The one hectare flood risk assessment threshold

A site of one hectare or more needs a flood risk assessment even in Flood Zone 1, where the mapped risk of river and sea flooding is low. The one hectare flood risk assessment threshold catches a lot of self-builders and small developers who have checked the flood map, seen Zone 1 and assumed the subject was closed. It is not a flood zone test. It is an area test, and the reason for it is surface water: a site of that size changes how rain leaves the land, and the assessment is as much about what you send downstream as about what arrives on you.

Why area triggers an assessment in Flood Zone 1

National planning guidance requires a site-specific flood risk assessment for all proposals of one hectare or more in Flood Zone 1, and for all proposals in Flood Zones 2 and 3 whatever their size.

The logic is straightforward. Replacing a hectare of field with roofs, roads and drives increases the volume and the rate of runoff, and that water has to go somewhere. Without an assessment nobody has checked whether the receiving ditch, sewer or watercourse can take it, or whether the scheme itself will flood from surface water in a heavy storm.

The threshold is measured on the application site area, the same red line area that decides whether a scheme is major development. Access strips, visibility splays and drainage land all count, so committee referral thresholds and delegated decisions and this test often bite at the same moment.

The other triggers people miss in Flood Zone 1

Area is only one route into an assessment. In Flood Zone 1 an assessment is also needed where any of the following apply.

• The site is in an area with critical drainage problems, as notified by the Environment Agency.
• The land has been identified as at risk from other sources of flooding, including surface water, groundwater, sewers or reservoirs.
• The development involves a change of use to a more vulnerable class, such as a store becoming a dwelling.
• The local planning authority's own strategic flood risk assessment identifies an issue with the site.

Surface water is the common one. The long term flood risk service shows surface water risk that the Flood Map for Planning does not, and a site can sit in Zone 1 with a clear band of high surface water risk running through it.

Critical drainage areas and local requirements

Critical drainage areas are mapped locally, usually through the council's strategic flood risk assessment, and they mark places where the drainage system is already under stress. Development in one attracts tighter requirements on discharge rates and on attenuation volumes.

Check the council's strategic flood risk assessment before you design the layout. That document also refines the flood zones locally, splits Zone 3 into 3a and 3b where the work has been done, and sets out what the authority expects from an assessment in different situations.

Local validation lists then say what has to be submitted. They differ between neighbouring authorities more than most applicants expect.

Scope of the assessment at this scale

For a one hectare residential site in Zone 1 with no other flood sources, the assessment is mostly a drainage exercise with a flood risk wrapper.

Exceedance routes are the section most often missing. When the drainage system is overwhelmed, water takes a path across the site, and the layout should direct it to open space rather than through front doors.

Run-off calculations and greenfield rates

The starting point is the greenfield runoff rate for the site, estimated using the standard hydrological methods, for a range of return periods. The design then limits the discharge from the developed site to that rate or, in a critical drainage area, to a lower figure the authority sets.

Attenuation makes up the difference: ponds, basins, permeable paving with storage, oversized pipes or tanks, sized to hold the volume that cannot discharge during the storm. The non-statutory technical standards for sustainable drainage set the framework, and the council or the lead local flood authority will state which climate change allowance and which return period to design for.

An illustrative example: a one hectare greenfield site might have a greenfield runoff rate in the order of a few litres per second, while the developed site would produce many times that without control. The attenuation volume follows from the difference, and the numbers should come from the hydrological calculation for your catchment rather than from an example in an article.

Infiltration is worth testing early. Where soils allow, soakaways reduce the attenuation volume and the connection costs, and where they do not, you need to know before the layout is fixed. Percolation testing costs little compared with redesigning around a basin you did not plan for, and the results date quickly if the ground is disturbed, so test where the feature will actually go.

Lead local flood authority consultation

For major development, the lead local flood authority is a statutory consultee on surface water drainage. Their comments carry real weight, and an application submitted without a drainage strategy will usually sit unresolved until one arrives.

Engage before submission. Ask which discharge rate they expect, which climate change allowance, what they want to see on maintenance, and whether they require an outline or detailed design at application stage. Where sewer connection is proposed, the water undertaker's position matters too, and water and sewerage consultation thresholds sets out when they are formally consulted.

Adoption and maintenance are the questions that get deferred and should not be. Somebody has to maintain an attenuation basin for the life of the development, and the answer affects both the section 106 and the service charge.

What to submit at validation

1. The flood risk assessment, with a dated extract from the flood map and the other flood source checks recorded.
2. A drainage strategy with the discharge rate, attenuation volume and outfall arrangement.
3. A levels drawing with existing and proposed levels and finished floor levels.
4. Infiltration test results where soakaways are proposed.
5. Confirmation of the outfall, including any third party consent needed.
6. A maintenance plan naming the responsible party.

Where the site area is close to a hectare, state the measured area on the form and in the assessment. An application that says 0.98 hectares, with a red line that measures 1.02, invites a validation query on the first day.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

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