Advice for applicants, agents and developers

Urban environments can cause more rainwater to run off hard surfaces instead of soaking into the ground naturally. The increased runoff can lead to flooding by overloading existing drainage systems or raising groundwater levels. Development can also cause soil erosion and increase water pollution because pollutants from the urban environment flow into watercourses and the ground.

To make sure that development does not increase flood risk or harm the environment, it is important that sustainable drainage methods are used. These are known as 'Sustainable Drainage Systems,' referred to as SuDS.

In the development industry we talk about four key pillars of SuDS design:

  • water quantity,
  • water quality,
  • amenity (attractive spaces for people to visit), and
  • biodiversity (habitats for plants and wildlife).

Water quantity is often thought about first, however it is important to think about the other pillars and how to include them together in the design. This means that their benefits can be combined in meaningful and beneficial way.

Why SuDS should Be Considered Early in the Design Process

National planning policy and our Local Plan require SuDS to be considered early in the design process  The benefits of this are:

  • better design outcomes, 
  • wider environmental benefits,
  • an efficient design, 
  • less land take; making the land work harder by giving it more than one purpose,
  • less chance of refusal of any planning application, and
  • costs savings by avoiding delays, redesigns, and budget overruns.

Carrying out site and ground investigations before development starts makes it easier to get accurate results and plays an important part in the drainage design. Similar investigations are often needed for other elements such as foundation design and to consider contamination. Detailed investigation and reporting make the results more useful and reduce costs overall. If it is found early that water can soak into the ground naturally (infiltration), the size and scale of the SuDS can inform the initial development layout.

When to consider SuDS design

It is sensible to think about how a site will drain before buying the land and at least in the early layout design stages. Where drainage options are limited, this can affect what can be built on a site, which will affect costs and ultimately profits.

Detailed design is not expected from the beginning. However, what you need to understand and show early on, is where the rainwater is going and how much storage space you need for it. As you progress your early designs, they will become more detailed and ultimately reach a standard where drawings can be used for construction purposes.

How SuDS impact scale and layout

The space needed for rainwater storage depends on how you drain the site, for example, to ground, to a piped network, or to a water body. Designers cannot simply select the cheapest drainage option; they must prioritise options in accordance with national guidance. This is important because different drainage methods and ground conditions will result in different designs.

Efficient SuDS fulfil more than one benefit. Combining benefits can lead to different design options. For example, a SuDS pond can provide water storage, treatment, amenity, and biodiversity benefits, but they take up space in a development layout and impact the public open space. In contrast, an underground tank does not take up surface space on the site but only provides storage. Tanks like this are discouraged as they do not provide multiple benefits and therefore more features must be provided elsewhere on the site to make up for this.

It is important to find out early if your site can soak water into the ground (infiltration). The only way to accurately do this is by monitoring the groundwater levels and testing the infiltration rate of the ground, at appropriate times of year. If groundwater is high or infiltration rates are slow then the area needed for storage will increase. Unsuitable ground conditions can also rule out an infiltration scheme and in that case other ways of draining the site may have to be explored. Your designer should be familiar with these investigations and help you to organise them.

Existing drainage features like watercourses and pipes should also be considered as these often have legal buffers (easements) which cannot be built within.  It is particularly important to work out if there are any pipes on the site early on, as consent may be needed to make any alterations to them or build over them.  

Working together to achieve the best design

Working with key partners is very important and can help create SuDS features that achieve multiple purposes. By working together and combining different goals relating to planning policy, a more efficient and sustainable development is created. For example, ecologists can help identify current habitats and the wildlife present on site which require protection. SuDS features and landscaping can then be designed to provide new and improved habitats that complement this.

Why SuDS cannot always be dealt with by Planning Condition

SuDS can have a major impact on the scale and layout of a development, as discussed elsewhere in this guidance. This may impact where buildings, structures and other features could be located on the development. The position of SuDS depends entirely on how and where the water will be drained. This means that an acceptable SuDS design cannot always fit into a development layout that has not considered drainage early on.   

Due to the strong link between the layout of SuDS and the layout of development, it is not possible to consistently secure one by condition without the other also being known. This applies to all applications except outline applications - where the development layout is not fixed. Important aspects such as infiltration potential, groundwater levels and where runoff will go should be submitted with a drainage design to support a full or outline planning application.

If there aren't many options for where to drain water to, this can seriously affect whether the land can be developed. It would not be appropriate for us to approve a scheme which has not been shown to be able to be adequately drained, or where changes to the drainage of the site could lead to changes in the scale and layout of the development.

Where a site has been previously developed

In the past national guidance was less strict for brownfield (or previously developed) sites. This was because redevelopment often improved drainage compared to the old site, which might have had poor drainage or be increasing flood risk.

However, this ignored how much worse the brownfield runoff may be compared with the expected greenfield scenario. National guidance now expects brownfield sites to base their SuDS designs on the greenfield conditions, which can be relaxed to a degree. These relaxations often result in runoff rates and volumes that are still much lower than would have been allowed in the past. This can have a bigger impact on smaller or restricted sites.

It is important that developers of brownfield sites check how the site is currently drained. This will not help determine the storage that will be needed for the new site or always affect the future SuDS design. However, it is important to understand the existing drainage regime and whether this affects the SuDS design or the development layout.

Our Advice has Changed

In May 2024, we updated our guidance to mirror advice from the Lead Local Flood Authority and national planning policy. This change ensures that SuDS proposals meet both local and national guidance. The revised view has since been tested at appeal and Planning Inspectors have agreed with our reasons for refusal in cases where SuDS schemes were not fully proven. This supports the importance of submitting detailed and policy-compliant drainage strategies early in the application process.

We offer a range of detailed surface water drainage design guidance on these webpages.   These will support your designer so that you can be more confident in submitting an application which meets our requirements and national standards.  

Page last updated

2 December 2025