Flood Risk Assessment for New Build Homes

Residential development is classified as “more vulnerable” under the NPPF flood risk vulnerability categories, placing it in one of the more restrictive categories for flood zone compatibility. This means that new homes proposed in flood-risk areas face a more rigorous assessment process than many other development types, and the Flood Risk Assessment must provide compelling evidence that the development can be made safe.

For developers of new-build homes — whether a single dwelling or a major housing scheme — understanding the FRA process, the policy framework, and the technical requirements is essential for securing planning permission efficiently and avoiding costly delays.

The Policy Framework

The NPPF

The National Planning Policy Framework sets the overarching policy for flood risk and development. The key provisions for residential development are:

Paragraph 165 requires that inappropriate development in areas at risk of flooding should be avoided by directing development away from areas at highest risk. Where development is necessary in such areas, it should be made safe for its lifetime without increasing flood risk elsewhere.

Paragraph 168 establishes the Sequential Test — requiring development to be directed to areas at lowest flood risk.

Paragraph 170 establishes the Exception Test — required for more vulnerable development in Flood Zone 3a and for highly vulnerable development in Flood Zone 2.

Paragraph 173 requires that all plans should apply a sequential, risk-based approach to the location of development, taking into account all sources of flood risk and the current and future impacts of climate change.

The Planning Practice Guidance

The PPG provides detailed guidance on how to implement the NPPF’s flood risk policies. Key sections for residential development include:

• How to apply the Sequential Test (paragraphs 023-029)
• How to apply the Exception Test (paragraphs 030-034)
• What a site-specific FRA should contain (paragraph 030)
• Climate change allowances and how to apply them
• Safe access and egress requirements
• The role of sustainable drainage systems

Flood Zone Compatibility

The NPPF compatibility matrix for residential development is:

Flood Zone	Residential (More Vulnerable)
Zone 1	Appropriate
Zone 2	Appropriate
Zone 3a	Exception Test required
Zone 3b	Not appropriate

This means residential development is not appropriate in Flood Zone 3b (functional floodplain) under any circumstances. In Flood Zone 3a, it is only appropriate if the Exception Test is passed.

The Sequential Test

Purpose

The Sequential Test aims to steer new development to areas with the lowest risk of flooding. It is the first hurdle that residential development in Flood Zones 2 and 3 must clear.

How It Works

The Sequential Test is applied by the local planning authority, but the evidence is usually provided by the developer. The process involves:

1. Defining the area of search — the geographic area within which alternative sites are considered. For housing, this is typically the LPA’s administrative area, or in some cases a wider area defined by the housing market area.

2. Identifying reasonably available alternative sites — sites that are available, suitable for the proposed development, and achievable within the relevant time frame.

3. Comparing flood risk — demonstrating that the proposed site is at a lower (or equal) risk of flooding compared to the alternative sites, or that the alternative sites cannot accommodate the development.

Common Issues

The LPA’s role. The LPA should provide guidance on the area of search and may maintain a list of available sites. However, in practice, many LPAs expect the developer to carry out the Sequential Test assessment and present the evidence.

Allocated sites. If the site is allocated for residential development in the adopted Local Plan, the Sequential Test is considered to have been passed at the plan-making stage (provided the Local Plan was subject to its own Sequential Test through the Strategic Flood Risk Assessment). The developer does not need to re-apply the Sequential Test at the application stage.

Previously developed land. The NPPF gives weight to the development of brownfield (previously developed) land, and this can be a factor in the Sequential Test. A brownfield site in a flood zone may be sequentially preferable to a greenfield site in a lower flood zone, depending on the overall sustainability of the development.

Windfall sites. For windfall sites (not allocated in the Local Plan), the developer must apply the Sequential Test. This requires demonstrating that there are no reasonably available sites at lower flood risk within the area of search. This can be a challenging exercise, particularly in areas where land at lower flood risk is available.

The Exception Test

When It Applies

The Exception Test is required for more vulnerable development (including residential) in Flood Zone 3a, and for highly vulnerable development (including basement dwellings) in Flood Zone 2.

