1 Overview
Managing surface water helps reduce flooding and keep water clean. It also helps make sure new buildings and developments are safe and can cope with future changes in the weather.
As the Lead Local Flood Authority (LLFA), West Sussex County Council provides legal advice on surface water drainage for major planning applications in West Sussex.
Documents you submit to the Local Planning Authority (LPA) for surface water flood risk should follow the guidance and standards below:
- National Planning Policy Framework
- Flood risk and coastal change PPG
- National standards for sustainable drainage systems (SuDS)
- Local Plan for the relevant LPA
- Strategic Flood Risk Assessment (SFRA) for the relevant LPA
- Neighbourhood Plan
- any relevant Supplementary Planning Document (SPD) for the LPA such as:
- surface water drainage specific documents
- urban design documents
- landscaping documents
We will update this guidance regularly to reflect any changes to national or local policy and standards.
Pre-application advice
The LLFA runs a Flood Risk Management pre-application advice service. This provides guidance on surface water drainage and local flood risk. Further information, including how to apply, is available on our Flood Risk Management Pre-application advice page.
2 What a Flood Risk Assessment is and when it must happen
A Flood Risk Assessment (FRA) explains the risk of flooding at a development site. It shows how the site will manage flooding during and after development. It must also show that the development will not increase flood risk in nearby areas.
The assessment should also consider how climate change could affect flood risk, and how vulnerable the development is to flooding. It should explain how the development could help reduce flood risk overall. The assessment should follow the National Planning Policy Framework (NPPF) and Planning Practice Guidance (PPG).
A site-specific FRA must happen when development sites:
- are in River and Sea Flood Zones 2 and 3
- exceed 1 hectare or more within Flood Zone 1
- on land that the Environment Agency has identified as having critical drainage problems
- on land identified in a strategic flood risk assessment as being at increased flood risk in the future
- on land that may be subject to other sources of flooding (including surface water, groundwater and artificial sources), where development would introduce a more vulnerable use
To assess current and future surface water flood risk, you must use the Environment Agency’s flood map for planning. These include climate change allowances appropriate for planning purposes.
What must happen in an FRA
You must consider all sources of flooding (both present day and future) for all development proposals. You must assess future flood risk by considering the impact of climate change.
This includes flooding from:
- main rivers
- ordinary watercourses
- surface water
- groundwater
- the sewerage system
- reservoirs
- other artificial sources
The assessment must identify and assess where surface water could flow across the site. Groundwater flooding can be hard to predict and manage and effects can last for a long time. To assess the risk of groundwater flooding, you need information about the site, including the results of ground investigations.
If a nearby reservoir or canal is higher than a development site, you must assess breach scenarios. Where development would result in a loss of flood storage, on-site level-for-level compensation is required when there is:
- loss of floodplain storage (the LLFA should check this from ordinary watercourses, and any associated surface water flood risk)
- deflection or constriction of surface water flow paths
- inadequate management of surface water
This is because development in these areas can displace floodwater and increase flood risk within the site or elsewhere.
Some applications may need a 'sequential' or 'exception' test. If the test is required, the application must pass this test before an FRA is carried out. The LLFA are not responsible for reviewing the tests. We recommend contacting the LPA for further assistance on this matter.
You must apply the sequential approach where a site is at risk from any source of flooding. This means locating development in areas of no or low flood risk wherever possible, especially for the most vulnerable uses. You must consider flood risk early in the planning process to help ensure the site layout is suitable.
To show there is safe access for the lifetime of the development, we recommend using flood risk assessment guidance for new development.
For information on measures to reduce flood risk, we recommend using CIRIA's code of practice for property flood resilience and the Environment Agency standing advice for selecting finished floor levels.
The following websites give further guidance on preparing FRAs:
4 Sustainable drainage systems (SuDS)
Sustainable drainage systems (SuDS) manage surface water runoff at its source and close to the surface. They copy natural drainage processes and provide a range of benefits, including:
- managing surface water runoff rates and volumes
- water quality treatment
- amenity and biodiversity improvements
These benefits help developers to create sustainable developments that can manage flood risk. Traditional piped drainage systems do not achieve the same benefits.
We consider the following features most effective at achieving the outcomes set out in the National Standards for SuDS:
- green roofs
- swales
- rain gardens or bioretention areas
- pen storage features (basins)
- permeable surfacing
Following the July 2025 update, the county council expects all major planning applications to comply with the standards. If an application does not meet the standards the Local Planning Authority (LPA), as approving body, must agree to the departure. As the Lead Local Flood Authority (LLFA), we advise LPAs when applicants do not follow the standards.
Developers should consider SuDS early in the planning process to avoid conflict with other land uses. We will not accept high density housing or lack of space will not be accepted as reasons to exclude source control SuDS.
The following resources give further guidance on incorporating SuDS into major planning applications:
- CIRIA - Getting SuDS right from the start
- CIRIA - SuDS manual
- LPA design guidance provided by your local district or borough council
Complying with the National Standards for SuDS
Different types of planning application require different levels of information. As LLFA, we have produced guidance to help applicants consider SuDS fully throughout the planning process.
Complying with the National Standards for SuDS (PDF, 588KB)To make sure you address the National Standards for SuDS adequately, you can download the major development template from CIRIA.