National Standards for Sustainable Drainage Systems (SuDS) 2025

On 19 June 2025, DEFRA launched a refreshed set of non-statutory national standards for Sustainable Drainage Systems (SuDS), aimed at delivering consistent, multifunctional surface-water management across England. While not legally mandatory, these standards are published as broader national standards aligned with legal frameworks such as the Environment Act 2021 and the Flood and Water Management Act 2010. They signal a significant shift in how developers, planners, and engineers approach drainage.

Key Takeaways from SuDS 2025 Standards

• Shift to Formal Standards: SuDS are no longer just ‘good practice’ guided by policy; they are now defined by a clear set of seven national standards, increasing expectations for compliance and consistency.
• Runoff Hierarchy is Mandatory: Developers must now follow a strict hierarchy for discharging runoff, prioritising on-site reuse and infiltration. Deviating from this requires robust evidence.
• Greater Technical Rigour: New, specific design criteria for flood risk — including storm event caps (1-in-2 and 1-in-100 year) and a 10% urban creep factor — demand more detailed hydraulic modelling and design.
• Green Infrastructure is Central: Amenity and biodiversity are now formal standards, requiring SuDS to be designed as multi-functional spaces that deliver measurable environmental net gain, not just as drainage assets.
• Whole-Lifecycle Focus: A major emphasis is placed on long-term performance, with mandatory requirements for detailed maintenance plans, construction method statements, and designing for structural durability.

The 7 National SuDS Standards

There are two types of standards:

• Standard 1 is a hierarchy standard and gives the criteria for prioritising the choice of final runoff destination.
• Standards 2-7 are fixed standards, which state the minimum design criteria of all surface water drainage systems, including how they should be built, maintained and operated.

Standard 1: Runoff Destinations

The hierarchy is:

1. Collected for non-potable use (e.g. rainwater harvesting for toilet flushing, irrigation)
2. Infiltrated to the ground
3. Discharged to an above-ground surface water body
4. Discharged to a surface water sewer or another piped drainage system
5. Discharged to a combined sewer

Standard 2: Management of Everyday Rainfall (Interception)

For most rainfall events, the first 5mm of rainfall must be intercepted within the development.

Standard 3: Management of Extreme Rainfall and Flooding

Sets out requirements for designing infiltration features and allowable discharge rates during heavy rainfall events, including 50% AEP (1 in 2 year) and 1% AEP (1-in-100 year) storm events with a 10% urban creep factor for developments with curtilage.

Standard 4: Water Quality

Any proposed SuDS must be based on a thorough water quality risk assessment using a recognised method such as the Simple Index Approach.

Standard 5: Amenity

SuDS should be designed to be aesthetically pleasing and provide recreational value to the community.

Standard 6: Biodiversity

SuDS should be designed to support and enhance local wildlife and habitats, contributing to Biodiversity Net Gain (BNG) and Local Nature Recovery Strategies (LNRS).

Standard 7: Design of Drainage for Construction, Operation, Maintenance, Decommissioning and Structural Integrity

SuDS must be designed with their entire lifecycle in mind, including long-term operation, maintenance schedules, detailed Construction Method Statements (CMS), and potential failure risks.

How SuDS 2025 Differs from 2015 Guidance

Key differences include:

• Status: From policy guidance only to published official national standards (non-statutory)
• Applicability: From major developments only to intended for all development types
• Runoff Hierarchy: From encouraged to mandatory with justification required
• Everyday Rainfall: No specific requirement previously; now formal standard to manage first 5mm
• Water Quality: From general encouragement to formal risk assessment requirement
• Amenity & Biodiversity: From vague/optional to explicit standards tied to BNG
• Flood Risk Design: From basic attenuation to specific discharge rate caps with urban creep uplift
• Maintenance: From general consideration to detailed plans, CMS, and structural design required

What Does This Mean For Future Developments?

These new non-statutory standards mark a turning point for developments in England. SuDS are no longer a simple drainage calculation tacked on at the end of a design; they are a central, defining feature of a project’s success.

For developers, designers, and planners, this means:

• SuDS move from the margins to the master plan. Surface water management must now be considered from day one, influencing site layouts, green space, and road design.
• Collaboration is essential. Engineers and Consultants will need to work more closely than ever with landscape architects and ecologists to create schemes that are not only functional but also beautiful and biodiverse.
• The paper trail is more important. Planning applications will require a new level of detail, with robust evidence justifying your drainage strategy and clear, long-term maintenance plans.
• A fundamental shift from grey to green. The era of relying solely on underground pipes and drainage features is now discouraged as much as is feasible. The future is in creating resilient, visible, and valuable green infrastructure that serves the community and the environment.