Environmental · Mandatory · Pre-Construction

Flood Risk Assessment

An assessment of flood risk to and from a development site.

Last reviewed: 29 March 2026 — This guide reflects UK law as of this date. NPPF / Town and Country Planning remains current with no amendments enacted as of 29 March 2026. Next scheduled review: 29 March 2027.

Legal basis	National Planning Policy Framework (NPPF) / Town & Country Planning
Status	Mandatory in Flood Zones 2/3 and sites >1 ha in Flood Zone 1
Prepared by	Flood Risk Engineer
Flood Zones	1 (low), 2 (medium), 3a (high), 3b (functional floodplain)
Climate change	Climate change allowances must be applied to all flood level and flow calculations

1. Flood Risk Assessment — Ensuring Development Is Safe from Flooding

A flood risk assessment (FRA) is a fundamental requirement of the planning system for any development in an area at risk of flooding or any major development regardless of flood zone. The primary function of the FRA is public safety — ensuring that new development does not place people or property at unacceptable risk of flooding and does not increase flood risk elsewhere. With climate change increasing the frequency and severity of flood events across the United Kingdom, the importance of robust flood risk assessment has never been greater.

The National Planning Policy Framework requires a site-specific flood risk assessment for all development proposals in Flood Zones 2 and 3, for all development proposals on sites of one hectare or more in Flood Zone 1, and for any development in Flood Zone 1 where there is a known risk of flooding from other sources (surface water, groundwater, sewers, or artificial sources). The FRA must assess flood risk from all sources — not just rivers and the sea, but also surface water, groundwater, sewers, canals, reservoirs, and other artificial sources.

The Environment Agency is a statutory consultee on planning applications in Flood Zones 2 and 3 and has the power to object to applications where the FRA is inadequate or where the proposed development would place people at unacceptable risk. An objection from the Environment Agency on flood risk grounds is very difficult to overcome and will usually result in the refusal of planning permission.

Engage the Environment Agency early

Engage the Environment Agency via pre-application advice before preparing the FRA — their views on flood risk and SuDS strategy will shape the assessment.

2. Key Sections of the Report

A compliant flood risk assessment must address each of the following areas. The scope and depth of assessment required depends on the flood zone, the size and vulnerability of the development, and the sources of flood risk identified.

Section	What it covers
Site description	A description of the site location, topography, geology, hydrology, and existing land use. The site description must identify all watercourses on or adjacent to the site, existing drainage features, ground levels, and any historical flood records. Ordnance Survey mapping, LiDAR data, and site survey data are used to establish the topographic context.
Flood risk data	A review of all available flood risk data for the site and surrounding area, including Environment Agency flood maps, historical flood records, strategic flood risk assessments (SFRAs) prepared by the local planning authority, surface water flood maps, groundwater vulnerability maps, and reservoir flood maps. The Environment Agency's product data (detailed flood risk information available on request) should be obtained for sites in Flood Zones 2 and 3.
Sequential Test	The Sequential Test aims to steer new development to areas with the lowest risk of flooding. The applicant must demonstrate that there are no reasonably available alternative sites at lower flood risk where the development could be located. The scope of the sequential search area and the definition of “reasonably available” are agreed with the local planning authority. If the Sequential Test is not passed, planning permission should not be granted.
Flood sources	An assessment of flood risk from all relevant sources: fluvial (river) flooding, tidal flooding, surface water (pluvial) flooding, groundwater flooding, sewer flooding, and flooding from artificial sources such as canals, reservoirs, and raised defences. Each flood source must be assessed separately, and the combined risk from multiple sources must be considered.
Flood levels	The predicted flood levels for the site for a range of return periods, including the 1 in 100-year fluvial event and the 1 in 200-year tidal event, with climate change allowances applied. Climate change allowances are published by the Environment Agency and must be applied to all flood level calculations to account for the projected increase in peak river flows and sea level rise over the lifetime of the development.
Flood vulnerability	An assessment of the vulnerability classification of the proposed development in accordance with the NPPF flood risk vulnerability classification. Developments are classified as essential infrastructure, highly vulnerable, more vulnerable, less vulnerable, or water-compatible. The vulnerability classification determines which flood zones the development is appropriate in and whether the Exception Test is required.
Mitigation measures	The proposed flood risk mitigation measures, including finished floor levels set above the predicted flood level with an appropriate freeboard allowance, flood-resistant and flood-resilient construction techniques, safe access and egress routes that remain above flood level, flood warning and evacuation procedures, and any proposed flood defences or flood storage compensation. The FRA must demonstrate that the development will be safe for its lifetime.
SuDS strategy	A surface water drainage strategy demonstrating how surface water run-off from the development will be managed to prevent increasing flood risk elsewhere. The strategy must follow the drainage hierarchy (infiltration, attenuation and discharge to a watercourse, discharge to a surface water sewer, discharge to a combined sewer) and must limit post-development run-off rates to greenfield rates for the site.
Exception Test (if req'd)	Where the Sequential Test has been passed but the development is proposed in a flood zone that is not normally appropriate for its vulnerability classification, the Exception Test must be applied. The Exception Test requires the applicant to demonstrate that the development provides wider sustainability benefits to the community that outweigh the flood risk, and that a site-specific FRA demonstrates that the development will be safe for its lifetime without increasing flood risk elsewhere and, where possible, will reduce flood risk overall.

