Climate Change Allowances in SuDS Design: What's Changed in 2025Martin Young

Sustainable drainage systems (SuDS) design has undergone significant transformation in 2025 with the introduction of new National SuDS Standards. These changes represent a crucial shift in how drainage professionals must approach climate resilience, particularly regarding climate change allowances and their integration into modern drainage design.

Understanding Climate Change Allowances

Climate change allowances are essential factors that account for the anticipated increase in rainfall intensity and frequency due to changing weather patterns. These allowances ensure that drainage systems remain effective throughout their design life, typically spanning 100 years for most developments.

Historically, designers have applied percentage increases to rainfall data to account for climate change impacts. However, the 2025 National SuDS Standards have introduced more sophisticated approaches that better reflect our evolving understanding of climate science and its practical implications for drainage infrastructure.

Key Changes in the 2025 National SuDS Standards

Enhanced Runoff Destination Hierarchy

One of the most significant changes involves the establishment of a clear hierarchy for runoff destinations. The new standards prioritise sustainable water management through a five-tier system:

1. Collected for non-potable use

2. Infiltrated to ground

3. Discharged to surface water body

4. Discharged to surface water sewer

5. Discharged to combined sewer

This hierarchy fundamentally changes how climate change allowances are applied, as designers must now consider the most sustainable option first, rather than defaulting to traditional sewer connections.

Mandatory Rainwater Harvesting Considerations

The 2025 standards now mandate rainwater harvesting consideration in three specific circumstances:

• Developments with demand for non-potable water and available catchment area

• Projects requiring landscape irrigation

• Developments in seriously water-stressed areas as defined by the Environment Agency's 2021 classification

This requirement directly impacts climate change allowance calculations, as harvested rainwater can significantly reduce peak flow rates and provide additional storage capacity during extreme weather events.

Practical Implications for Drainage Design

Revised Calculation Methods

The new standards require designers to adopt more nuanced approaches to climate change allowances. Rather than applying blanket percentage increases, professionals must now consider:

• Regional climate projections specific to the development location

• The interaction between harvesting systems and peak flow management

• Long-term maintenance implications of increased rainfall intensity

• The cumulative effect of multiple SuDS components working together

Water-Stressed Area Classifications

Developments in seriously water-stressed areas face additional requirements that directly influence climate change allowance calculations. The Environment Agency's 2021 classification document provides clear guidance on which areas fall into this category, requiring enhanced water conservation measures that affect overall system sizing.

Design Life Considerations

The emphasis on rainwater harvesting and sustainable runoff management means that climate change allowances must now account for the operational efficiency of these systems over extended periods. This includes considering how harvesting systems perform during prolonged dry periods followed by intense rainfall events.

Technical Design Considerations

Storage Calculations

With the new hierarchy prioritising collection and infiltration, designers must recalculate storage requirements to account for:

• Reduced discharge rates to traditional drainage systems

• Increased reliance on natural infiltration processes

• The buffering effect of rainwater harvesting systems

System Integration

Climate change allowances must now consider the integrated performance of multiple SuDS components. This holistic approach requires more sophisticated modelling to ensure that the combined system can handle projected climate scenarios effectively.

Maintenance and Longevity

The new standards emphasise the importance of long-term system performance. Climate change allowances must therefore account for potential degradation in system efficiency over time, particularly for infiltration-based solutions that may become less effective due to siltation or ground condition changes.

Regional Variations and Local Authority Requirements

Local authorities are increasingly adopting the new National SuDS Standards, but implementation varies across different regions. Designers must stay informed about specific local requirements while ensuring compliance with the overarching national framework.

The seriously water-stressed area classifications create additional complexity, as developments in these regions face enhanced requirements that directly impact climate change allowance calculations and system design approaches.

Future-Proofing Drainage Systems

The 2025 changes reflect a more sophisticated understanding of climate resilience in drainage design. By prioritising sustainable runoff management and mandatory rainwater harvesting considerations, the new standards help create systems that are better equipped to handle future climate scenarios.

Designers must now think beyond traditional approaches and consider how climate change allowances interact with sustainable water management principles. This includes evaluating the long-term performance of integrated systems and ensuring that designs remain effective throughout their operational life.

Conclusion

The 2025 National SuDS Standards represent a significant evolution in sustainable drainage design, with climate change allowances now forming part of a more integrated and forward-thinking approach.

The emphasis on runoff destination hierarchy and mandatory rainwater harvesting considerations requires designers to adopt more sophisticated calculation methods and consider the long-term resilience of their systems.

For drainage designers, staying current with these changes is essential for delivering compliant and effective designs.

The new standards not only address immediate regulatory requirements but also contribute to broader climate adaptation strategies that will benefit communities for generations to come.

As the industry continues to adapt to these changes, ongoing professional development and collaboration with local authorities will be crucial for successful implementation of the new National SuDS Standards and their associated climate change allowance requirements.