The publication of the national standards for SuDS (NSfS) on 19 June 2025 introduced stricter requirements for the drainage of surface water on previously developed sites.
For most sites it is no longer necessary to demonstrate the existing discharge rates or volumes from the developed site. This is because the approved discharge rates and volumes will only relate to the greenfield runoff equivalents.
All previously developed sites must demonstrate that they have followed the hierarchy for sustainable drainage as prescribed by standard 1 of the NSfS.
This means that water reuse and infiltration to ground will be prioritised over discharging surface water to a waterbody or piped network.
Where reasonably useable space is likely to limit the potential for infiltration it is suggested that pre-application advice is sought. This enables the applicant to present the design challenges and agree a departure from the national standards in advance of making their application.
This represents the cases where the scale and layout of a development were approved prior to the publication of the NSfS. In these instances, discharge rates and volumes must be reduced to as close to the greenfield runoff equivalents as possible.
It is recognised that the approved scale and layout of the site may prevent the discharge rates and volumes from being restricted to 5 times the greenfield runoff rate or less, or 3l/s/ha. In these cases, the site must restrict rates and volumes as much as practicably possible and demonstrate at least 30% betterment over the existing (developed) equivalents. This is the minimum requirements suggested by The SuDS Manual.
If this approach is followed, then the existing (developed) discharge rates and volumes must be submitted to demonstrate compliance.
If the quantum of the development has been explicitly approved, then the approach shall be the equivalent to that for full or reserved matters applications made before the same date.
If the quantum of development approved is for 'up to' a maximum level of development, then the applicant is expected to adhere to the NSfS wherever possible.
The national standards for SuDS allow for a relaxation factor to be applied to the greenfield runoff rates where evidence is provided that demonstrates why greenfield runoff or 3l/s/ha rates cannot be achieved.
The maximum relaxation factor that shall be agreed is 5 times the greenfield runoff rate. This is a maximum rather than a target. An applicant should aspire to achieve greenfield equivalents or 3l/s/ha, with a relaxation factor only applied where necessary and the scale of relaxation kept to a minimum.
24 October 2025