1. Your Site Is in Flood Zone 2 or 3

This is the most straightforward trigger. If any part of your site falls within Flood Zone 2 (medium probability) or Flood Zone 3 (high probability), a flood risk assessment is required regardless of the size of the development or the type of application.

You can check your site’s flood zone designation using the Environment Agency’s Flood Map for Planning. Bear in mind that the EA’s mapping is based on broad-scale modelling and does not account for local defences or site-specific conditions. The actual risk may be higher or lower than the mapped zone suggests.

Even minor developments — extensions, change of use, or single dwellings — trigger the FRA requirement if they fall within Flood Zone 2 or 3. There are no exemptions based on project scale within these zones.

2. Your Site Is Larger Than 1 Hectare in Flood Zone 1

Sites in Flood Zone 1 (low probability of fluvial and tidal flooding) are generally not required to submit an FRA. However, if the site area exceeds 1 hectare, the NPPF requires a flood risk assessment regardless of the zone.

The rationale is sound: larger sites have a greater impact on surface water runoff, and the development itself can alter drainage patterns and increase flood risk elsewhere. The FRA for a large Flood Zone 1 site will typically focus on surface water management, drainage strategy, and the potential impact on downstream receptors rather than fluvial or tidal risk.

This 1-hectare threshold catches many developments that applicants assume are exempt. A site may be in the lowest risk zone, well away from any river or coast, but if it exceeds 10,000 square metres in area, an FRA is required. This is a common cause of validation delays.

3. Your Site Is Near a Main River or the Coast

If your site is in the immediate vicinity of a main river, ordinary watercourse, or the coast, the local planning authority (LPA) or the Environment Agency may require a flood risk assessment even if the site is technically in Flood Zone 1 and below the 1-hectare threshold.

This is particularly common where the EA’s flood mapping is known to be out of date, where there is historical evidence of flooding in the area, or where the watercourse is culverted beneath or adjacent to the site. Culverted watercourses present a particular risk because they can surcharge during storm events, and the blockage of a culvert inlet can cause rapid, localised flooding.

It is always worth checking the EA’s Risk of Flooding from Surface Water mapping alongside the Flood Map for Planning. Surface water mapping often reveals risk that the fluvial flood zones do not capture.

4. Local Policy or Site-Specific Requirements

Many local planning authorities have adopted policies that extend the FRA requirement beyond the national triggers. Common local requirements include:

• Critical Drainage Areas (CDAs): Some LPAs require FRAs for any development within a designated CDA, regardless of site size or flood zone. London boroughs are particularly active in this regard.
• Surface Water Flood Risk Areas: Where the Risk of Flooding from Surface Water mapping shows medium or high risk, LPAs may require a flood risk assessment even for small sites in Flood Zone 1.
• Strategic Flood Risk Assessment (SFRA) recommendations: The LPA’s SFRA may identify specific areas or site typologies that require site-specific assessment.
• Neighbourhood Plan policies: Some neighbourhood plans include flood risk policies that go beyond the NPPF requirements.

Always check the LPA’s Local Plan, validation checklist, and pre-application advice before concluding that an FRA is not required. A quick conversation with the planning officer or a review of the validation requirements can prevent costly delays.

A Decision Flowchart

Working through the following sequence will help you determine whether your project needs a flood risk assessment:

Step 1: Check the Flood Map for Planning. Is any part of the site in Flood Zone 2 or 3? If yes, an FRA is required.

Step 2: If the site is in Flood Zone 1, measure the site area. Is it greater than 1 hectare? If yes, an FRA is required.

Step 3: Check the surface water flood risk mapping. Does the site show medium or high surface water risk? If yes, check the LPA’s local policy — an FRA may be required.

Step 4: Is the site adjacent to a main river, ordinary watercourse, or the coast? If yes, check with the LPA and EA — an FRA may be required.

Step 5: Review the LPA’s validation checklist and Local Plan policies. Are there any local triggers (CDA, SFRA recommendations, neighbourhood plan policies)? If yes, an FRA is required.

If the answer to all five steps is no, then a standalone FRA is unlikely to be required. However, you may still need to address surface water drainage as part of your planning application, and a drainage strategy or SuDS statement may be necessary.

What Happens If You Get It Wrong?

Submitting a planning application without a required FRA will result in the application being invalidated at registration. The LPA will return the application and ask for the missing assessment. This adds a minimum of four to six weeks to your programme, and often longer if the FRA reveals issues that require design changes.

Conversely, commissioning an FRA that is not strictly required is rarely wasted money. A flood risk assessment provides valuable information about site constraints, drainage requirements, and potential mitigation measures that can inform the design from the outset. It is far cheaper to address flood risk at the design stage than to retrofit solutions after planning conditions are imposed.

When to Involve a Specialist

If you are unsure whether your site triggers the FRA requirement, it is worth seeking advice early. A brief site screening — checking flood zones, surface water risk, local policy, and watercourse proximity — can be completed quickly and inexpensively. At Aegaea, we offer a free initial site assessment that will confirm whether an FRA is needed and, if so, what level of assessment is appropriate.

The earlier you identify the requirement, the more effectively you can programme the work and avoid delays to your planning submission. A flood risk assessment is not something that can be rushed at the last minute — particularly if hydraulic modelling is required.

Key Takeaways

• Flood Zone 2 or 3: FRA always required, no exceptions.
• Flood Zone 1, site > 1 hectare: FRA required.
• Near a main river or coast: FRA may be required — check with the LPA and EA.
• Local policy triggers: Always check the LPA’s validation checklist and Local Plan.
• When in doubt, ask early: A quick screening saves weeks of delay.

Getting the FRA requirement right at the outset is one of the simplest ways to keep your planning programme on track. If you are unsure, get in touch and we will confirm what you need within 24 hours.