New National Standards for SuDS: Rainwater Harvesting Takes Priority

The drainage design landscape has undergone a significant transformation with the publication of the National Standards for Sustainable Drainage Systems (SuDS) on 19 June 2025. These new standards, which apply across England, represent a fundamental shift in how we approach surface water management, with rainwater harvesting now positioned as the top priority in the drainage hierarchy.

The New Drainage Hierarchy: Rainwater Harvesting First

Under Standard 1: Runoff Destinations, the new regulations establish a clear hierarchy for managing surface water runoff. For the first time, rainwater harvesting sits at Priority 1 – the highest level in the drainage hierarchy:

Priority 1: Collected for non-potable use (rainwater harvesting) Priority 2: Infiltrated to ground Priority 3: Discharged to an above ground surface water body Priority 4: Discharged to a surface water sewer Priority 5: Discharged to a combined sewer

This represents a revolutionary change from traditional approaches that typically prioritised infiltration or direct discharge to sewers.

When Rainwater Harvesting Must Be Considered

The new standards mandate that rainwater harvesting shall be considered in all circumstances where any of the following apply:

  • There is a demand for non-potable water and available contributing catchment area that will deliver safe and efficient water savings
  • There is a need for landscape irrigation
  • The development is in an area identified as seriously water stressed

This means that for most commercial developments, educational facilities, residential complexes, and industrial projects, rainwater harvesting systems must now be actively evaluated and implemented where feasible.

Why This Means More Work at Every Stage

Design Stage

The elevation of rainwater harvesting to Priority 1 significantly increases the complexity of initial design work. Drainage designers must now:

  • Conduct detailed water demand assessments to identify non-potable water requirements
  • Calculate optimal storage volumes using BS EN 16941 standards
  • Design systems that can handle both everyday rainfall interception and extreme weather events
  • Integrate rainwater harvesting with other SuDS features in a comprehensive management train
  • Demonstrate compliance with performance criteria against appropriate rainfall time series

Planning and Approval Stage

The new standards require robust evidence that higher priority destinations have been “utilised to the maximum extent practicable” before considering lower priority options. This means:

  • Comprehensive feasibility studies for rainwater harvesting systems
  • Detailed justification if rainwater harvesting is deemed unsuitable
  • Integration with biodiversity net gain requirements
  • Coordination with multiple stakeholders including local planning authorities, lead local flood authorities, and water companies

Construction and Implementation

Rainwater harvesting systems are inherently more complex than traditional drainage solutions, requiring:

  • Sophisticated storage and distribution infrastructure
  • Water treatment systems for intended end-uses
  • Monitoring and control systems
  • Integration with building services and landscape irrigation
  • Compliance with public health and security requirements for communal systems

Ongoing Management and Maintenance

The new standards place significant emphasis on long-term performance, requiring:

  • Comprehensive management and maintenance plans
  • Regular monitoring of system performance
  • Maintenance of water quality standards
  • Coordination between multiple responsible parties
  • Rehabilitation procedures following extreme weather events

Compliance and Evidence Requirements

The standards are particularly stringent about evidence requirements. Developers must demonstrate that rainwater harvesting has been thoroughly evaluated and, where not implemented, provide “appropriate evidence” that it’s not practicable. Importantly, “higher cost alone shall not be a reason to utilise lower priority final destinations.”

For systems that are implemented, compliance must be demonstrated through:

  • Calculations to BS EN 16941 standards
  • Performance modelling against appropriate rainfall data
  • Integration with interception requirements (Standard 2)
  • Water quality risk assessments (Standard 4)
  • Biodiversity and amenity considerations (Standards 5 and 6)

The Broader SuDS Approach

While rainwater harvesting now takes priority, it must be integrated within the broader “SuDS Approach” that delivers multiple benefits. The standards emphasise that surface water drainage systems should work as a “management train” – a series of different drainage features operating together to control flow rates, reduce volumes, provide water quality benefits, and encourage biodiversity and amenity.

Implications for The Drainage Designers

For drainage design professionals, these new standards represent both a challenge and an opportunity. Projects will require:

  • Enhanced technical expertise in rainwater harvesting system design
  • Stronger collaboration with architects, engineers, and building services professionals
  • More comprehensive project planning and stakeholder engagement
  • Greater emphasis on long-term performance and maintenance planning
  • Integration of multiple regulatory requirements and standards

The shift to prioritising rainwater harvesting reflects the government’s commitment to sustainable water management, climate resilience, and resource efficiency. While this will undoubtedly increase the complexity and scope of drainage design projects, it also positions the UK as a leader in sustainable water management practices.

For developers and property owners, early engagement with qualified drainage designers will be more critical than ever to navigate these new requirements successfully and deliver compliant, sustainable drainage solutions.

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