Walkthrough: a new dwelling in Flood Zone 2

A single new dwelling in Flood Zone 2 needs a sequential test and a flood risk assessment, and it does not need an exception test. That is the shape of the whole application, and knowing it on day one saves a great deal of wasted effort. Dwellings are classified as more vulnerable development, and more vulnerable development is appropriate in Flood Zone 2 without the exception test. What follows is an illustrative walkthrough of how such an application is put together. It is not a record of a real case, and no levels or figures in it are taken from one.

The plot and the first check on the map

A 0.08 hectare infill plot in a village, between two existing houses, with a garden boundary onto a small ordinary watercourse. The vendor described it as having outline potential. The first two checks took ten minutes and cost nothing.

The Flood Map for Planning showed the rear third of the plot in Flood Zone 2, with the building footprint area in Zone 1 but close to the boundary between them. The long-term flood risk service showed a surface water flow path running diagonally across the plot from the lane, which is a separate matter from the river zone and one that catches people out.

Two observations from that. Flood zones on the published map ignore existing defences, and they are drawn at a resolution that is coarse for a plot this size, so the boundary line on screen is not a survey line. And where any part of a site falls in Zone 2, a flood risk assessment is required for the whole application, even if the house itself sits on the higher ground.

The sequential test for a single dwelling

The sequential test asks whether there are reasonably available sites at lower flood risk that could accommodate the development. It applies to a new dwelling. It does not apply to minor development such as householder extensions, and the exception test is not engaged for more vulnerable development in Zone 2.

The hard part is the search area, and the applicant does not get to choose it alone. The local planning authority defines the area of search, and in most districts that means the whole authority area unless there is a good reason to narrow it, such as a policy that supports infill within defined settlement boundaries.

Here the case for a narrower area was put in writing before submission. The scheme was a self-build on a plot the applicant already owned, within the village settlement boundary, of a size and form the local plan supported as infill. The council accepted a search area of the parish and the two adjoining parishes. The Sequential Test then listed the sites available in that area from the council's own housing land supply schedule and the brownfield register, with the reason each was not reasonably available: allocated for a different use, under option to another party, or already permitted.

Put the search area agreement in an email and keep it. Officers change, and a sequential test assessed against a wider area than the one agreed is a sequential test that fails.

Getting modelled levels from the Environment Agency

The published flood map shows extent, not level. To set a floor level you need modelled water levels for the relevant return periods, and those come from the Environment Agency's flood risk data products, requested through its data request service.

The detailed product gives modelled levels and flows at nodes along the watercourse for a range of annual exceedance probabilities. Allow several weeks. Where no detailed model exists for a small ordinary watercourse, which is common, the alternative is a hydraulic assessment by a drainage engineer or, on very small sites, an agreed approach using the nearest modelled node and local survey levels.

Two things to request at the same time. Historic flood outlines for the reach, which often tell a different story from the modelled map. And confirmation of who is the risk management authority for that watercourse, since an ordinary watercourse falls to the lead local flood authority rather than the Environment Agency, and land drainage consent may be needed for anything within or near the channel.

Climate change allowances then get applied to the modelled flows. Check the current allowances on gov.uk at the time of the assessment, because they are revised and they vary by river basin district and by epoch.

Setting floor levels and reconciling them with access

With a design flood level established, the finished floor level is set above it with a freeboard allowance. Three hundred millimetres of freeboard is the figure most commonly used in practice, and local authorities and the Environment Agency will state what they expect.

Then the design problem starts. A floor level raised above ground produces steps at the front door, which collides with the accessibility requirements in the Building Regulations and with the street scene in a village of low-set cottages. The usual answers are a graded approach ramp within the plot, a raised platform with the level change taken in the driveway, or a change in house type.

Safe access and egress is assessed separately from the building itself. The question is whether people can reach the house and leave it during a flood without passing through hazardous depths and velocities. On this plot the lane to the north stayed dry in the modelled event, which resolved it. Where access floods, the argument gets much harder, and an evacuation plan is a weak substitute for dry access on a single dwelling.

Flood resilient construction covers the rest: solid floors, closed-cell insulation below the design level, services routed high, and sacrificial finishes at low level. A Flood Risk Assessment should describe these in specification terms rather than as aspirations, because the condition will be drafted against what the assessment says.

Surface water and the discharge rate

The surface water flow path across the plot mattered as much as the river zone.

The drainage hierarchy starts with infiltration. Soakaway testing to the recognised method showed slow percolation in the clay subsoil, so infiltration was discounted with evidence rather than by assertion, which matters because councils reject unsupported statements that infiltration is unsuitable.

The next option was discharge to the ordinary watercourse at the greenfield rate, with attenuation storage sized for the 1 in 100 year event plus a climate change allowance, and with an exceedance route across the garden away from the house. The Drainage Strategy set out the calculation, the storage volume, the control device and the maintenance responsibility.

One practical detail. Discharging into an ordinary watercourse needs land drainage consent from the lead local flood authority, which is a separate consent from planning permission and has its own timescale. Apply for it in parallel.

The consultee response

For minor development in Flood Zone 2 the Environment Agency generally directs authorities to its published standing advice rather than commenting individually, so the application was assessed by the case officer against that advice with input from the council's drainage engineer.

The drainage engineer raised three points. The soakaway test results needed the test pit locations shown on a plan. The attenuation calculation used a runoff coefficient that was not stated. And the exceedance route needed to show where water goes once storage is full, including the effect on the neighbouring garden.

All three were answerable in a two-page addendum. None of them would have arisen if the original strategy had shown its workings.

The conditions and what they cost to discharge

Permission was granted with conditions covering finished floor levels tied to a stated datum, flood resilient construction in accordance with the submitted assessment, a detailed surface water drainage scheme with a maintenance plan, and a prohibition on raising ground levels within the Zone 2 area without further approval.

Two of those were pre-commencement. Discharging them took one round of correspondence and a fee per request, and the sequencing is the same as in the note on discharging pre-commencement conditions. The levels condition was the one that mattered on site, because the groundworker set out from the drawing rather than the datum and had to be stopped.

Check the flood position before you buy the plot, not after. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.

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