TAN15 Flood Consequence Assessment Guidance and Advice

TAN15 Wales — Technical Advice Note 15: Development, Flooding and Coastal Erosion — is the national planning policy that decides whether development can go ahead on land at flood or coastal erosion risk anywhere in Wales. The version in force today took effect on 31 March 2025, and it changed the rules substantially: a new Flood Map for Planning replaced the old Development Advice Map, Defended Zones appeared for the first time, and a Flood Consequence Assessment (FCA) is now required across a far wider range of applications.

This page explains how the current TAN15 works: what the policy asks of a development site, how the Flood Map for Planning and Defended Zones fit in, which vulnerability category your proposal falls into, and the acceptability criteria your evidence has to satisfy. It is the Welsh counterpart to our NPPF flood risk guidance for England — the two systems share a philosophy but differ in almost every mechanism, so evidence prepared for one rarely satisfies the other.

Applications made on or after 31 March 2025 are determined under the new TAN15; anything submitted earlier still falls under the 2004 rules.

What is the TAN15 guidance in Wales?

TAN15 is Welsh Government planning guidance that sets out how flood risk and coastal erosion must be addressed in development decisions across Wales. It defines the flood zones, classifies development by vulnerability, determines when a Flood Consequence Assessment is required, and sets the acceptability criteria a proposal must meet to gain planning permission on land at risk.

It sits beneath Planning Policy Wales and operates within the national framework set by Future Wales: The National Plan 2040. Although it is guidance rather than primary legislation, TAN15 is a material consideration in every planning decision where flood risk arises — Local Planning Authorities and Natural Resources Wales (NRW) apply it directly, and an application that ignores it invites objection, delay or refusal. The full policy text is published on the Welsh Government website.

The policy applies to residential, commercial and industrial schemes, infrastructure, changes of use, and land allocations in Local Development Plans. If your site is in England, different rules apply — start with our Flood Risk Assessment for planning service instead.

What changed on 31 March 2025?

The 2025 TAN15 is the biggest reform of Welsh flood planning policy in over two decades, replacing both the 2004 TAN15 and its 1998 predecessor TAN14. We unpacked the full detail in our TAN15 2025 policy update and examined the story behind the reform in our analysis of the new TAN15 as a policy U-turn — but the essentials sit in the comparison below.

The Welsh Government was explicit about the intent behind the reform. In the written statement announcing the new policy, the Cabinet Secretary for Economy, Energy and Planning said:

Two further changes matter for anyone bringing a scheme forward. The 2025 Notification Direction requires councils to notify Welsh Ministers of any highly vulnerable development proposed on previously undeveloped land in Flood Zone 3, opening a 21-day window in which the application can be called in. And design for resilience is now explicit policy: raised floor levels, flood-compatible materials, sustainable drainage and emergency access have to be designed in, not bolted on — including designing for exceedance, which asks where the water goes when the drainage system is overwhelmed.

TAN15 sets the policy — which assessment does your project need?

TAN15 is not a report you commission. It is the planning policy that determines what evidence a Welsh application must submit — and for most schemes affected by flood risk, the evidence it calls for is a Flood Consequence Assessment. That said, people arrive at this page from every direction, and not every situation is governed by TAN15 at all. Here is the right starting point for each:

• Applying for planning permission in Wales? TAN15 governs your application, and where flood risk affects the site the report it requires is a Flood Consequence Assessment.
• Developing in England? TAN15 does not apply — the NPPF does. You need a Flood Risk Assessment for planning, and possibly Sequential and Exception Test reports.
• Buying a property flagged for flood risk? That sits outside TAN15 altogether — our homebuyer flood risk survey assesses the actual property rather than a planning proposal.
• Insurer raising flood concerns? Also outside the planning system — an independent Flood Risk Assessment for insurance addresses underwriting questions instead.
• Chasing BREEAM credits? The Pol 03 credit needs a BREEAM-specific FRA structured around pre-development classification.
• Mapped flood extents look wrong for your site? Site-specific flood modelling can refine or challenge the national mapping that TAN15 decisions rely on.
• Drainage rather than flooding? Wales has statutory SuDS approval — a surface water drainage strategy covers that side, and groundwater questions sit with our groundwater flood risk assessments.