Part 1: Wider Sustainability Benefits

The development must provide wider sustainability benefits to the community that outweigh the flood risk. For residential development, relevant sustainability benefits include:

• Meeting housing need — particularly where the LPA cannot demonstrate a five-year housing land supply
• Affordable housing provision — delivering affordable homes in areas of need
• Regeneration — developing brownfield land, bringing derelict sites back into use
• Sustainable location — proximity to public transport, services, and employment
• Design quality — high-quality, energy-efficient homes
• Biodiversity net gain — delivering measurable biodiversity improvements

Part 2: Safe Development

The FRA must demonstrate that the development will be safe for its lifetime (taken as at least 100 years for residential development), taking account of the vulnerability of its users, without increasing flood risk elsewhere, and where possible reducing flood risk overall.

This is the most technically demanding part of the assessment and requires detailed evidence on:

• Flood risk from all sources
• Design flood levels with climate change allowances
• Proposed mitigation measures
• Residual risk after mitigation
• Emergency planning

What the FRA Should Cover

Site-Specific Flood Risk Assessment

The FRA should assess flood risk from all sources:

Fluvial (river) flooding. Using Environment Agency flood zone maps, detailed flood modelling data (Product 4), and any site-specific hydraulic modelling. The FRA should establish the 1% annual probability flood level (1 in 100 year) and apply the appropriate climate change allowance.

Tidal flooding. Where relevant, using the 0.5% annual probability level (1 in 200 year) with climate change allowances for sea level rise.

Surface water flooding. Using the Risk of Flooding from Surface Water maps and any available detailed surface water modelling. Surface water flood risk is increasingly the primary risk in many urban areas.

Groundwater flooding. Using British Geological Survey data, SFRA information, and site-specific ground investigation data.

Sewer flooding. Using water company DG5 records and any available sewer capacity modelling.

Artificial sources. Including reservoir inundation mapping (available on GOV.UK) and any risk from canals or other artificial water bodies.

Climate Change Allowances

For residential development with a 100-year design life, the climate change allowances are significant. The Environment Agency publishes allowances for:

• Peak river flow — percentage increases to be added to the 1% annual probability peak flow. These vary by river basin district and time horizon, and range from +12% to +95% for the 2080s epoch.
• Peak rainfall intensity — percentage increases to be added to design rainfall depths. Currently +5% to +40% depending on the time horizon.
• Sea level rise — absolute values in millimetres to be added to the design tidal level. Currently ranging from approximately 0.5m to over 1.2m by 2125 depending on the scenario.

The FRA should clearly state which allowances have been applied and why, and should present the flood risk assessment for both the “central” and “upper end” scenarios.

Finished Floor Levels

The FRA should specify the minimum finished floor level for all residential units. The standard requirement is:

• At least 300mm above the 1% annual probability flood level with the appropriate climate change allowance (for river flooding)
• At least 300mm above the 0.5% annual probability flood level with climate change (for tidal flooding)
• Where possible, 600mm above the design flood level is preferred

For ground-floor residential units, this often means raising the floor level above surrounding ground level. This has design implications — including ramped access, raised gardens, and interface with adjacent properties — that should be addressed in the architectural design.

Safe Access and Egress

Safe access and egress is a critical requirement for residential development. The EA requires that:

• A safe route to and from the development must be available during a 1% annual probability flood event (with climate change)
• “Safe” means the route is dry, or if it is not dry, the depth of flooding does not exceed 300mm with a velocity that keeps the depth x velocity product below 0.5 m2/s
• The route must lead to an area outside the flood zone

Where safe dry access cannot be achieved, the developer may need to demonstrate that:

• Flood warning lead time is sufficient for evacuation before the access route becomes unsafe
• The development includes a safe internal refuge above the flood level
• An emergency flood plan is in place

Flood Resilience and Resistance

For residential development in flood-risk areas, the FRA should recommend a package of flood resilience and resistance measures:

Resistance measures:

• Flood barriers for all ground-floor openings
• Waterproof construction below the design flood level
• Non-return valves on all drainage connections
• Waterproof membrane to the building envelope below the flood level

Resilience measures:

• Waterproof plaster to walls below the design flood level
• Tiled or sealed concrete flooring at ground floor level
• Raised electrical sockets, switches, and consumer units (at least 300mm above the design flood level within the building)
• Raised boiler, washing machine, and other white goods
• Kitchen units in plastic or marine-grade materials below the flood level

