Updated planning guidance on flood risk and coastal change highlights some of the challenges faced when considering SuDS for new schemes. In part one of a three-part series, Stuart Crisp, UK manager of Advanced Drainage Systems (ADS), sets the scene by summarising the new guidance, its implications and ambiguities.
On September 17th 2025, the Government issued updated guidance on managing flood and coastal change within the planning system. The revisions place particular emphasis on the sequential approach to development, clarifying what constitutes a ‘reasonably available’ site. Developers are now encouraged to consider a broader set of alternative locations – including smaller, amalgamated plots – before progressing applications in higher-risk areas.
The guidance rightly recognises that sustainable drainage systems (SuDS) can make a significant contribution to reducing the risk of flooding, devoting a section to the deployment of SuDS. However, it also raises some questions around issues such as the interplay between water quantity and water quality, problems regarding maintenance and adoption and the consideration of engineered or grey SuDS alongside green infrastructure.
The guidance reiterates that developments should first be directed to Flood Zone 1, then to Flood Zone 2 when necessary, and only to Flood Zone 3 when no other suitable land exists and the Exception Test is met. Crucially, it stresses that flood risk is not limited to rivers or the coast; surface water, drainage exceedance and overwhelmed sewers must also be treated consistently as sources of hazard, particularly as climate change intensifies rainfall.
Contained in just seven paragraphs, a dedicated section sets out the purpose of sustainable drainage and aims to highlight the issues around SuDS that need to be considered, further investigated or approved by other bodies and authorities. While it cannot be expected to be comprehensive, it does contain discrepancies and anomalies which could cause some confusion for the reader.
Green and grey
Paragraph 055 explicitly acknowledges that SuDS often blend nature-based and built components, and that engineered systems may be necessary to mimic natural drainage. However, beyond this acknowledgement, the guidance largely reverts to an emphasis on vegetative, green infrastructure. This risks misleading readers into assuming that engineered interventions have limited value, when in reality they are often crucial where space is constrained, pollution loads are high or groundwater conditions limit infiltration.
For instance, where runoff contains elevated contaminants – such as from busy roads or industrial premises – pre-treatment through proprietary engineered devices is essential to protect receiving water bodies and downstream SuDS assets. Without this, vegetative features alone may become saturated with pollutants, degrading their function and harming ecosystems. Evidence from Chartered Institution of Water and Environmental Management (CIWEM) and Stormwater Shepherds underscores the scale of the problem: thousands of highway outfalls discharge untreated runoff directly into watercourses.
The guidance also touches on the need for environmental permits where polluted water enters surface or groundwater systems. Yet historically, regulatory enforcement has been inconsistent, particularly relating to discharges from major highways.
While the new guidance attempts to clarify responsibilities, its treatment of green and grey SuDS leaves important gaps. Part 2 explores these omissions further – examining how SuDS design, maintenance planning and early-stage decision-making determine long-term performance.
