SuDS in Flood Areas

Local Planning Authority Position Statement on Sustainable Drainage Systems in Flood Areas 

This position statement applies to all proposed development, irrespective of scale or previous land use. For the purposes of this position statement, the term Flood Areas refers to fluvial Flood Zone 2, Flood Zone 3a, Flood Zone 3b and any area at risk of surface water flooding in the 0.1% annual exceedance probability [AEP] event. If related national guidance or local policy becomes available then this Position Statement will be updated accordingly or revoked.

There should be no siting of sustainable drainage systems in Flood Areas, as during a flood event this would result in the loss of either Flood Zone or surface water SUDS storage. Meaning the features would be inundated by flood water prior to runoff entering them, or vice versa, thereby rendering them ineffective. It also has the possibility of altering/blocking conveyance routes and flow paths.

SuDS will only be considered in Flood Areas where the site is entirely within those Areas.  On all other sites, all SuDS storage features must be located within Flood Zone 1 and outside of any area of surface water flood risk.  It is expected that the layout of a proposed development is designed with this in mind.

Exceptions

In certain circumstances, SuDS storage features could potentially be located in Flood Areas. For example:

  • when the Flood Zone has been modelled to show that the SuDS area will not flood during the 1% AEP plus climate change event. The modelling needs to be agreed with the Environment Agency and appropriate climate change figures used within the modelling.
  • SuDS are allowed in tidal Flood Zones where the risk is isolated to tidal sources and there is no other flood risk. For example, no additional risk from watercourses or surface water.
  • SuDS are permitted in fluvial Flood Zones where all of the following criteria is met:
    • the applicant has demonstrated that area is defended for the lifetime of the development. For example, the site is protected by existing Environment Agency or Local Authority Schemes. The standard of protection and asset condition must be evidenced.
    • the features are not located in an area at risk of surface water flooding.
  • Conveyance SuDS can be sited in Flood Areas as ultimately they need to discharge into a watercourse.
  • where an area of surface water flood risk can be evidenced to only receive flow from within the site boundaries.
  • SuDS storage features within a proposed building's footprint. For example, green or blue roofs or tanked storage elsewhere within a structure.
  • where sealed underground attenuation tanks demonstrate that they can be operational during flood events and are maintainable.

Further information

Where SuDS are proposed in Flood Areas they may require erosion protection measures as well as enhanced maintenance regimes to ensure the features are 'fit for purpose' after flood events. Any control structures should be suitable for submerged conditions. Consideration should also be given to the impact on the banks as well as third party land.

Any Flood Area displaced will need to be identified and compensated for within the design.

Under no circumstances will we accept SuDS in flood storage areas as these areas are essentially designated functional floodplain (Flood Zone 3b). Nor will we accept SuDS in Flood Compensation Areas.

Page last updated

24 October 2025