In June 2025, the UK government published a major update to the National Standards for Sustainable Drainage Systems (SuDS), the most significant change in a decade. While the standards bring in wide-ranging improvements to surface water management, a standout development is the new emphasis on rainwater harvesting.
Rainwater harvesting is now explicitly recognised as a frontline method of compliance, particularly in meeting requirements for capturing the initial rainfall from smaller, more frequent events. This makes rainwater harvesting not just a sustainability bonus but a practical drainage solution that can improve project outcomes across planning, performance, and water resource efficiency.
From Drainage to Green Infrastructure
A core theme of the new standards is treating SuDS features as integral components of site design, not afterthoughts. The 2025 guidance positions SuDS as multifunctional infrastructure that contributes not only to water management but also to public amenity, biodiversity, and climate adaptation. In fact, there is now a strong emphasis on the four pillars of SuDS design: water quantity (flood attenuation), water quality (pollution control), amenity, and biodiversity. This means that drainage solutions in new developments are encouraged to provide multiple benefits. For example, a swale or pond that both stores runoff and creates a natural habitat. Two new core standards (Standard 5 on Amenity and Standard 6 on Biodiversity) underline this shift by requiring designers to incorporate green and natural features that benefit people and wildlife.
This holistic approach encourages architects to integrate SuDS early in the site planning process, aligning drainage with landscape architecture and urban design. Rather than relying on underground pipes alone, the standards promote visible, nature-based solutions (green roofs, rain gardens, permeable paving, ponds, etc.) that make developments more resilient and attractive. In short, SuDS are no longer just about engineering away water; they are about designing water in to create greener, healthier places for communities.
Key Updates in the 2025 SuDS Standards
The updated National SuDS Standards introduce several points for consideration in drainage design. Here’s some of the key updates to note.
First Flush” Capture (Everyday Rainfall):
New guidance under Standard 2 advises managing at least the first 5 mm of rainfall on-site for the majority of rain events. In practice, this means the initial runoff from small storms should not discharge to sewers or rivers. Designers are encourage to show that this first flush is collected for reuse, infiltrated into the ground, or stored in SuDS features so it can evaporate later. This is a more rigorous approach to intercepting everyday rainfall and reducing pressure on traditional drains. Rainwater harvesting systems are now recognised in the standards as compliant methods to meet the initial rainfall interception requirement.
Stronger Climate Resilience
The standards recommend using the latest climate change allowances (the “Upper End” scenario for 1-in-30 and 1-in-100 year storms) appropriate to the development’s lifespan. Standard 3 suggests designing for resilience against extreme rainfall, ensuring the site’s drainage can handle more intense downpours without increasing flood risk off-site. For example, any 1% annual probability (1 in 100 year) storm (plus climate uplift) to be managed so that post-development runoff does not worsen downstream flooding.
Infiltration and Discharge Limits
The updated standards formalise criteria for using infiltration. If you plan to drain all runoff into the ground, the soil’s infiltration rate should be at least 1×10⁻⁶ m/s (a rule of thumb now made official). Additionally, any infiltration-based system must empty (drain down) within 24 hours after a large storm (to be ready for the next event). These criteria aim to ensure that soakaways or infiltration basins are viable and won’t remain waterlogged. The guidance also clarifies that if infiltration alone is insufficient, designers must combine it with attenuation storage to meet runoff control targets.
Runoff Rates for Brownfield Sites
One notable change is a simplification of how runoff from previously developed (“brownfield”) sites is handled. Instead of trying to calculate a pre-existing brownfield runoff rate, the new standards suggest applying a “relaxation factor” up to 5× the greenfield (natural) runoff rate for the site. In other words, allowable discharge from a redevelopment can be up to five times what it would be if the site were greenfield. This is a more consistent and easy-to-apply method, replacing the often murky calculations of past practice. It aims to make SuDS design more feasible in dense urban areas while still substantially reducing runoff compared to older drainage systems.
Clearer Design Requirements
The document is structured more clearly than the old 2015 standards. Each standard now has specific clauses using “shall” (mandatory), “should” (advisory), or “may” (optional), with sub-clauses and guidance notes. This clarity eliminates ambiguity and should lead to more consistent design and approval of SuDS across different regions. There is also explicit clarification that SuDS features themselves count toward a site’s drainage catchment (resolving a grey area about whether storage areas needed additional volume for self-draining). Additionally, the standards align with current best practice guides (such as the CIRIA SuDS Manual) and emphasise using the “SuDS management train”—a ”sequence of source, site, and regional controls to treat and slow water.
