S6: Biodiversity

S6

What S6 requires

Source: Defra Non-Statutory Technical Standards for Sustainable Drainage Systems (2015, refreshed 19 June 2025), Standard S6. Reproduced in CIRIA C753 §6 (Designing for biodiversity) and aligned with the Environment Act 2021 mandatory Biodiversity Net Gain (BNG) requirements.

S6 requires design that maximises biodiversity benefits throughout the development lifecycle. SuDS features should create diverse, self-sustaining ecosystems that contribute to biodiversity net gain (BNG) targets and support local nature recovery strategies.

Ecosystem Design

Rather than simple grass verges or monoculture planting, SuDS should be designed as complex, diverse habitats: retention ponds with marginal plants, emergent vegetation, and submerged aquatic plants supporting fish and invertebrates; swales with native wildflower mixes attracting pollinators; bioretention with diverse shrub and perennial layers.

Biodiversity Net Gain (BNG)

New developments in England must deliver at least a 10% biodiversity net gain, a measurable increase in habitat value compared to the baseline. SuDS features should be designed to contribute significantly to this target. Retention ponds and wetlands are particularly valuable as they create aquatic and semi-aquatic habitats. Native plant selection, habitat connectivity, and long-term management are essential.

Local Nature Recovery Strategies (LNRS)

Local Nature Recovery Strategies set out priorities for habitats and species in each region. SuDS design should align with these priorities: for example, supporting water voles in chalk rivers, or creating wildflower meadows in lowland grassland regions. This ensures that SuDS contribute to landscape-scale biodiversity objectives.

Biodiversity Risk and Opportunity Assessment

Designers are required to undertake a biodiversity risk and opportunity assessment for the site: identifying existing habitats and species, assessing potential impacts of development, and designing SuDS and landscaping to maximise opportunities for enhancement. This assessment may require ecological surveys if the site has potential for protected species.

Long-Term Management: Biodiversity benefits are only realized if habitats are appropriately managed long-term. The Management and Maintenance Plan (required under S7) must specify the management regime for planted and aquatic features, including seasonal cutting, invasive species control, and monitoring.