Flood Risk Assessment for a Single Dwelling: What You Need to Know

If you are planning to build a single house — whether as a self-builder, a small developer, or someone converting a plot — the question of whether you need a flood risk assessment (FRA) will come up early in the planning process. For many people, the answer is not immediately obvious. The rules depend on where the site sits relative to the Environment Agency’s flood zones, the size of the site, and how the proposed use is classified under national planning policy.

This article explains when a flood risk assessment is required for a single dwelling, what it needs to cover, how the Sequential Test and Exception Test apply, what mitigation measures are typically needed, and what you can expect to pay and how long it takes.

When Is a Flood Risk Assessment Required?

The requirement for an FRA is set out in the National Planning Policy Framework (NPPF) and its accompanying Planning Practice Guidance (PPG). For a single dwelling, an FRA is required in two main circumstances.

Flood Zone 2 or 3

If any part of your site falls within Flood Zone 2 or Flood Zone 3 on the Environment Agency’s Flood Map for Planning, an FRA is required regardless of the site area. There is no minimum threshold — even a small infill plot entirely within the flood zone needs an FRA.

Flood Zone 2 represents land with a 1 in 100 to 1 in 1,000 annual probability of flooding from rivers, or a 1 in 200 to 1 in 1,000 annual probability from the sea. Flood Zone 3 is higher risk: a 1 in 100 or greater annual probability from rivers, or 1 in 200 or greater from the sea.

The flood zone designation is based on the EA’s broad-scale modelling and does not account for local flood defences, so the zone shown on the mapping does not always reflect the actual risk at a specific site. That is one of the things a good FRA will interrogate.

Flood Zone 1 Sites Over One Hectare

In Flood Zone 1 (low probability of flooding), an FRA is only required for sites of one hectare or more. A single house plot will almost never reach this threshold, so if your site is entirely in Flood Zone 1 and smaller than a hectare, you will not need a formal FRA.

However, if the site has known drainage issues, is at risk from surface water flooding, or the local planning authority has specific requirements, they may still request a flood risk assessment or at least a drainage strategy. It is worth checking the local validation requirements before you submit your application.

Other Triggers

Local planning authorities can impose their own validation requirements that go beyond the national minimum. Some LPAs require FRAs for sites in Critical Drainage Areas or areas at risk of surface water flooding, regardless of the EA flood zone. Always check the LPA’s local validation checklist and their Strategic Flood Risk Assessment (SFRA) before assuming an FRA is not needed.

How a Single Dwelling Is Classified Under Planning Policy

This is where many self-builders and small developers get caught out. Under the NPPF flood risk vulnerability classification, residential dwellings — including a single house — are classified as “more vulnerable” development. This has significant implications for what is and is not possible in each flood zone.

The vulnerability classification determines which combinations of development type and flood zone are appropriate:

• Flood Zone 1: More vulnerable development is appropriate without restriction (subject to the usual planning considerations).
• Flood Zone 2: More vulnerable development is appropriate, but the Sequential Test must be passed.
• Flood Zone 3a: More vulnerable development requires both the Sequential Test and the Exception Test to be passed.
• Flood Zone 3b (functional floodplain): More vulnerable development, including a single dwelling, is not appropriate. This is effectively a prohibition.

One common misconception is that building a “small” single house is somehow treated more leniently than a larger residential development. It is not. The vulnerability classification applies equally to a single self-build plot and a 200-unit housing estate. The policy tests are the same; what changes is the scale of evidence required to satisfy them.

What a Flood Risk Assessment for a Single Dwelling Should Cover

A flood risk assessment for a single dwelling is not a tick-box exercise. A well-prepared FRA needs to address a specific set of questions, and the depth of analysis required will depend on the flood zone.

Site-Specific Flood Risk

The FRA should establish the flood risk at the site from all relevant sources: river (fluvial) flooding, tidal flooding, surface water flooding, groundwater flooding, and sewer flooding. The EA flood zone tells you about river and tidal risk; the other sources need to be assessed separately using the EA’s updated flood mapping, the lead local flood authority’s (LLFA) records, and information from the sewerage undertaker.

Design Flood Levels

For sites in Flood Zone 2 or 3, the FRA must determine the relevant design flood levels — typically the 1 in 100 year (1% annual probability) flood level for rivers, or 1 in 200 year (0.5%) for tidal, plus an appropriate climate change allowance. The EA’s climate change guidance requires that FRAs consider future flood risk over the development’s lifetime, typically using a central or higher-end allowance depending on the vulnerability of the development.

