Biodiversity Net Gain Meets SuDS: Maximising Environmental Benefits

The convergence of Biodiversity Net Gain (BNG) requirements and Sustainable Drainage Systems (SuDS) presents a unique opportunity for developers and designers to create developments that deliver multiple environmental benefits. Understanding how these two frameworks complement each other is essential for maximising value whilst meeting regulatory obligations.

Understanding the Dual Framework

Biodiversity Net Gain became mandatory in England in February 2024, requiring developments to deliver a minimum 10% measurable increase in biodiversity. Meanwhile, SuDS have long been the preferred approach for managing surface water drainage, mimicking natural processes to reduce flood risk and improve water quality. When designed together from the outset, these systems can achieve far more than the sum of their parts.

The key lies in recognising that many SuDS features are inherently biodiversity-rich. Rain gardens, swales, detention basins, and wetlands don't just manage water; they create habitats. By integrating BNG principles into SuDS design, you transform functional infrastructure into thriving ecosystems that deliver measurable environmental gains.

The Strategic Benefits of Integration

Combining BNG and SuDS offers several compelling advantages. Firstly, it maximises land use efficiency. Rather than allocating separate areas for drainage infrastructure and biodiversity enhancement, integrated designs serve both purposes simultaneously. This is particularly valuable on constrained urban sites where every square metre counts.

Secondly, integrated approaches reduce overall project costs. Installing a biodiverse rain garden costs little more than a conventional one, yet it contributes to both drainage requirements and BNG targets. This dual functionality means developers can meet multiple obligations without proportionally increasing expenditure.

Thirdly, these combined systems enhance resilience. Biodiverse SuDS features with varied vegetation and soil structures are more robust, better able to withstand extreme weather events, and require less intensive maintenance over their lifetime. The diversity of plant species ensures that if some fail, others continue to provide functionality.

Design Principles for Maximum Impact

Successful integration begins at the earliest design stages. Start by conducting a thorough baseline biodiversity assessment alongside your drainage strategy. Understanding existing habitats, species, and ecological connectivity helps identify opportunities to enhance rather than simply mitigate impacts.

Select SuDS features that offer the greatest biodiversity potential. Wetlands and detention basins with varied water depths create diverse aquatic and marginal habitats. Swales planted with native wildflower mixes support pollinators whilst managing runoff. Green roofs, particularly biodiverse variants with varied substrate depths, provide habitat in otherwise unused space.

Plant selection is critical. Prioritise native species that support local wildlife, particularly those that provide food and shelter throughout the year. Include a mix of grasses, wildflowers, shrubs, and where space permits, trees. Aim for structural diversity: varied heights, densities, and growth habits create more ecological niches and support a wider range of species.

Consider connectivity. Isolated habitat patches have limited ecological value. Design SuDS features as part of a connected network, creating corridors that allow wildlife to move through the development and link to surrounding green spaces. This connectivity is essential for species survival and contributes significantly to BNG scores.

Practical Implementation Strategies

Rain gardens exemplify the integration potential. These shallow depressions capture and filter runoff whilst providing habitat. Design them with varied topography to create different moisture zones, each supporting distinct plant communities. Use locally appropriate native plants that tolerate both wet and dry conditions. The result is a feature that manages water, improves water quality, supports biodiversity, and enhances visual amenity.

Detention basins offer substantial biodiversity opportunities. Rather than designing sterile grass basins, incorporate varied topography with deeper permanent pools, shallow seasonal wetlands, and drier margins. Plant these zones with appropriate native species to create a mosaic of habitats. Include features like log piles, rock stacks, and varied substrate to further enhance habitat diversity.

Swales, when designed with biodiversity in mind, become linear wildlife corridors. Plant them with native wildflower and grass mixes rather than amenity grass. Vary the swale profile to create different moisture conditions. Allow some areas to develop naturally, creating structural diversity that supports invertebrates, small mammals, and birds.

