One hectare and the automatic flood risk requirement

A site of 1 hectare or more needs a site-specific flood risk assessment even where the whole of it sits in Flood Zone 1 with no river or coastal risk at all. That catches self-builders and small developers regularly, because the instinct is to check the flood map, see Zone 1, and move on. The one hectare flood risk assessment requirement comes from national planning guidance rather than from the flood zone, and it exists because surface water is the risk that area brings with it.

Everything in Flood Zones 2 and 3 needs an assessment whatever its size. The 1 hectare figure is the additional trigger that applies inside Zone 1.

Where the one hectare trigger comes from

Planning Practice Guidance on flood risk and coastal change sets out when a site-specific flood risk assessment is required. Proposals in Zones 2 and 3 always need one. Proposals in Zone 1 need one where the site is 1 hectare or more, and also where the strategic flood risk assessment or other information shows a risk from another source.

That second limb is easy to miss and it applies to small sites. A 0.2 hectare plot in Zone 1 sitting in a critical drainage area, or over a groundwater flooding hotspot identified in the council's strategic assessment, needs an assessment even though it is nowhere near the threshold.

Check the strategic flood risk assessment for your authority before concluding that Zone 1 and a small site means nothing is required. It is a published document and it is where the local sources are recorded.

Sites in Flood Zone 1 above the threshold

The purpose of the assessment on a large Zone 1 site is different from its purpose in Zone 3. There is no meaningful fluvial or tidal risk to manage, so the document deals with what the development does to surface water, and with sources other than rivers and the sea.

An assessment that recites the flood zone, states that the site is at low risk and stops there will be returned. What the lead local flood authority wants to see is the runoff position: what the site does now, what it will do after development, and how the difference is controlled.

The scale also matters for another reason. A hectare of land generates enough runoff that the drainage design has real consequences for third parties downstream, which is the policy reason the threshold exists at all.

What the assessment must cover at this scale

For a site of 1 hectare or more in Zone 1, the content that satisfies most authorities is:

1. All sources of flooding considered and dismissed with a reason, covering surface water, groundwater, sewers, artificial sources such as reservoirs and canals, and overland flow from adjoining land.
2. Existing runoff rates and volumes, with the method stated, and the greenfield or brownfield baseline explained.
3. Proposed discharge rates and the storage needed, including the drainage hierarchy worked through from infiltration downwards.
4. Climate change allowances applied to rainfall intensity, with the version and date recorded.
5. Urban creep allowance where the development is residential.
6. Exceedance flow routes for events beyond the design standard, shown on a plan.
7. Finished floor levels relative to surrounding ground and to any overland flow route.
8. Safe access and egress in an exceedance event.
9. Maintenance responsibility for every drainage feature.

Infiltration testing to a recognised method, with groundwater levels, is what supports the hierarchy argument. An assertion that infiltration is unsuitable without a test is the single most common reason a drainage condition is not discharged.

Surface water and critical drainage areas

Surface water risk is mapped separately from the flood zones, and the two often disagree on the same field. A site in Zone 1 can have a defined surface water flow path running across it that the flood zone map does not show.

Where the authority has designated a critical drainage area, expect tighter discharge rates and a requirement to demonstrate betterment rather than no worsening. The designation comes from the strategic flood risk assessment and is usually shown on the council's constraint mapping.

Overland flow from adjoining land is the source most often left out. Where the site sits at the bottom of a slope, the assessment needs to say where that water goes once the development is built, and a boundary bund or an interceptor swale is a design decision rather than an afterthought.

Lead local flood authority consultation

The lead local flood authority is a statutory consultee on surface water drainage for major development. A site of 1 hectare or more will normally be major under the general area limb of the definition, so the two thresholds tend to arrive together.

That consultation is where large Zone 1 applications are most often delayed. The authority's checklist is published in most areas, and working to it directly is faster than submitting a generic assessment and responding to comments.

Three items recur in holding responses: no infiltration testing, no exceedance plan, and no named maintaining party for the drainage features. Each of those can be resolved before submission.

Sequential test interaction

The sequential test is not required for proposals in Flood Zone 1 unless the strategic flood risk assessment identifies risk from other sources. Crossing the 1 hectare threshold does not by itself bring the test into play.

Where other sources are identified, the position changes and the test may apply. The Sequential Test (Flood Risk) page sets out what that involves, and the test is about alternative sites rather than about site design.

So a large Zone 1 site normally needs a Flood Risk Assessment and a Drainage Strategy without needing the sequential test at all. Keep the three documents consistent on rates, volumes and levels, because inconsistency between them is what generates further rounds of comment.

Reducing the red line to stay below

The temptation is obvious and the answer is usually no.

A red line has to include all the land where works are proposed, all the land needed for access and visibility, and all the land carrying the drainage features. A site of 1.05 hectares cannot be trimmed to 0.98 hectares by excluding the attenuation basin, because the basin is part of the development.

Where land forms no part of the proposal, it belongs in the blue line, and doing so is legitimate. A paddock retained in the same ownership, with no works proposed and no drainage function, is not part of the application site.

Two practical points. If the site is close to the threshold, measure the area properly from the survey rather than from a scaled drawing, and state it to two decimal places on the form. And if the answer is 0.97 hectares, do the drainage evidence anyway, because a site of that size will need a drainage strategy for the planning condition whether or not a formal flood risk assessment is triggered. Our note on statutory consultees that appear as a scheme grows sets out who else arrives as the area increases.

This is general guidance, not legal advice. Flood risk and drainage design is work for a suitably qualified engineer, and for specialist technical reports, qualified professional input may be required. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.