Flood Zone 3a and 3b on the same site: what changes

Flood Zone 3a and Flood Zone 3b are both high probability flood risk, and the development you can put in them is completely different. Zone 3a is land with a 1 in 100 or greater annual probability of river flooding, or 1 in 200 or greater from the sea. Zone 3b is the functional floodplain: land where water has to flow or be stored in times of flood, identified as land that would flood with an annual probability of 1 in 20 or greater. Housing can go in 3a if it passes the exception test. Housing cannot go in 3b at all.

The practical difficulty is that the national Flood Map for Planning does not show the 3a and 3b split. It shows Flood Zone 3 as a single extent. Where that line falls across your site is a question for the authority's strategic flood risk assessment or for site-specific modelling, and on many sites nobody has answered it.

How the two zones are defined

Flood zones come from the tables in planning practice guidance on flood risk and coastal change, and they describe the probability of flooding ignoring the presence of defences. That last point is the one most commonly misunderstood: a site behind a defence built to a 1 in 200 standard is still in Flood Zone 3 because the zone describes the undefended probability.

The 1 in 20 figure is indicative. The guidance also treats land designed to flood in an extreme event, such as a flood storage area, as functional floodplain, and it allows the authority and the Environment Agency to agree a different probability where local circumstances justify it. So the boundary is a planning judgement informed by hydraulics rather than a purely technical output.

The functional floodplain and the 1 in 20 event

What makes land functional floodplain is its role in conveyance or storage, not simply that it gets wet.

A field alongside a river that fills every few years and holds water that would otherwise go downstream is functional floodplain. A hollow in the middle of an urban site that ponds because the drainage is inadequate is not, even if it floods more often, because it plays no part in the river's flood mechanism. Conversely, a narrow strip of land between two buildings that carries flow in a flood can be functional floodplain despite being small.

Some strategic flood risk assessments take a simplifying approach and treat the whole of Flood Zone 3 as functional floodplain where no modelling exists. That is a defensible precaution on the authority's part and it is frequently wrong on individual sites, particularly on previously developed land where buildings have long occupied part of the zone. Developed land that is already built on is generally not functional floodplain, because water does not flow or store across it, and that argument has won a lot of brownfield applications.

Check what your authority's assessment actually says before accepting the position. Some define 3b by modelled extent, some by a precautionary rule, and some do not define it at all.

What each zone permits by vulnerability

Development is classified by vulnerability in the guidance, and the compatibility table then tells you which classifications are appropriate in which zone.

In Flood Zone 3a, less vulnerable development is appropriate, more vulnerable development requires the exception test, highly vulnerable development is not appropriate, and essential infrastructure requires the exception test. Residential development is more vulnerable, so almost every housing scheme in 3a needs the Exception Test as well as the sequential test.

In Flood Zone 3b, only water-compatible development and essential infrastructure are appropriate, and essential infrastructure requires the exception test. Everything else, including less vulnerable development, is inappropriate.

That last sentence catches designers out regularly. Car parking falls into the less vulnerable classification, so a car park is not appropriate development in functional floodplain even though it has no sleeping accommodation and floods harmlessly. Water-compatible development covers amenity open space, outdoor sport and recreation, nature conservation and flood control infrastructure. Put the open space in 3b and the parking somewhere else.

Basement dwellings are highly vulnerable, which rules them out of both 3a and 3b. Basement car parking and plant rooms are treated differently, but a bedroom below ground in Flood Zone 3 is not going to be acceptable.

Why the mapped boundary is often wrong

The Flood Map for Planning is a national product built from broad-scale modelling. On any individual site it can be out by a surprising margin, in either direction.

Four common reasons:

• The underlying model used coarse terrain data and did not pick up a flood wall, a raised road or a change in level that actually controls the flood extent.
• The site has been raised, or a building has been there for 80 years, and the mapping has not caught up.
• A culvert or structure through the site is not represented, so the model routes water across land it never reaches.
• The mapping shows the undefended extent, which is correct for zone definition, and the actual flood behaviour on the ground is different.

