Sustainable Drainage (SuDS) Requirements for London Planning Applications

What SuDS Are and Why London Requires Them

Sustainable drainage systems, commonly abbreviated as SuDS, are drainage features that slow the flow of surface water runoff, allow some water to infiltrate into the ground or evaporate, and reduce the volume and rate of water entering the sewer network from a developed site. Traditional drainage design connected new development directly to the combined sewer network, which has contributed to sewer overflows and surface water flooding in London as the city has grown and more of its surface area has been covered with impermeable paving, roads, and roofs.

The London Plan, which is the strategic planning framework for Greater London, requires all major development and all applications involving significant new hard surfaces to demonstrate that runoff will be managed sustainably. Local planning authorities across the 33 London boroughs have adopted supplementary planning documents that set out specific SuDS standards for their areas, and the Environment Agency is a statutory consultee for drainage-sensitive applications. The general principle required by London planning policy is that post-development runoff rates and volumes should be no greater than the pre-development greenfield runoff rate, which means that new development must demonstrate a significant reduction in the rate at which surface water leaves the site.

SuDS Techniques Applicable to London Sites

The most commonly used SuDS techniques on London residential and mixed-use development sites include permeable paving, green roofs, rain gardens, swales, and underground attenuation tanks. Permeable paving allows water to pass through the surface into a sub-base that stores water temporarily before releasing it slowly to the drain or into the ground. Green roofs retain rainfall in the growing medium and release it slowly through evapotranspiration, significantly reducing peak runoff rates from flat roofs. Rain gardens are planted areas with a specially designed soil profile that allows them to receive and store surface water from adjacent impermeable areas and then release it gradually.

In many parts of inner London, infiltration to ground is not possible because the clay subsoil does not allow water to drain through at a rate sufficient to manage storm events. In these locations, attenuation storage such as underground tanks or oversized pipe runs is used to hold water temporarily and release it to the sewer at a controlled rate that does not exceed the pre-development greenfield rate. The storage volume required depends on the impermeable area of the site, the design rainfall event, and the release rate permitted by the drainage design.

SuDS in Planning Applications for London Householders

For householder applications in London, such as rear extensions, garage conversions, and loft conversions that involve changes to the roof drainage, some London boroughs require a drainage strategy demonstrating that the SuDS hierarchy has been followed. This typically requires considering infiltration first, then attenuation and reuse, and only then a controlled connection to the sewer as a last resort. A simple rear extension in a London terrace that does not significantly increase the impermeable roof area may not trigger a detailed SuDS requirement, but the creation of a new basement or the installation of a large paved area in a front garden may require a formal drainage strategy report.

Prestige Engineers can assist London developers and homeowners with the drainage design required to support planning applications, including CCTV surveys of existing drainage, assessment of infiltration potential, and preparation of surface water drainage strategies that meet the requirements of the relevant London borough planning authority.