Sustainable Drainage Systems (SuDS)

The NPPF requires that all major development incorporates sustainable drainage systems. For residential development in flood-risk areas, the drainage strategy is a critical component of the FRA:

• Attenuation — surface water runoff from the development must be attenuated to greenfield rates (or as close to greenfield as practicable for brownfield sites)
• SuDS hierarchy — the drainage design should follow the SuDS hierarchy, prioritising infiltration, then attenuation and discharge to a watercourse, then to a surface water sewer, and only as a last resort to a combined sewer
• Climate change — the drainage design must account for climate change, applying the appropriate peak rainfall intensity allowance
• Exceedance — the drainage design must include an exceedance strategy, demonstrating what happens when the design capacity is exceeded (e.g., in an event larger than the 1% annual probability event)
• Water quality — SuDS should provide appropriate treatment of surface water runoff before discharge

Flood Risk Elsewhere

A fundamental requirement of the NPPF is that development must not increase flood risk elsewhere. The FRA must demonstrate that:

• The development does not reduce floodplain storage (or that any reduction is compensated on a level-for-level, volume-for-volume basis)
• The development does not impede flood flow routes
• Surface water runoff from the development is managed so that it does not increase flood risk to downstream properties
• The development does not increase groundwater or sewer flood risk

Emergency Planning

For residential development in flood zones, the FRA should include or be accompanied by a Flood Warning and Evacuation Plan (or Flood Emergency Plan). This should cover:

• How flood warnings are received and disseminated to residents
• The evacuation procedure, including assembly points and safe routes
• The role of residents, management company, and emergency services
• Arrangements for vulnerable residents
• Maintenance and review of the plan

Common Pitfalls

Underestimating the Sequential Test

The Sequential Test is the most common reason for FRA-related refusals of residential planning applications. Many developers underestimate the rigour required and submit a superficial assessment that the LPA finds unconvincing. Invest time in a thorough Sequential Test assessment, and engage with the LPA early to agree the area of search and methodology.

Ignoring Surface Water Risk

Many FRAs focus on river flooding (because this is what the flood zone maps show) and give insufficient attention to surface water flood risk. Surface water flooding is the most common source of flooding in urban areas and is increasingly scrutinised by LPAs and LLFAs.

Inadequate Climate Change Allowances

The climate change allowances have been updated several times in recent years, and it is essential to use the current figures. An FRA that uses outdated allowances will be rejected.

Poor Drainage Design

The drainage strategy is as important as the flood risk assessment itself. A poorly designed drainage scheme can be a reason for refusal or for onerous planning conditions. Engage a drainage engineer early in the design process.

Ignoring the LLFA

The Lead Local Flood Authority is a statutory consultee on major development applications for surface water drainage. Ignoring the LLFA’s requirements — which vary between authorities — is a common cause of delays. Pre-application consultation with the LLFA is highly recommended.

Cost and Timescale

FRAs for new-build residential development vary significantly in cost depending on the scale and complexity:

Development Size	Typical FRA Cost	Typical Timescale
Single dwelling	GBP 800 - GBP 1,500	3-4 weeks
Minor development (2-9 units)	GBP 1,500 - GBP 3,000	4-6 weeks
Major development (10+ units)	GBP 3,000 - GBP 10,000+	6-12 weeks

These costs assume that existing Environment Agency modelling data is available. Where new hydraulic modelling is required — for example, where the EA’s data does not cover the site, or where the existing modelling is outdated — the cost can increase significantly.

How Aegaea Can Help

Aegaea prepares Flood Risk Assessments and drainage strategies for residential developments of all sizes across England, Scotland, and Wales. From single plots to strategic housing sites, we provide the technical evidence needed to secure planning approval.

Our team of Chartered engineers has extensive experience of navigating the Sequential Test, Exception Test, and the detailed technical requirements of the NPPF, PPG, and EA guidance. We work closely with developers, architects, and planning consultants to ensure that flood risk and drainage are addressed from the earliest design stages.

Contact us for a no-obligation discussion about your residential development project.