SuDS and nutrient neutrality: A combined approach to sustainable housing development

In recent years, legislation and regulatory advice from the Environment Agency have been introduced to protect designated catchment areas across England from the impacts of increased nutrient run-off. Given that watercourses create essential ecosystems – supporting aquatic organisms (fauna and flora), natural habitats and recreation for humans – there is a need to protect them from pollution. Various land uses (agriculture, wastewater treatment and housing developments) have the potential to contribute to nutrient pollution by releasing nitrogen and phosphates into watercourses.

As a solution, and to prevent further deterioration of affected watercourses, new housing developments within the designated catchments must demonstrate nutrient neutrality (i.e., not increase nutrient pollution in the catchment). Where the housing development cannot demonstrate this, the developer must create a nutrient mitigation project (on-site or off-site) or purchase nutrient credits from markets within the catchment to offset the excess pollution. This has made nutrient neutrality a planning permission requirement of new housing developments within certain catchments. 

A housing boom is anticipated in the coming years, so getting nutrient neutrality right is vital to achieving sustainable growth

The limited policy guidance on nutrient neutrality has led to uncertainty for local authorities and developers alike. This came to a head when the government in office at the time sought to introduce an amendment to the Levelling-up and Regeneration Bill (now Act) that would have scrapped the nutrient neutrality obligation for new developers. The amendment was voted down in the House of Lords in December 2023. This prompted the government to reiterate its commitment to providing financial support to enable the creation of nutrient mitigation schemes. A housing boom is anticipated in the coming years, so getting nutrient neutrality right is vital to achieving sustainable growth.

While nutrient neutrality became mainstream, so too did measures around flood management. Sustainable draining systems (SuDS) have now become a requirement for new housing developments in England. This ensures surface water is effectively managed in line with the four pillars of SuDS: water quantity, water quality, biodiversity and amenities.

The first pillar, water quantity, seeks to ensure that the volume of surface water is effectively managed at the source and on-site to prevent flooding. This requires the introduction of drainage systems to reduce the flow rate of surface water run-off and encourage infiltration (via retention basins and permeable paving). The second pillar, water quality, aims to manage the quality of surface water run-off and prevent pollution. This involves the establishment of constructed wetlands, SuDS ponds and swales to function as filtration measures. The third pillar, biodiversity enhancement, benefits habitats via the creation of green spaces (grasslands, woodlands, green roofs, rain gardens and wetlands). This has the added advantage of increasing the value of the development and promoting the fourth pillar, amenities, aimed at recreation and wellbeing for residents.

Ultimately, sustainable drainage ensures effective management of water quality and quantity during construction and throughout the lifespan of the development while promoting biodiversity and wellbeing. Integrating these systems during the design and construction phases of new housing developments prevents the hassles associated with retrofitting existing housing developments.

Given the diverse benefits of SuDS identified above, they can also be designed to promote nutrient neutrality, acting as mitigation measures for new housing developments. Surface water run-off from construction sites usually contains nitrogen and phosphates. 

“Sustainable drainage should be integrated at the design stage of new housing developments”

Sustainable drainage systems can be carefully designed to remove excess nutrients from the catchment. This is why they have been identified as an important nutrient mitigation measure by Natural England, the Department for Environment, Food and Rural Affairs (Defra) and the Construction Industry Research and Information Association (CIRIA).

Sustainable drainage should be integrated at the design stage of new housing developments, ensuring the benefits outlined in the four pillars are achieved from the outset and that the statutory requirements for both SuDS and nutrient neutrality for new housing developments are met. Ideally, SuDS Approving Bodies (SABS), established pursuant to the Flood and Water Management Act 2010, should be equipped with relevant skills to ensure that these systems are designed to prioritise nutrient mitigation.

>>See also: Housebuilders hail ‘very welcome’ shake up of nutrient neutrality rules

This approach has been shown to be effective in promoting water quality management and pollution prevention in new housing development sites in Scotland. In a new housing development site close to Glasgow, surface water from the site is channelled through the sustainable drainage management train, which includes site drainage fitted with gully bags, a SuDS retention pond, grassland and swales. This ensures that site run-off entering the drainage is filtered through the SuDS management train before being released into a nearby watercourse.

Although the water entering the site drainage may be silty and cloudy, having been filtered through the several levels of the SuDS management train, clear water is released into the watercourse. This demonstrates the effectiveness of sustainable drainage systems in tackling pollution, including nutrient pollution from housing.

The methods used on this site, such as vegetated grassland and swales, also provide habitats for wildlife. When construction work is complete, the SuDS pond will become a wetland supporting various species of plants, invertebrates, birds and other animals. It could also become an aesthetic feature of the area for residents.

This combined approach has many benefits. Not only does it solve flood and surface water management issues faced by developers but also it secures the wider goals of achieving improved water quality and environmental protection. In addition, where sustainable drainage is designed as a nutrient mitigation measure, this can address any shortfall in the nutrient neutrality requirement for new developments and save developers from purchasing nutrient credits.