Emphasis on Water Quality, Amenity, and Biodiversity
Beyond controlling flows, the new standards heighten the focus on water quality treatment (Standard 4) and introduce dedicated standards for Amenity (5) and Biodiversity (6). Designers are encouraged to incorporate features that provide habitat value and community benefits in addition to managing water. For example, a detention pond might be planted with wildflowers and have walking paths, making it a dual-purpose park area. The standards advise performing pollution risk assessments for higher-risk land uses and encourage natural filtration (e.g., swales, bioretention areas) to remove contaminants from runoff. All SuDS schemes should aim to deliver co-benefits wherever possible, rather than simply building underground tanks that only handle water.
Lifecycle Maintenance and Adoption
Recognising that drainage schemes can fail if not maintained, Standard 7 is all about long-term sustainability. It encourages early consideration of maintenance plans, ownership, and adoption of SuDS features. Local authorities and regulators will expect clear evidence of who will maintain each SuDS component and how, whether it’s a private management company, the water utility, or the local council. By planning for the whole lifecycle (design through maintenance to eventual replacement), developers can avoid the common pitfall of SuDS that work on paper but later get clogged or neglected. In practice, this might mean setting up a maintenance schedule for permeable paving (to prevent clogging) or ensuring there’s a legal agreement on who is responsible for a communal attenuation pond in a housing development. The new standards push for these arrangements to be ironed out at the design stage.
Rainwater Harvesting: A Key Solution Under SuDS
One particular aspect of the new standards is the spotlight on rainwater harvesting. The idea of reusing rainwater on-site is not new, but the 2025 SuDS standards formally recognise rainwater harvesting systems as an effective means of managing rainfall. In fact, the guidance says that if a development uses a properly designed rainwater harvesting system such that no runoff leaves the site for routine rain events, it can be deemed compliant with the first 5 mm interception requirement (Standard 2). In simple terms, capturing rainwater for reuse kills two birds with one stone: it reduces storm runoff (helping drainage compliance) and supplies a source of non-potable water that can be used for toilet flushing, irrigation, vehicle washing, etc., easing demand on mains water.
However, not all rainwater harvesting setups automatically meet the new SuDS criteria. The standards specify that rainwater harvesting systems should be designed to British Standard BS EN 16941 and have a regular daily draw-down of water. This ensures the storage tank is kept available to absorb rainfall in typical events. (For example, a system supplying a building’s toilets will draw down water each day, making room for the next rain, whereas an intermittently used garden water butt might often be full and unable to take more runoff.) Notably, the standards call out that simple water butts alone are not considered sufficient for compliance unless they are actively managed or designed with reliable emptying between storms. This underscores the need for more robust, plumbed-in harvesting solutions if you want to claim the runoff reduction credit.
For architects and developers, incorporating rainwater harvesting is a win-win. It can count toward SuDS requirements for interception, improve a project’s sustainability profile, and potentially lower water bills for the end users. For instance, using harvested rainwater to flush toilets in a commercial building can significantly cut potable water use. On residential schemes, collected rainwater can keep landscaping watered during dry spells. Given these benefits and the new policy support, we anticipate many upcoming projects will include some form of rainwater harvesting storage as part of their drainage strategy.
Conclusion
The 2025 update to the UK’s SuDS standards represents a paradigm shift in drainage philosophy from “grey” infrastructure to “green,” from handling water as a nuisance to embracing it as a resource. For architects, developers, and builders, this shift brings both challenges and opportunities. In the short term, it means coming up to speed with new design criteria and possibly investing more effort in drainage planning and implementation. In the long term, it promises more resilient developments that are better equipped to handle climate change impacts and that offer enhanced amenities for communities.
It’s important to note that as of now these national standards are non-statutory guidance. Unlike Wales, which has a legal requirement for SuDS on new developments, England (and the rest of the UK) has not yet fully mandated SuDS through law. Instead, implementation relies on the planning system and the goodwill of developers. This lack of a formal mandate or a SuDS approval body (as envisioned in Schedule 3 of the 2010 Flood Act) means there may still be inconsistencies in enforcement. Nonetheless, policy momentum is clearly moving toward wider adoption of sustainable drainage. The updated standards have strong support from environmental bodies and are referenced in national planning policy, so it’s quite possible that a tougher legal requirement will follow in the coming years.
For those in the building industry, the best course is to get ahead of the curve: start designing and constructing to these standards now. By doing so, you not only ease the path through planning approvals but also contribute to developments that are safer from floods, cheaper to service, and greener for the community. The evolution of SuDS is turning drainage from an afterthought into a central part of design and placemaking.
At Graf UK, our stormwater management experts and rainwater harvesting team are here to help—whether it’s navigating the new guidelines or supplying proven, high-performance products. We offer below-ground tanks, filters, pump packages, and more, all tailored to meet BS EN 16941 and designed for a wide range of domestic and commercial applications.
Want to make sure your next project meets SuDS standards efficiently?
Browse our rainwater harvesting systems here, or get in touch with our technical team for tailored advice.
Posted by Callum Vallance-Poole, on September 5, 2025.