None of those is a reason to ignore the map. All of them are reasons to model, if the amount of developable land at stake justifies the cost.

Start with a detailed flood risk data enquiry to the Environment Agency, which gives the model that produced the mapping, the modelled levels for each return period and the model's limitations. Sometimes that alone shows the 1 in 20 extent and resolves the question without new modelling.

Modelling to refine the split

Where new modelling is needed, three things determine whether the Environment Agency and the authority will accept it.

The first is the terrain. A ground model built from a current topographic survey of the site, tied to Ordnance Datum and merged with national terrain data for the wider floodplain, is the baseline. Spot levels taken off an old drawing will not do.

The second is the hydrology. Flows for each return period should be derived using an accepted method, consistent with the Environment Agency's existing model where one exists, with any difference explained. An applicant's model producing lower flows than the Agency's own, with no stated reason, is going nowhere.

The third is the hydraulics and its presentation. Run the 1 in 20 event to define the functional floodplain, plus the 1 in 100 and the 1 in 100 with the relevant climate change allowance, and present depth and velocity across the site rather than an extent line alone. Include a sensitivity run on roughness and on the downstream boundary.

Agree the modelling scope with the Agency before you start. A pre-application discussion about which events, which allowances and which model to build from costs a fraction of a model that has to be redone. The modelling then feeds the Flood Risk Assessment rather than sitting beside it, and the assessment's conclusions have to be traceable to the model outputs.

Layout consequences on a split site

A site divided between 3a and 3b has to apply the sequential approach internally: the most vulnerable uses go to the lowest risk part of the site, and the highest risk land takes the least vulnerable use.

In practice that produces a recognisable layout. Built development concentrates on the 3a land and on any Zone 2 or Zone 1 land. The 3b land becomes public open space, attenuation that works with the flood mechanism, playing field or habitat, and it carries the biodiversity and amenity contribution rather than the development quantum. Access roads crossing 3b need justification as essential infrastructure, with no loss of conveyance.

Three design consequences follow that affect viability directly.

1. Floodplain compensation. Building on 3a land within the 1 in 100 plus climate change extent requires level for level, volume for volume compensatory storage, hydraulically connected to the floodplain and ideally within the red line. Finding that land is often the binding constraint, and it cannot come out of the 3b area, which is already doing a job.
2. Finished floor levels. Set from the modelled 1 in 100 plus climate change level with freeboard, which on a site with a steep flood gradient may mean different levels in different parts of the scheme.
3. Safe access and egress. A route out of the site to land above the design flood level, dry if possible, and the Drainage Strategy has to show that the surface water system and the exceedance flow routes do not undermine it.

Where the application cannot avoid 3b for built form, the honest answer is usually that the layout needs to change rather than that the policy needs to bend.

The evidence the Environment Agency expects

Submit a package rather than a report. The Agency's flood risk consultees review a handful of applications a week and respond fastest to submissions that let them check the arithmetic.

Include the flood risk assessment itself, the modelling report with model schematics and the flows used, the topographic survey, a plan showing the 1 in 20, 1 in 100 and 1 in 100 plus climate change extents over the proposed layout, a compensatory storage calculation with before and after cross-sections, a levels plan with finished floor levels as absolute values, and the access and egress route with levels along it. Where the sequential test has been applied, the Sequential Test should state the area of search agreed with the authority.

Then say plainly which parts of the site you contend are functional floodplain and which are not, and why. A submission that leaves the 3a and 3b boundary ambiguous invites the consultee to adopt the most precautionary reading, and that reading will cost you developable area.

For specialist technical reports, qualified professional input may be required, and hydraulic modelling is specialist work. This is general guidance rather than legal advice, so check the position with your local planning authority or a qualified professional before relying on it.

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