Not sure which regime or report applies? Tell us about the site and we will point you the right way — it costs nothing to ask.

When does TAN15 require a Flood Consequence Assessment?

An FCA is required wherever flood risk could influence the acceptability of a proposal — and the 2025 policy widened that net considerably. Expect to need one where:

• The site falls within Flood Zone 2 or 3 on the NRW Flood Map for Planning, from rivers, the sea or surface water.
• The site sits in a TAN15 Defended Zone. An FCA is required regardless of the defences — the assessment must address residual risk, including overtopping and breach scenarios.
• The site lies in a lower-risk zone adjacent to a small watercourse — one of the 2025 additions that catches sites the old Development Advice Map ignored.
• The proposed use is vulnerable — housing, schools, hospitals and emergency services infrastructure all sit in the highly vulnerable category.
• The development could affect flood risk elsewhere, by displacing floodwater, changing flow routes or increasing runoff.

The technical bar rose as well as the scope. A compliant FCA must now address breach, blockage and exceedance scenarios and apply climate change projections over the development's lifetime — a step up from the desk studies that once passed. Templated reports are exactly what NRW and councils have learned to push back on; if you want the fuller picture of what assessors now expect, our TAN 15 explained guide walks through the framework in detail.

Flood zones, Defended Zones and the Flood Map for Planning

The Flood Map for Planning (FMfP) is the evidence base for every TAN15 decision. Managed by Natural Resources Wales, it launched in September 2021 and became the sole planning flood map on 31 March 2025, when it replaced the Development Advice Map. Updated twice a year, in May and November, it shows risk from rivers, the sea, surface water and small watercourses, together with areas susceptible to coastal erosion. Land falls into Flood Zone 1 (low probability), Zone 2 (medium), Zone 3 (high) — and, new in 2025, TAN15 Defended Zones: areas protected by flood defences built to at least a 1-in-100-year standard for river flooding or 1-in-200-year for tidal flooding. Development in a Defended Zone is not prohibited, but it must confront the residual risk of defences overtopping or failing.

One recurring trip hazard: NRW publishes two flood maps, and only one of them counts for planning. The public-facing Flood Risk Assessment Wales map is a general-awareness tool with no planning status — a point worth checking before you conclude your site is safe, or doomed.

Note the terminology trap for anyone working both sides of the border: England has its own, entirely separate Flood Map for Planning run by the Environment Agency. Same name, different datasets, different policy regimes — and different climate change allowances, which in Wales are embedded in the FMfP extents rather than applied separately.

Development vulnerability under TAN15

TAN15 classifies development into three categories, and the classification does a lot of work: crossed with the site's flood zone, it determines whether a proposal is acceptable in principle before any detailed assessment begins.

• Highly vulnerable: all residential buildings, hospitals, schools and emergency services infrastructure. The most restricted category — new highly vulnerable development on previously undeveloped land in Zone 3 should not be permitted under policy.
• Less vulnerable: most commercial and industrial uses, acceptable across a wider range of locations subject to the acceptability criteria.
• Water-compatible: uses that belong near water — marinas, some open space and water-management infrastructure — acceptable in most zones.

Establishing your classification early is one of the cheapest pieces of planning risk management available. It shapes the whole strategy: which sites are worth pursuing, what evidence the application needs, and whether the scheme collides with the Notification Direction. The 2025 framework is notably more generous to redevelopment of previously developed land than to greenfield sites — a distinction that decides schemes before a single calculation is run.

The acceptability criteria: what happened to the justification test?

Under the 2004 policy, development in higher-risk areas had to pass a three-part justification test — necessity of location, wider regeneration or sustainability benefit, and manageable consequences. The 2025 TAN15 retired that blanket mechanism. In its place sits a plan-led, risk-based framework: flood risk is meant to be resolved strategically through Local Development Plans and their Strategic Flood Consequence Assessments, while individual applications are judged against acceptability criteria.

