Flood risk in Wales: TAN 15 and the development advice maps

This article covers Wales only. Flood risk in Wales is not assessed with the sequential and exception tests used in England. The Welsh framework works through Technical Advice Note 15, a set of mapped zones, a series of justification tests for development in the higher-risk zones, and a Flood Consequences Assessment rather than a Flood Risk Assessment. The difference is more than terminology. An English assessment asks whether development can be steered away from risk and then made safe. The Welsh assessment asks what the consequences of flooding would actually be for the people in the building, measured in depth, velocity, rate of rise and the ability to get out.

One practical warning before anything else. Wales has been moving between the long-standing version of Technical Advice Note 15 with its Development Advice Maps and a revised version with a new flood map for planning, and the implementation date has been deferred more than once. Check on gov.wales which version is currently in force and which map the authority is using, and confirm it with the case officer, before you commission anything.

The Welsh flood zone framework

The zones people still work to under the long-standing version of the advice note are lettered rather than numbered, which is the first thing to adjust to.

Zone A is land considered at little or no risk of fluvial or tidal flooding. Zone B is land shown as having been flooded in the past by evidence of alluvial or tidal deposits. Zone C is land within the extreme flood outline, subdivided into C1, areas served by significant infrastructure including flood defences, and C2, areas without such defences.

The zone matters because the policy response is keyed to it. Development in zone A raises no flood question. Development in zone B raises a drainage and surface water question. Development in zone C engages the justification tests and requires a Flood Consequences Assessment.

The zone is a starting point, not a conclusion. The map is strategic, drawn at a scale that cannot tell you whether a particular plot sits above or below a level, and a site-specific assessment routinely refines it.

The justification tests for development

Under the established framework, new development in zone C that falls into the more vulnerable categories has to be justified, and the tests are cumulative rather than alternatives.

The site has to be located in an area already developed, so the framework is not a route to new isolated development in the floodplain. The development has to be necessary to assist or be part of a strategy of regeneration, or necessary to contribute to employment objectives supported by the authority. And the assessment has to demonstrate that the consequences of flooding can be managed to an acceptable level over the lifetime of the development.

For a self-builder, the second test is the hard one. A single dwelling on an infill plot inside a village can sometimes be argued as contributing to a local strategy, and often cannot. Get a view on it at pre-application stage, because it determines whether the rest of the work is worth commissioning.

Highly vulnerable development, which includes housing, in undefended zone C2 is where the framework is at its most restrictive.

Flood consequences assessments and what they contain

A Flood Consequences Assessment is a different document from an English flood risk assessment, and submitting the latter with a Welsh application invites a request for more information.

The content that has to be there:

• The sources of flooding relevant to the site, fluvial, tidal, surface water, groundwater and sewer.
• Probability, expressed as annual likelihood, for the relevant return periods.
• Predicted flood depth on the site, and separately at the proposed building.
• Velocity, and the hazard rating that follows from depth and velocity together.
• Rate of rise, since a fast-rising flood removes the option of evacuation.
• Duration, because a long-duration flood raises different issues from a flash event.
• Access and egress, including whether a dry route exists and whether it remains dry through the event.
• Emergency response and evacuation, including who would be responsible.
• Residual risk after mitigation, including failure or overtopping of any defence.

Finished floor levels, flood resilient construction and safe refuge above flood level are mitigation, and they answer some of those points and not others. A first-floor refuge does nothing for a rate of rise that traps a car on the access road.

Tolerable conditions for different development types

The advice note sets out tolerable conditions by category of development, which is the mechanism that turns a set of modelled numbers into an acceptability judgement. Emergency services, hospitals and similar uses have the tightest thresholds. Housing sits in the more vulnerable group. Less vulnerable uses such as general industry and storage have wider tolerances, and water-compatible uses wider still.

Read the categories and the thresholds in the current version of the advice note and design to them, rather than designing the building and then measuring it against them. The depth and hazard limits usually dictate the finished floor level, and the finished floor level dictates the ground floor arrangement, the entrance, the access ramp and the relationship to the street.

For a self-build on a sloping plot, that sequence can also change where on the plot the house sits.

Climate change allowances used in Wales

Wales applies its own climate change allowances for peak river flow, rainfall intensity and sea level rise, published by the Welsh Government, and they are not the same as the Environment Agency allowances used in England. Using the English figures in a Welsh assessment is a common and avoidable error.

The allowances are revised periodically, so check the current published figures at the point the modelling is done rather than relying on a number in an earlier report. Where a report is more than a couple of years old, expect the authority or Natural Resources Wales to ask whether the allowances are current.

The lifetime of the development matters too, since the allowance applied depends on the period being assessed. A residential scheme is normally assessed over a long horizon.

Natural Resources Wales as consultee

Natural Resources Wales is the statutory consultee for development in flood risk areas in Wales, and it publishes the flood mapping. Its role corresponds to the Environment Agency's in England, and its advice carries substantial weight with authorities.

Two practical points. Engage before submission where the site is in zone C, because an objection from Natural Resources Wales is much harder to answer during determination than a comment resolved beforehand. And where the organisation holds modelling for the watercourse in question, ask for it. Building an assessment on the same model the consultee uses removes an entire category of argument.

Where Welsh practice diverges from England

The last row is worth repeating because it catches English developers. In Wales, surface water drainage needs its own approval alongside planning permission, so the Drainage Strategy and the flood case have to be consistent with a design that will also pass that separate consent.

The contrast with Scotland is sharper still, where the avoidance principle in the national framework does the work that the justification tests do in Wales. The comparison in flood risk under NPF4 in Scotland is a useful reminder that these three systems cannot be treated as variants of one another, and the English Flood Risk Assessment and Sequential Test pages describe a structure that does not transfer.

PlanScope's drafting is grounded in English national policy and local plans, so a Welsh flood case needs the reviewer to check Technical Advice Note 15, Planning Policy Wales and the current Welsh climate change allowances, and the Flood Consequences Assessment itself needs a suitably qualified author. Reports should be reviewed before submission, and PlanScope does not guarantee planning approval. For specialist technical reports, qualified professional input may be required. Browse the report types by category to scope the supporting documents your application needs. This is general guidance, not legal advice; check the position with the local planning authority or a qualified professional before relying on it.