Permeable paving can contribute too. Use systems that allow vegetation growth in joints or voids. Select native low-growing plants that tolerate foot traffic. Whilst the biodiversity contribution is modest compared to wetlands, every element counts towards your BNG target.

Meeting Regulatory Requirements

Both BNG and SuDS are subject to regulatory oversight, and integrated designs must satisfy both frameworks. For SuDS, your design must meet local authority requirements and Lead Local Flood Authority standards, demonstrating adequate capacity, appropriate treatment, and acceptable discharge rates.

For BNG, you must use the Defra Biodiversity Metric to calculate baseline biodiversity units and demonstrate a minimum 10% net gain. This requires detailed habitat surveys and careful design documentation. Integrated SuDS features must be assessed using appropriate habitat types within the metric, typically wetland, grassland, or mixed habitats depending on design.

Crucially, BNG gains must be secured for at least 30 years through planning conditions or conservation covenants. This long-term commitment aligns well with SuDS, which also require ongoing maintenance. Develop a combined management plan that addresses both drainage functionality and biodiversity objectives, ensuring features deliver their intended benefits throughout their design life.

Maintenance Considerations

Biodiverse SuDS require different maintenance approaches than conventional systems. Traditional intensive mowing and vegetation clearance are replaced with sensitive management that supports both drainage function and wildlife. This typically means less frequent intervention but requires greater ecological understanding.

Establish a maintenance regime that balances hydraulic performance with biodiversity objectives. Remove excessive silt and vegetation that impedes flow, but retain diverse plant communities and structural features that support wildlife. Time maintenance activities to avoid sensitive periods such as bird nesting season or peak flowering.

Monitor both drainage performance and biodiversity outcomes. Regular inspections should assess water management effectiveness and habitat condition. This dual monitoring ensures the system continues to deliver against both objectives and allows early identification of issues requiring intervention.

Common Challenges and Solutions

One frequent challenge is convincing stakeholders that biodiverse SuDS are as effective as conventional systems. Address this by presenting evidence from successful projects and emphasising that biodiversity enhancement doesn't compromise hydraulic performance when properly designed. Detailed calculations and modelling demonstrate that vegetated systems often outperform sterile alternatives.

Another concern is maintenance complexity. Whilst biodiverse systems require different management, they're not necessarily more demanding. Develop clear, practical maintenance schedules and provide training for maintenance teams. Many clients find that once established, biodiverse SuDS require less intensive intervention than conventional landscaping.

Securing BNG units from SuDS features requires careful metric application. Work with ecologists familiar with both the Biodiversity Metric and SuDS design to ensure features are correctly classified and assessed. Document design decisions thoroughly to support metric calculations and demonstrate compliance.

The Future Landscape

As both BNG and SuDS requirements become more established, integrated approaches will shift from innovative to standard practice. Developments that embrace this integration early will benefit from streamlined approvals, reduced costs, and enhanced marketability. Buyers and tenants increasingly value developments with strong environmental credentials.

Looking ahead, expect greater emphasis on measurable outcomes. Monitoring requirements for both BNG and SuDS will likely increase, with developers required to demonstrate that features deliver promised benefits. Designing for measurability from the outset (with clear baseline data, defined targets, and practical monitoring protocols) will become essential.

The integration of BNG and SuDS represents a fundamental shift towards multifunctional green infrastructure. By viewing drainage systems as opportunities for biodiversity enhancement, we create developments that are more sustainable, resilient, and valuable. The question is no longer whether to integrate these approaches, but how to do so most effectively.

Taking Action

For architects, developers, and engineers embarking on new projects, the message is clear: consider BNG and SuDS together from day one. Engage ecologists and drainage specialists early in the design process to identify integration opportunities. Challenge conventional approaches and explore innovative solutions that deliver multiple benefits.

The regulatory landscape demands it, environmental imperatives require it, and increasingly, the market rewards it. Developments that successfully integrate Biodiversity Net Gain with Sustainable Drainage Systems will set the standard for sustainable development in the years ahead, delivering environmental benefits that extend far beyond regulatory compliance.