For a residential dwelling, the relevant climate change epoch is typically 2070 (or the end of the development’s design life). The appropriate allowance varies by river catchment and is set out in EA guidance.

Sequential Test Evidence

For sites in Flood Zone 2 or 3, the FRA or an accompanying planning statement must address the Sequential Test. This requires demonstrating that there are no reasonably available alternative sites at a lower risk of flooding where the proposed development could be accommodated.

Exception Test Evidence (Flood Zone 3a Only)

For a single dwelling in Flood Zone 3a, the Exception Test must also be addressed. This has two limbs:

1. The development must provide wider sustainability benefits to the community that outweigh the flood risk.
2. The development must be safe for its lifetime without increasing flood risk elsewhere.

The first limb is typically addressed in the planning statement rather than the FRA. For a single dwelling, demonstrating sustainability benefits is harder than for a larger development — housing delivery on an allocated site is the most straightforward argument. Brownfield regeneration and the removal of a derelict structure are also valid considerations.

The second limb is addressed in the FRA through the mitigation strategy and flood risk impact assessment.

Residual Risk and Flood Warning

The FRA should identify the residual risk to future occupants — what happens if the flood defences fail or the design flood is exceeded — and set out how that risk will be managed. For most single dwelling applications, this means confirming that the site is within the EA’s flood warning coverage and recommending that the future occupant registers with Flood Warnings Direct.

The Sequential Test for a Single Dwelling

The Sequential Test is a genuine policy hurdle for single dwellings in Flood Zone 2 or 3, and it is one that a number of self-build applications fail to address adequately.

The test asks: could this development go somewhere with a lower flood risk? The LPA’s starting point is generally that if you want to build a house in a flood zone, you need to demonstrate that you have genuinely considered lower-risk alternatives.

For self-builders, the most common arguments for passing the Sequential Test are:

• Site-specific constraints: The applicant has been seeking a plot in a specific location (for family, employment, or school reasons) and the sites available within that search area are all at flood risk.
• Brownfield development: The site is previously developed land and providing a replacement dwelling on a site that is already built on arguably does not represent a net increase in flood risk exposure.
• Allocated site: The site is allocated for residential development in the local plan, which means the LPA has already implicitly applied a Sequential Test at the strategic level.

The LPA will assess the Sequential Test evidence against the alternatives available in their area. The search area is defined by the LPA, not the applicant, and in most cases it will be the LPA’s administrative area. Some LPAs will accept a narrower search area where the specific locational need can be demonstrated.

It is important to engage with the LPA early on the Sequential Test. Some authorities have standing guidance in their SFRA that sets out how they expect the test to be applied for different types of development. Others are more flexible but will want to see a credible methodology.

Mitigation Strategies

Once the Sequential and Exception Tests are addressed, the FRA must demonstrate that the development will be safe. For a single dwelling, the mitigation strategy typically draws on some combination of the following measures.

Finished Floor Levels

Raising the finished floor level (FFL) above the design flood level — including climate change — is the most important mitigation measure. The standard approach is to set the FFL at least 300mm to 600mm above the design flood level as a freeboard allowance, though some LPAs and the EA will specify a particular freeboard requirement.

For a single dwelling, this typically means raising the ground floor level. This affects the design of the building — the approach to the front door, the garage level, the relationship of the house to the street — so it needs to be considered early in the design process, not added at the FRA stage.

Flood-Resistant and Flood-Resilient Construction

Where the finished floor level cannot be raised above the design flood level — for example, on a constrained infill site or in a conversion — flood-resistant (keeping water out) or flood-resilient (accepting water in but limiting damage and enabling quick recovery) construction may be required.

Flood-resistant measures include reinforced doorways, air brick covers, waterproof membranes to external walls, and non-return valves on drainage connections. Flood-resilient measures include raised electrical sockets and consumer units, concrete or tiled ground floors rather than timber, and materials that can be dried out and returned to use quickly.

The EA’s guidance on flood resistance and resilience is a useful reference. For a single dwelling in Flood Zone 3, the FRA should specify which measures are required and at what levels.

Compensatory Floodplain Storage

If the development involves raising ground levels within the floodplain — for example, to create a raised platform for the house — this displaces floodwater storage. The planning policy requirement is that compensatory storage must be provided on a level-for-level, volume-for-volume basis within the same flood cell.

