Reports for one new dwelling in Flood Zone 2

One new house on a plot in Flood Zone 2 needs a site-specific flood risk assessment and, in almost all cases, a sequential test. It does not need an exception test, because a dwelling is classified as more vulnerable development and more vulnerable development is appropriate in Flood Zone 2. That distinction saves a great deal of work, and plenty of applicants pay for an exception test they never needed.

The one-hectare threshold that triggers a flood risk assessment in Flood Zone 1 is irrelevant here. In Zones 2 and 3 the assessment is required regardless of site area, so a 0.06 hectare infill plot is caught exactly as a five-hectare site would be.

What Flood Zone 2 triggers for a single plot

Flood Zone 2 is land assessed as having a medium probability of river or sea flooding, between the 1 in 100 and 1 in 1,000 annual chance from rivers, or between 1 in 200 and 1 in 1,000 from the sea, ignoring flood defences.

For a single dwelling that produces three requirements.

• A site-specific Flood Risk Assessment proportionate to the scale of the proposal.
• A sequential test, unless the council's own policy or strategic flood risk assessment removes the need for one on this type of site.
• Surface water drainage details showing how run-off will be managed, which most validation lists now require for any new dwelling.

What it does not trigger is an exception test, an Environment Agency consultation as a statutory consultee (which applies to development in Zone 3 and to specific categories), or a lead local flood authority consultation, since the LLFA is a statutory consultee on major development only.

That last point cuts both ways. No statutory consultee means no expert advocate for your drainage design either, so the case officer will read it themselves and will be conservative if it is unclear.

Sequential test and exception test

The sequential test asks whether there are reasonably available sites in areas at lower risk of flooding that could accommodate the proposed development. It is applied to the development, not to your ownership, and "reasonably available" is defined by the council, normally as sites within the same settlement or the same parish, available on the market and capable of taking the same number and type of homes.

For a single dwelling on an infill plot the test is usually short, and it usually succeeds. Set it out properly anyway.

1. Define the area of search and justify it, using the council's own approach if the strategic flood risk assessment sets one out.
2. Identify the sites considered, using the housing land supply schedule, brownfield register and current market listings at a stated date.
3. Explain why each is not reasonably available, unsuitable or would not deliver the same development.
4. Conclude that no reasonably available site exists at lower risk.

A Sequential Test written in that form takes a few pages. One written as a paragraph asserting that no alternative exists gets asked for again, and the request costs three weeks.

The Exception Test is a different matter and applies where more vulnerable development is proposed in Flood Zone 3a, or in other combinations set out in the national guidance. If someone quotes it to you for a house in Zone 2, ask which table they are reading.

Inside the flood risk assessment for a small infill site

A proportionate assessment for one house runs to perhaps fifteen to twenty pages and covers a predictable sequence.

The site description and the existing levels, ideally from a topographical survey rather than from published contours, because 300 millimetres matters here and mapping resolution does not deliver it.

The flood zone classification taken from the Flood Map for Planning, with the date of the extract, and a note of any local mapping in the council's strategic flood risk assessment that supersedes it.

All sources of flooding, rivers and everything else: surface water, groundwater, sewers, artificial sources such as canals and reservoirs. Surface water is the source that most often changes the answer on an infill plot, and it is mapped separately.

Climate change allowances, taken from the current Environment Agency peak river flow and peak rainfall allowances for the relevant river basin district and epoch. These are updated periodically, so cite the version and the date you accessed it.

Finished floor levels, usually set above the modelled flood level for the design event including climate change, with freeboard. Many councils apply a 300 millimetre freeboard as a starting point and will say so in their strategic flood risk assessment.

Safe access and egress, flood resilience measures where levels cannot be raised, and a residual risk section covering what happens in an event exceeding the design standard, including whether the occupants can leave or should shelter above the flood level.

Surface water, SuDS and drainage

Even without a statutory consultee, the drainage strategy is where a small application most often stalls.

Work down the discharge hierarchy in order and evidence each rejection: infiltration first, then a watercourse, then a surface water sewer, then a combined sewer as a last resort. Infiltration needs a soakage test carried out to a recognised method, and a test dug in August tells you less than one dug in February.

A short Drainage Strategy should show the impermeable area before and after, the discharge rate proposed, the attenuation volume and how it is provided, and who will maintain it. On a single plot the answer is often a permeable driveway and a modest attenuation crate, which is fine, provided the calculation is shown.

Two things to avoid. Do not propose infiltration where the flood risk assessment identifies high groundwater, because the two documents will contradict each other. And do not size attenuation for the 1 in 30 event when the local standard asks for the 1 in 100 plus climate change.

An illustrative bundle for a 0.06 ha infill plot

Illustrative example. A single detached house on a 0.06 hectare garden plot in Flood Zone 2, in a village with a settlement boundary, mains drainage available, no watercourse on site.

• Full planning application forms, certificates and fee for one dwelling.
• Location plan, site layout, floor plans, elevations and a topographical survey with levels.
• Flood risk assessment, site-specific, with the flood map extract dated.
• Sequential test, set out as above.
• Drainage strategy with a soakage test result and the discharge hierarchy evidenced.
• Biodiversity net gain metric and a biodiversity gain plan, unless a statutory exemption applies, because a new dwelling is not householder development.
• Ecology: a preliminary appraisal if the plot holds anything more than mown grass.
• A design and access statement, since the site is in a village and the design will be scrutinised.

No exception test. No transport assessment. No environmental impact assessment screening at this scale.

Checking the flood map before you buy

The Flood Map for Planning is free, takes two minutes, and is the single best due diligence step on a plot purchase. Check it alongside the long-term flood risk service, which shows surface water and reservoir risk that the planning map does not.

Then read the council's strategic flood risk assessment for the parish. It often holds modelled levels, climate change mapping and a locally defined Zone 3b functional floodplain that the national map does not show, and land in Zone 3b is effectively undevelopable for housing.

A plot priced low in Zone 2 with a surface water flow path across it is not a bargain. It is a plot with a drainage problem and a sequential test attached.

Enter your postcode on PlanScope to see which reports your site 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, and for specialist technical reports qualified professional input may be required. This is general guidance rather than legal advice.