Flood risk assessment or a drainage note on a small site
Flood zone and site area decide whether you need a flood risk assessment. Surface water drainage is a separate question that almost every new dwelling now has to answer, whatever the zone.
For a small site the flood risk assessment question has a short answer: if any part of the site is in Flood Zone 2 or 3, or the site is one hectare or more, or another source of flooding is known to affect it, you need a site-specific assessment. Otherwise a proportionate drainage note dealing with surface water is what the council is actually asking for, and confusing the two is why validation stalls.
They are different documents answering different questions. A flood risk assessment asks whether the development is safe from flooding and whether it makes flooding worse elsewhere. A drainage note asks where the rain that lands on the roofs and drives is going to go.
When a full assessment is required
The triggers, drawn from national guidance:
| Situation | Site-specific FRA | Sequential test |
|---|---|---|
| Flood Zone 1, under 1 ha, no other source | Not normally | No |
| Flood Zone 1, 1 ha or more | Yes | No |
| Flood Zone 2, minor development | Yes, proportionate | No |
| Flood Zone 2 or 3, new dwellings | Yes | Yes for non-minor development |
| Any zone, mapped surface water or groundwater risk | Yes, focused on that source | Depends on the source and scale |
Check the flood map for planning for the zone, then check the long-term flood risk service for surface water and reservoir risk, then check the council's strategic flood risk assessment for local detail such as critical drainage areas. Three sources, ten minutes, and it settles the scope of work.
A point people miss: the zone is based on the presence of a flood source without defences taken into account for Zone 3a and 3b definitions, so a site behind a defence can still sit in a high risk zone. The Flood Risk Assessment then has to deal with the residual risk of overtopping or breach rather than asserting that the defence solves it.
What a proportionate drainage note covers
On a scheme of a few dwellings in Flood Zone 1, the drainage content the council expects:
- Existing site drainage and whether the site is greenfield or previously developed.
- Infiltration testing to a recognised method, with results and the soakaway sizing that follows.
- The discharge hierarchy applied in order, with reasons where infiltration is rejected.
- Discharge rate and attenuation volume for the design events, including a climate change allowance.
- Water quality treatment where run-off comes from parking or hardstanding.
- Exceedance routing, so the water that leaves the system in a larger event has somewhere to go that is not a habitable room.
- Maintenance responsibility, naming who maintains the features and how.
That is two or three pages plus calculations and a drainage layout, not a chapter. A Drainage Strategy at that scale earns its keep by making the infiltration and rate calculations explicit, because an assertion that surface water will discharge to a soakaway without a test result invites a condition at best and a refusal reason at worst.
Site area and flood zone thresholds
Two numbers do most of the work. One hectare is the threshold at which a site in Flood Zone 1 needs an assessment, because at that scale the development itself can alter run-off enough to matter. Ten or more dwellings, 1,000 square metres of floorspace or 0.5 hectares makes the scheme major development, which brings the lead local flood authority in as a statutory consultee.
Sites straddling a zone boundary need care. If any part of the application site falls within Zone 2, treat the application as being in Zone 2 for scoping purposes and explain in the assessment which parts of the development sit where. Keeping the built form in the lowest risk part of the site is the single most effective design response, and it is what the sequential approach within a site is for.
Surface water risk outside the flood zones
Flood zones are fluvial and tidal. Surface water flooding, which is where most property flooding in England actually happens, is mapped separately and does not appear on the flood map for planning zones at all.
So a site can be in Zone 1 and sit in a hollow that collects run-off from a road. The right response is not a full fluvial assessment. It is a focused note: the mapped extent and depth from the long-term flood risk service, the topography from a levels survey, finished floor levels set above the modelled depth with freeboard, and an exceedance route that keeps water moving past the building.
Groundwater and sewer flooding follow the same logic. The water company's records and the council's strategic flood risk assessment are the evidence sources, and the design response is levels and resilience rather than modelling.
Who reviews each document
The Environment Agency is the statutory consultee for fluvial and tidal flood risk, and it comments on major development in Zones 2 and 3 rather than on householder schemes. For minor development it publishes standing advice that the council applies itself. The lead local flood authority is the statutory consultee for surface water on major development. On minor schemes the council's own drainage engineer takes the view.
That changes how you write. A document destined for an Environment Agency officer needs modelled levels, climate change allowances by river basin district and epoch, and a clear statement of residual risk. A document destined for a council drainage engineer needs infiltration results, rates, volumes and a maintenance plan. Both should be legible to a case officer who is neither.
Illustrative example: four houses on a 0.2-hectare plot, wholly in Flood Zone 1, with the long-term flood risk map showing a low surface water flow path along the rear boundary. No flood risk assessment is required by the zone or the area. The submission carries a drainage statement with infiltration tests showing poor permeability, a rate-limited connection to the surface water sewer agreed in principle with the undertaker, attenuation sized for the design event with a climate change allowance, finished floor levels 150 millimetres above surrounding ground, and a rear swale retained as the exceedance route. Illustrative only.
Validation and scoping before submission
Read the local validation list before you commission anything. Many authorities require a drainage statement on every application for a new dwelling, and some require a completed surface water drainage checklist or pro forma that has to be signed by the designer. Submitting a technically sound report in the wrong format still fails validation.
Where the position looks borderline, the quickest resolution is a scoping enquiry. Send the council or the lead local flood authority the site boundary, the proposed use and your reading of the maps, and ask what level of assessment they expect. Where a sequential test is in play, how the sequential and exception tests differ is worth reading first, and the Sequential Test page sets out what the document contains. The wider evidence base sits across all 66 report types.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it, and check the current climate change allowances at the time of writing because they are revised.
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Frequently asked questions
Does every new house need a flood risk assessment?
No. A site-specific assessment is expected where the site lies in Flood Zone 2 or 3, where the site is one hectare or more even in Flood Zone 1, or where another source of flooding such as surface water, groundwater or sewers is known to affect it. Outside those situations a drainage note usually suffices.
What is minor development for flood risk purposes?
National guidance treats householder development, non-residential extensions below a stated floorspace figure and alterations that do not increase the footprint as minor development for these purposes. The sequential test is not applied to minor development or to most changes of use, though the assessment of flood risk itself can still be required.
Who reviews the flood documents?
The Environment Agency comments on fluvial and tidal risk for major development in Flood Zones 2 and 3, the lead local flood authority comments on surface water for major development, and the council's own drainage engineer handles minor schemes using standing advice. The sewerage undertaker deals separately with connections.
Is a drainage note the same as a drainage strategy?
In substance it is a shorter version. The content is the same: how surface water leaves the site, in the order of preference set by the technical standards, with evidence for the chosen route, rates, volumes and maintenance. A five-plot scheme needs calculations and infiltration results, not a page of description.
What if the site is in Flood Zone 1 but shows surface water risk?
Then address it. Flood zones cover rivers and the sea only, and surface water risk is mapped separately. Councils increasingly ask for the long-term flood risk check as well as the flood map for planning, and a site in Zone 1 with a mapped surface water flow path needs finished levels and exceedance routing thought through.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.