For a single dwelling, this typically means identifying an area of the site (usually open space or garden) where levels can be lowered to compensate. The FRA should demonstrate that the compensation is adequate and that the net effect on flood storage is neutral.

Safe Access and Egress

One of the requirements for the Exception Test is that the development is safe for its lifetime. A key component of this is demonstrating that a safe route of access and egress is available during the design flood event.

Safe access is defined in terms of flood depth, velocity, and the resulting hazard rating. The EA uses a standard methodology for calculating flood hazard, and the threshold for “safe” pedestrian access is broadly a depth of 0.25 metres or a hazard rating of less than 0.75. For vehicles, the thresholds are higher.

Where the access road is within the flood zone, the FRA needs to assess flood levels along the route and demonstrate that the hazard rating is acceptable. Where a fully dry access cannot be achieved, the EA may accept a route through shallow, slow-moving water provided the hazard rating is satisfactory — but this requires clear evidence, not an assumption.

Working With the Environment Agency

For planning applications in Flood Zone 2 or 3, the EA is a statutory consultee. This means the LPA is required to notify the EA and must consider their response before deciding the application. In practice, the EA has 21 days to respond, and LPAs are advised not to grant permission against an EA objection without good reason.

The EA will object if the FRA is inadequate, if it does not address the Sequential or Exception Tests, if the mitigation is insufficient, or if there is a concern about the impact of the development on flood risk elsewhere. An EA objection is a serious obstacle — it does not make the application impossible, but it will require either addressing the EA’s concerns or appealing the LPA’s decision.

The best way to avoid an EA objection is to engage early. Many LPAs offer pre-application advice services where you can discuss the approach to the FRA before you submit. Some LPAs will also facilitate pre-application engagement with the EA. This investment of time and money at the early stage almost always saves time and cost later.

Typical Costs

Flood risk assessments for single dwellings vary in cost depending on the flood zone, the complexity of the site, and what the LPA and EA are likely to require.

For a straightforward site in Flood Zone 2 with no particular complexities, a basic FRA prepared by a qualified flood risk specialist will typically fall at the lower end of the professional services market — this is a relatively contained scope of work. For a site in Flood Zone 3 where the Sequential Test, Exception Test, and detailed mitigation design are all required, the cost is meaningfully higher. Where detailed hydraulic modelling is needed to establish design flood levels or to demonstrate no increase in flood risk, the cost increases further still.

These assessments represent real professional work — site-specific research, technical analysis, and a document that will be scrutinised by the LPA, the EA, and potentially an inspector on appeal. Cutting corners on the FRA to save money is a false economy: a weak FRA leads to an EA objection, which at best delays the application and at worst sinks it.

Timeline

For a single dwelling in Flood Zone 2, a straightforward FRA can typically be completed within two to four weeks of instruction, assuming the flood risk data is available without delay.

For a site in Flood Zone 3, particularly where hydraulic modelling is required or the Sequential Test needs to be carefully argued, allow four to eight weeks. If the EA raises queries during the application — which is common for Flood Zone 3 — you should budget additional time to prepare and submit responses.

Planning applications with FRAs are typically determined within the standard eight or thirteen week period for residential development, assuming the EA does not object and no other issues arise. In practice, where the EA raises queries or requests additional information, it is not uncommon for the flood risk element to add several weeks to the overall determination.

Key Takeaways

• A flood risk assessment is required for a single dwelling on any site touching Flood Zone 2 or 3, regardless of the site area.
• A single dwelling is classified as “more vulnerable” development — the Sequential Test applies in Flood Zone 2 and 3a; the Exception Test also applies in Flood Zone 3a.
• Residential development is not appropriate in Flood Zone 3b (functional floodplain).
• The FRA must cover all sources of flood risk, establish design flood levels including climate change, address the Sequential and Exception Tests, and specify a mitigation strategy.
• Mitigation typically involves raised finished floor levels, flood-resistant construction, compensatory floodplain storage, and safe access and egress.
• Early engagement with the LPA and EA is the single most effective way to de-risk the planning process.

---

If you are planning a single dwelling on a site in or near a flood zone and need a flood risk assessment, our residential team can advise on the scope of work required and prepare a robust FRA to support your planning application.

Find out more about our residential flood risk assessment services.