Flood risk assessment structure for a home in Flood Zone 2

A flood risk assessment for a single dwelling in Flood Zone 2 follows the same section order as one for a 500-home scheme, at a fraction of the length. Eight sections, ten to fifteen pages, and the whole document turns on three numbers: the modelled flood level, the climate change allowance and the finished floor level you set as a result.

Zone 2 is the medium probability zone, between the 1 in 100 and 1 in 1,000 annual probability of river flooding, or between 1 in 200 and 1 in 1,000 from the sea. A dwelling is classified as more vulnerable development, and more vulnerable development is appropriate in Zone 2 without the exception test. That single fact makes this a manageable document.

What the assessment has to prove

Four things, and the structure exists to deliver them in order. That the site's flood risk has been properly characterised from all sources. That the development will be safe for its lifetime, allowing for climate change. That it will not increase flood risk elsewhere. And that people can get in and out, or stay safely, when a flood happens.

An officer reading a flood risk assessment for a single plot is looking for those four conclusions and the evidence behind them. Anything else is padding.

Site description and the flood zone position

Open with the address, the National Grid reference, the site area and the red line boundary, then describe what is there now and what is proposed. Give existing ground levels to Ordnance Datum, surveyed rather than estimated, because everything downstream in the document depends on them.

State the flood zone from the Flood Map for Planning and say when you checked it. Name the watercourse and its distance, and say whether it is a main river or an ordinary watercourse, because that determines whether the Environment Agency or the lead local flood authority holds consenting powers over works near it.

Then request the modelled data. The Environment Agency supplies detailed flood risk information for a site on request, including modelled levels for a range of return periods where a model exists. Ask early. The response takes time and the whole assessment waits on it. Where no model covers the reach, say so plainly and explain what you used instead.

Sources of flooding, taken one at a time

Deal with each source under its own sub-heading and reach a conclusion on each rather than writing a general paragraph about flooding.

Fluvial and tidal come from the modelled data. Surface water comes from the national long-term flood risk service, read together with site topography, because a site that sits at the low point of a road will collect water regardless of what the mapping shows. Groundwater usually has no published mapping at all, so it is evidenced from geology, from any borehole records and from local history. Sewer flooding comes from the water undertaker's records, which you can ask for. Artificial sources mean reservoirs, canals and ponds upstream.

Where a source is not relevant, say it is not relevant and why, in one sentence. A conclusion of no risk with no reasoning is the commonest reason these sections get queried.

The climate change allowance

Apply the allowances published by the Environment Agency for the relevant river basin district and allowance category. They are set out as percentage uplifts to peak river flow and to peak rainfall intensity, they vary by catchment and by time period, and they are revised, so take them from the current published tables rather than from a previous report.

Two choices have to be stated and justified. The development lifetime, which for residential development is normally taken as 100 years from the date of construction. And the allowance category, which reflects the vulnerability of the development and the consequences of getting it wrong.

Show the arithmetic. Present the 1 in 100 annual probability level, the uplift applied, and the resulting design flood level, in a short table with levels to Ordnance Datum. That table is what the flood risk officer reads first.

The sequential test position

A new dwelling is not minor development, so the sequential test applies even to a single plot. Householder extensions and small non-residential extensions are exempt from it, which is why this document is more work than an extension in the same location.

The test asks whether there are reasonably available sites at lower flood risk that could accommodate the development. The local planning authority sets the area of search, and for a single dwelling that is often narrower than the whole district, but it is the authority's call rather than yours. Set out the search area you used, the sources you searched, the sites you found and why each was discounted, and ask the officer to confirm the area of search at pre-application if you can. The mechanics are covered in more depth under the sequential test.

The exception test does not apply to a more vulnerable dwelling in Zone 2. One exception matters: a basement bedroom makes the development highly vulnerable, and highly vulnerable development in Zone 2 does require the exception test. If the design has habitable basement accommodation, address this directly.

Finished floor levels and freeboard

Set the finished floor level above the design flood level, with a freeboard allowance on top. A freeboard of 300mm is the usual starting point, and authorities in sensitive locations ask for more.

State the resulting level to Ordnance Datum, not as a height above existing ground, and show it on the section drawings. Then check the consequences: a raised floor level changes the ridge height, the access gradients and the relationship with the neighbouring properties, and those changes have to work in the design as submitted. The detail of how to justify the figure is set out in the note on finished floor levels and freeboard.

Where raising the floor is not possible, say why and move to resistance and resilience measures instead.

Flood resilient construction

Resistance keeps water out, resilience limits the damage when it gets in, and a Zone 2 dwelling usually needs some of both. Specify them rather than listing them generically.

• Solid floors with the damp proof membrane detailed to link with the wall construction.
• Closed cell insulation, or insulation positioned above the design flood level.
• Electrical sockets and consumer unit raised above the design flood level, with ring circuits dropped from above.
• Water resistant internal finishes to the ground floor, with removable skirtings.
• Non-return valves on foul and surface water drainage connections.
• Flood resistant doors and any airbrick covers, referenced to the relevant British Standard for flood resistant and resilient construction.

Show the measures on a drawing. A condition requiring implementation is easier to discharge when the detail is already in the application.

Residual risk, access and evacuation

Residual risk is what remains after the design is built: a flood larger than the design event, a defence failure upstream, a blocked culvert. Describe the depth and the likely warning time in that scenario.

For access, state whether dry access exists to the highway in the design event. Where it does not, describe the depth and velocity on the access route and assess it against the safe access criteria the authority applies. Safe refuge above the design flood level within the dwelling, registration with the flood warning service, and a short household flood plan naming who does what and where the vehicle goes, together form a proportionate answer for a single dwelling.

Close the document with a one-page summary of conclusions against the four tests. Cross-refer it from the planning statement so the two documents agree.

This is general guidance, not legal advice. Flood risk requirements are set by the Environment Agency and your lead local flood authority, so confirm their expectations before finalising the assessment.

Draft the structure and the standard sections in minutes at PlanScope, then have the modelling and the levels checked by a suitably qualified engineer.

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