3. Common Mistakes

Not applying climate change allowances

The Environment Agency publishes climate change allowances that must be applied to all flood level and peak flow calculations in a flood risk assessment. These allowances account for the projected increase in peak river flows, peak rainfall intensity, sea level rise, and offshore wind speed and extreme wave height over the lifetime of the development. An FRA that uses current flood levels without applying climate change allowances will be rejected by the Environment Agency. The allowances are percentage increases applied to the baseline flood level or flow, and they vary by river basin, epoch, and the flood zone in which the development is located. Using the wrong allowance or failing to apply it at all is one of the most common reasons for Environment Agency objections to planning applications.

Not considering all sources of flood risk

A flood risk assessment must consider all sources of flooding, not just the most obvious one. Many FRAs focus exclusively on fluvial (river) flooding because the site is in a fluvial flood zone, but fail to assess surface water flooding, groundwater flooding, or sewer flooding. Surface water flooding is a significant and increasing risk in urban areas and can affect sites in Flood Zone 1 that are not at risk of river flooding. Groundwater flooding affects sites with high water tables, particularly chalk and limestone areas. Sewer flooding can occur when the capacity of the sewer network is exceeded during heavy rainfall. An FRA that does not assess all relevant flood sources is incomplete and may be rejected.

Not demonstrating the Sequential Test

The Sequential Test is a fundamental requirement of national planning policy for development in flood risk areas. The applicant must demonstrate that there are no reasonably available alternative sites at lower flood risk where the proposed development could be located. Many applicants either ignore the Sequential Test entirely or provide an inadequate assessment that does not properly consider alternative sites. The scope of the sequential search area and the criteria for assessing alternative sites should be agreed with the local planning authority before the FRA is prepared. Failure to pass the Sequential Test is a clear reason for refusal of planning permission, and the Environment Agency will object to any application where the Sequential Test has not been adequately addressed.

4. Frequently Asked Questions

Do flood zones affect permitted development rights?▾

Yes. Certain permitted development rights are restricted or removed in flood risk areas. Under the Town and Country Planning (General Permitted Development) (England) Order 2015 (as amended), some classes of permitted development do not apply in areas at risk of flooding. For example, Class Q permitted development rights (conversion of agricultural buildings to dwellings) do not apply if the site is in Flood Zone 2 or 3, or in an area identified as at risk of flooding by the Environment Agency. Other permitted development rights may be subject to conditions requiring a site-specific flood risk assessment or prior approval from the local planning authority on flood risk grounds. Developers relying on permitted development rights must check whether those rights are restricted by flood risk designations before commencing development. The flood zone of a site can be checked on the Environment Agency's Flood Map for Planning, which is available online.

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