DEFRA’s new National Standards for Sustainable Drainage Systems (SuDS)

By Reggie Wright and Ollie Manston

On 19 June 2025 the UK government published its new National Standards for Sustainable Drainage Systems (SuDS). The new standards do not drastically alter the current industry standards and best practice for how sustainable drainage and flood risk are considered for developments. The standards push slightly on design factors in minor ways that could lead to an increase in size of attenuation features.

The new standards do however present a more refined national blueprint for how smaller, and brownfield developments can be considered allowing more simplistic analysis, hopefully allowing for easier development in tight urban environments where SuDS can be difficult to implement.

It’s the first major revision in over a decade and appears to be a leap forward from the legislative perspective for England to consider SuDS as not only a flood control measure, but pieces of multifunctional green infrastructure.

But what are the key takeaways and how does it affect your development?

In general, the new standards outline that;

• SuDS are now mandatory for major developments – Surface water drainage must be considered upfront and designed in line with the new national criteria and local Lead Local Flood Authority (LLFA) guidance.
• There is a stronger emphasis on the four pillars of SuDS – Systems must demonstrate benefits across quantity, quality, amenity, and biodiversity — not just runoff reduction.
• Clarity on long-term adoption & maintenance – Local authorities will expect clearer strategies and stronger evidence of deliverability.
• Alignment with climate resilience – Designs must now account for future rainfall scenarios and integrate into wider flood risk strategies.

Pegasus has conducted a review of the standards and highlighted some of the key points which are relevant in how we approach our drainage design;

1. The “first flush” or first 5mm of rainfall – Thorough guidance on the management of everyday rainfall (interception) has now been provided under Standard 2. The standard provides a requirement to consider and follow guidance set out in the CiRIA SuDS Manual, with the target to show the first 5mm of runoff does not discharge off the site and is instead either collected for reuse, infiltrated into the ground, or else captured, conveyed, and stored within SuDS features.
2. Climate Change Allowances – The ‘Upper End Allowance’ for each of the 3.3%AEP and 1.0%AEP return period events should be applied to the relevant epoch based on the design lifetime of the development.
3. Infiltration guidance updates – Whilst considered a rule of thumb for drainage engineers for some time, the latest standards now formalise that the minimum permissible infiltration rate for discharging all runoff is 1.0E-06m/s. Additionally, the standards now add that the half drain down time for infiltrating features in the 3.3%AEP event should be less than 24hrs.
4. Brownfield (previously developed) Runoff Rates –The latest guidance states that a ‘relaxation factor’ should instead be applied to the target greenfield runoff rates (maximum relaxation factor should be no greater than 5x the greenfield rates). This system is much simpler and consistent than the previous method of ‘brownfield runoff rate’ and allows for easier implementation on heavily developed urban sites.
5. QBAR Definition – QBAR and the 50%AEP (1 in 2 year) can be regarded as similar allowing simple smaller schemes to discharge at the QBAR rate.
6. Catchment Area Clarification – The Standard confirms a previously grey area by stating that SuDS features should be included in the contributing catchment area. It also notes that there is no requirement to provide attenuation for external overland flows entering the site but these flows should be considered for capacity of the pipe network.
7. The Four Pillars of SuDS – Standards 4, 5, and 6 add much more emphasis to the importance of Water Quality, Amenity, and Biodiversity and how the ‘SuDS approach’ should be adopted to address each of these points. This aligns with the NPPF (para. 182) update from December last year which highlights that SuDS features should provide multifunctional benefits wherever possible.