Schedule 3 Wales: What Developers Need to Know in 2026

When Schedule 3 of the Flood and Water Management Act 2010 was commenced in Wales on 7 January 2019, it fundamentally changed how surface water drainage is approved and delivered in Welsh development. Over six years on, the system is well established — but developers new to Wales, or those with limited experience of SuDS Approval Bodies (SABs), still encounter avoidable delays and costs.

This article sets out the key requirements, the application process, and the most common pitfalls we see in practice.

What Schedule 3 Requires

Schedule 3 established a statutory requirement for SuDS on all new developments in Wales with drainage implications. Crucially, it introduced SuDS Approval Bodies (SABs) — typically the local authority — as a separate approval body with the power to approve or refuse drainage designs independently of the planning process.

The key requirements are:

Separate SAB Approval

SuDS approval is a separate consent process from planning permission. You need both. The SAB application must be approved before construction can commence on any drainage element, and in practice most developers submit the SAB application alongside or shortly after the planning application.

The SAB application is submitted to the local authority, which assesses the drainage design against the Welsh Government’s Statutory Standards for Sustainable Drainage.

Mandatory Standards

The Statutory Standards for Sustainable Drainage set out six mandatory standards that every SuDS scheme must meet:

1. Surface water runoff destination: Runoff must be managed using the hierarchy of infiltration, discharge to a watercourse, discharge to a surface water sewer, and discharge to a combined sewer. Each step down the hierarchy requires justification for why the preferred option is not feasible.
2. Surface water runoff hydraulic control: Runoff rates must be limited to the greenfield rate for all return periods up to the 1 in 100 year event plus climate change. Where the greenfield rate is impractical for low return periods, a minimum discharge rate of 5 litres per second may be applied.
3. Water quality: The SuDS scheme must provide adequate treatment of runoff before discharge. The required treatment level depends on the pollution hazard level of the drained surface — roofs require less treatment than car parks, which require less treatment than lorry yards.
4. Amenity: The SuDS design must maximise amenity benefits. This encourages above-ground SuDS features that contribute to the quality of the public realm rather than hidden underground tanks.
5. Biodiversity: The SuDS design must maximise biodiversity benefits. Features that create wildlife habitat — ponds, wetlands, rain gardens with diverse native planting — score more highly than inert hard infrastructure.
6. Design of drainage for construction, operation, and maintenance: SuDS features must be designed so they can be constructed, operated, and maintained effectively throughout their design life. Access for maintenance vehicles, appropriate gradients, and clear maintenance schedules are required.

Adoption by the SAB

A critical feature of Schedule 3 is that the SAB must adopt and maintain the approved SuDS features in perpetuity. This is a significant shift from England, where adoption of SuDS features by water companies or local authorities remains ad hoc and often contentious.

The corollary of mandatory adoption is that the SAB is rigorous about design quality and maintainability. They will reject designs that are difficult to maintain, that rely on complex mechanical components, or that do not meet their adoption standards.

The SAB Application Process

Pre-Application Advice

Most SABs offer pre-application advice, and we strongly recommend using this service. A pre-application discussion allows you to:

• Confirm the SAB’s expectations for the site.
• Discuss the preferred discharge hierarchy and any site-specific constraints.
• Identify potential issues early (such as ground conditions, water quality requirements, or maintenance access).
• Understand the SAB’s preferred SuDS features and design standards.

Pre-application advice is typically free or low cost and can save significant time and money by avoiding designs that the SAB will reject.

Full Application

The SAB application includes:

• Completed application form: Available from the local authority.
• Drainage layout plans: Showing the location, extent, and connectivity of all SuDS features.
• Drainage design calculations: Including runoff rate calculations, attenuation volume calculations, and water quality treatment assessments.
• Maintenance plan: Setting out the maintenance requirements for each SuDS feature, including frequency, method, and responsible party.
• Supporting information: Including site investigation results (soakaway tests, ground investigation), topographic surveys, and any other relevant technical data.
• Fee: The SAB application fee is calculated based on the number of dwellings or the site area, in accordance with the prescribed fee scale.

Determination Period

The SAB has a statutory determination period of 7 weeks for minor applications and 12 weeks for major applications (more than 10 dwellings or more than 0.5 hectares). In practice, determination often takes longer, particularly if additional information is requested or if the design requires revisions.

Conditions and Approval

SAB approval may be granted subject to conditions — for example, requiring detailed design of specific features, ground investigation, or a revised maintenance schedule. These conditions must be discharged before the relevant construction work can commence.

Common Pitfalls

1. Treating SAB as an Afterthought

The most common and most expensive mistake is treating SAB approval as a box-ticking exercise to be dealt with after the design is fixed. SAB requirements should inform the masterplan from the earliest design stage. If the layout does not accommodate above-ground SuDS features, the SAB will reject the drainage design — and redesigning the layout at that stage is far more costly than incorporating SuDS from the outset.

2. Over-Reliance on Underground Storage

Welsh SABs are particularly firm on the amenity and biodiversity standards. A design that relies primarily on geocellular underground storage, with minimal above-ground SuDS features, is unlikely to be approved. The standards require that amenity and biodiversity benefits are maximised, and underground storage provides neither.

We regularly see drainage designs prepared by engineers unfamiliar with the Welsh system that default to the underground-tank approach common in England. These designs require significant reworking to meet SAB standards, adding cost and delay.

3. Ignoring the Treatment Train

Water quality treatment must follow a logical treatment sequence — the management train discussed in our SuDS management train article. Runoff from polluting surfaces (car parks, roads) must pass through multiple treatment stages before discharge. A single SuDS feature cannot provide the full treatment train for heavily polluted runoff.

4. Inadequate Maintenance Plans

The SAB will adopt and maintain the SuDS features, so they need confidence that the features can be maintained efficiently and cost-effectively. Designs with poor maintenance access, overly complex features, or unrealistic maintenance schedules will be rejected.

Provide clear, practical maintenance plans for every SuDS feature. Specify access routes for maintenance vehicles, frequencies for routine tasks (mowing, sediment removal, inspection), and procedures for reactive maintenance (blockage clearance, structural repair).

5. Not Testing Infiltration Early Enough

The discharge hierarchy prioritises infiltration, and the SAB will expect evidence that infiltration has been properly investigated before allowing discharge to a watercourse or sewer. Commission soakaway testing (BRE365) as early as possible — ideally at the feasibility stage — so the drainage strategy can be designed around the actual ground conditions rather than assumptions.

Schedule 3 in England?

There has been long-standing discussion about commencing Schedule 3 in England. As of early 2026, the UK government has indicated its intention to implement Schedule 3 in England, but no commencement date has been confirmed. When it does commence, developers in England will face similar requirements to those already in place in Wales.

If you are a developer or planning consultant working in England, understanding the Welsh experience is valuable preparation. The SAB process, mandatory standards, and adoption requirements will likely form the basis of the English system when it arrives.

Getting It Right

Schedule 3 compliance is not difficult when the drainage design is considered from the outset and the designer understands the SAB’s expectations. At Aegaea, we have extensive experience designing SuDS schemes that satisfy SAB requirements in Wales while delivering commercially viable development.

If you have a project in Wales and need SAB-compliant drainage design, or if you want to understand how the Welsh system might apply to your projects when Schedule 3 is commenced in England, contact us to discuss your requirements.