Those criteria are demanding, and they are where an FCA earns its keep.

The four acceptability criteria

A compliant assessment must demonstrate all four:

1. Occupants stay safe for the development's lifetime — for highly vulnerable uses, the building must remain flood-free in the 1% annual probability river event (0.5% tidal) including climate change.
2. Consequences in extreme events are tolerable: in the 0.1% annual probability event plus climate change, flood depth, flow velocity and rate of rise on access routes and around the development must stay within the limits TAN15 sets for the site's vulnerability class.
3. Flood risk does not increase elsewhere — the scheme must not displace floodwater or worsen runoff onto neighbouring land.
4. Residual risk is managed — defended sites must assess overtopping and breach, and every scheme must design for exceedance.

The justification test has gone, but nothing got easier: the acceptability criteria demand more evidence, on more sites, to a higher technical standard than the 2004 policy ever did.

How TAN15 affects planning applications in Wales

Flood risk is a material consideration in every Welsh planning decision, and the practical machinery runs through Natural Resources Wales. NRW is a statutory consultee wherever a site touches Flood Zone 2, Zone 3 or a Defended Zone — it responded to around 2,100 planning consultations in 2024-25 where flood risk was flagged as a constraint. Its response can confirm the scheme is acceptable, recommend mitigation such as raised floor levels or safe access arrangements, or object. NRW does not grant or refuse permission — that stays with the council — but councils rarely approve against an unresolved NRW objection.

Timing matters more than most applicants expect. Flood risk evidence submitted late, or thin, is a common cause of invalidated applications, further-information requests and refusals. Considering TAN15 at site selection stage — before the option agreement, not after the design freeze — is consistently cheaper than retrofitting a strategy around a fixed layout. Works in or near a main river may also need a separate flood risk activity permit from NRW, which runs alongside planning rather than through it.

Drainage is the other half of the Welsh regime. Wales commenced Schedule 3 of the Flood and Water Management Act 2010 back in 2019, so most new development needs statutory SuDS approval from a SuDS Approving Body — a separate consent from planning permission, with its own national SuDS standards. If infiltration drainage is on the cards, our free ground conditions checker gives an early read on whether soakaways are plausible on your geology.

What a TAN15-compliant FCA involves — and what drives the cost

Where TAN15 catches your site, the assessment that follows identifies every source of flood risk affecting it — river, tidal, surface water and, where relevant, the less obvious sources such as groundwater and infrastructure failure — then tests the proposal against the acceptability criteria: flood levels against floor levels, safe access and egress, residual risk behind defences, climate change over the development's lifetime, and off-site impact. Where the national mapping is too coarse to answer those questions, site-specific modelling fills the gap.

We are asked about cost constantly, and the honest answer is that it depends on drivers rather than a rate card: the flood zone and sources involved, whether the site sits in a Defended Zone (which brings breach and overtopping analysis), the vulnerability of the proposed use, whether hydraulic modelling is needed, and how much NRW engagement the scheme demands. A straightforward desktop FCA typically takes five to ten working days; modelling and complex coastal or fluvial sites take longer. A scoped quote for your specific site — free, within the hour — is more useful than any generic figure, which is exactly why we work that way.

How Unda helps with TAN15 compliance

• Depth of record: more than 5,000 flood risk assessments delivered across England and Wales since 2014, spanning the full flood risk assessment range.
• Welsh framework fluency: our flood risk consultants work with TAN15, Planning Policy Wales and Future Wales daily, and know how NRW applies them in practice.
• In-house modelling: when the FMfP is too blunt for your site, we build the hydraulic case ourselves rather than subcontracting it.
• Proportionate scope: we tell you when a full FCA is not needed — pre-application advice that regularly saves clients the cost of a report they did not require.
• Local knowledge: published guides across our areas, rivers and local authority hubs feed real catchment understanding into every report.

TAN15 Wales: frequently asked questions

If you would like to talk a site through first, our experienced flood risk consultants are happy to advise on whether TAN15 applies, what evidence your application needs and the most effective route to approval. Call +44 (0) 1293 214444 or email enquiries@unda.co.uk.