What changes at one hectare

A one hectare site flood risk assessment is expected even in Flood Zone 1. That is the rule most developers meet first at this scale, and it surprises people who have checked the Flood Map for Planning, found the site entirely outside the river and sea zones, and assumed the flood work was done. National guidance sets the expectation by site area as well as by flood zone: proposals of one hectare or more in Flood Zone 1 need a site-specific assessment, because a site that size generates surface water risk whatever the river position.

One hectare also does other work. It is the area route into major development for non-residential schemes, and it brings the lead local flood authority in as a statutory consultee.

The flood risk assessment trigger at one hectare

The trigger is area, not use and not zone. A one-hectare employment site in Flood Zone 1 on free-draining chalk needs an assessment. So does a one-hectare residential site in the same position.

What the assessment has to cover is wider than the zone map suggests. All sources of flooding are in scope: rivers and the sea, surface water runoff from the site and from upstream, groundwater, sewer flooding, and artificial sources such as reservoirs, canals and raised watercourses. The Flood Map for Planning gives you the first of those and nothing else.

Scale the document to the risk. A Flood Risk Assessment for a hectare of Flood Zone 1 land with no surface water mapping on it is a short document covering the sources, the drainage strategy and residual risk. The same document for a site with a surface water flow path running through it is a different proposition and needs modelling.

Why it applies in Flood Zone 1

Flood zones are produced from modelling of fluvial and tidal flooding without defences. They say nothing about whether rainwater runs off a hectare of new roof and tarmac into a neighbour's garden.

That is the gap the area threshold fills. On a greenfield hectare the pre-development runoff is low, and the post-development runoff from buildings, roads and parking is high unless it is controlled. The assessment and the drainage strategy together demonstrate that the site is not exporting its new runoff downstream.

There is a second reason, less often stated. A site of a hectare is big enough to contain its own low ground, and the sequential approach within the site matters: put the vulnerable uses on the higher land and the parking, open space and attenuation on the lower land. That is a layout decision, and it is least costly to make it early.

Surface water drainage expectations

Expect to demonstrate discharge to the greenfield rate for the relevant return periods where the site is greenfield, or a betterment against the existing rate where it is previously developed. Expect attenuation sized for events up to the one in one hundred year return period with a climate change allowance applied to rainfall intensity. Expect exceedance routing for events beyond the design standard, and a maintenance plan with a named future maintainer.

Expect also to be asked about the drainage hierarchy: infiltration first, then discharge to a watercourse, then to a surface water sewer, and only then to a combined sewer. If you are proposing to go down the hierarchy, evidence why the options above were rejected. Infiltration testing to a recognised method is the usual evidence against the first option.

The Drainage Strategy should state the rainfall dataset used, the discharge destination with evidence of the right to discharge, the control device and its outlet size, and the storage volume with the critical duration.

An illustrative area and runoff comparison

For a hypothetical one hectare greenfield employment site, the shape of the result tends to look like this. These figures are illustrative only.

The attenuation volume follows from the gap between those first two rows and the third. Which is why reducing impermeable area, through permeable paving or green roofs, is usually the quickest way to shrink a tank.

Lead local flood authority consultation

For major development the lead local flood authority is a statutory consultee on surface water drainage. On a one hectare non-residential scheme the major threshold is met on area alone, so the consultation happens as of right.

Practical consequences. The authority's published local standards matter more than the national position, because its comments will be written against them. Discharge rates, climate change allowances, treatment requirements and adoption expectations all vary between authorities. Read the local guidance before you model, and where it is silent, write and ask.

Comments usually arrive mid-determination, which is late if they require a layout change. Pre-application engagement with the drainage team is one of the few pre-application discussions that reliably pays for itself.

Ecology and habitat baseline at this scale

Biodiversity net gain applies far below a hectare, so nothing switches on here. What changes is that the baseline becomes a real survey exercise. A hectare typically contains more than one habitat parcel, hedgerows or treelines on the boundaries, sometimes a ditch or pond, and the statutory metric is sensitive to the condition assessment of each.

The metric must be completed by a competent person, the biodiversity gain plan has to be approved by the authority before development begins, and the thirty-year management obligation has to be secured. Survey seasons govern the programme: botanical survey through the growing season, and protected species surveys in their own windows. Where protected species are a possibility, a licensed ecologist is legally required for the survey work and for any licence application, and an AI-assisted draft is only a starting point for that professional.

Our note on affordable housing tenure split on major sites covers the other obligation that bites once a residential scheme reaches major scale.

Construction environmental management

At a hectare, construction effects start to be conditioned rather than assumed. Expect requests for a construction environmental management plan covering surface water during construction, silt control, wheel washing, dust suppression, working hours, protected species precautions and tree protection.

Where the site drains to a watercourse, pollution control during earthworks is the issue the environmental regulator cares about most, and the discharge of silty water during construction has its own consent requirements. Deal with it in the plan rather than leaving it to the contractor.

Documents driven by area rather than units

Worth separating the triggers that follow area from those that follow quantum, because the two sets overlap untidily.

Area drives the flood risk assessment at one hectare, major status for non-residential development at one hectare, the fee band on outline applications, sustainable drainage expectations through major status, and the scale of the ecology baseline. Quantum drives affordable housing thresholds, the transport assessment trigger, statement of community involvement requirements on larger residential schemes, and the fire statement trigger for relevant buildings.

Environmental impact assessment sits apart from both. The screening thresholds for urban development projects are well above a hectare, so most schemes at this scale are outside Schedule 2 entirely. Sensitive area status changes that analysis, so screen the question and record the answer rather than assuming it. Our note on obligations and the levy at different scheme sizes sets out where the financial triggers fall.

This is general guidance rather than legal advice. Check the position with your local planning authority and the lead local flood authority before relying on it.

Enter your postcode on PlanScope to see which reports a site of a hectare is likely to need. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.