Surface Water Drainage Strategy For Planning

The overall outcome of a sustainable drainage strategy is to control a water source on your development by mimicking natural processes. The end goal is to provide a ‘betterment’ to the area and reduce the impact of your development on the surrounding site.

To satisfy planning, there are many Sustainable urban Drainage Systems (SuDS) that can be used to support achieving this goal. Each solution can be assessed on the 4 key SuDs benefits, but one solution does not need to meet all 4. You can use a variety of solutions together to meet these benefits. The 4 parts to assess the suitability and benefit of a SuDS solution are;

• Water Quantity
• Water Quality
• Amenity
• Biodiversity

The National Planning Policy Framework of 2019 introduced the use of SuDS as a requirement unless it is deemed to be inappropriate for the development.

When determining any planning applications, local planning authorities should ensure that flood risk is not increased elsewhere. Where appropriate, applications should be supported by a site-specific flood-risk assessments. Development should only be allowed in areas at risk of flooding where, in the light of this assessment (and the sequential and exception tests, as applicable) it can be demonstrated that:

a) within the site, the most vulnerable development is located in areas of lowest flood risk, unless there are overriding reasons to prefer a different location;

b) the development is appropriately flood resistant and resilient such that, in the event of a flood, it could be quickly brought back into use without significant refurbishment;

c) it incorporates sustainable drainage systems, unless there is clear evidence that this would be inappropriate;

d) any residual risk can be safely managed; and

e) safe access and escape routes are included where appropriate, as part of an agreed emergency plan.

This new policy means it’s highly encouraged to use sustainable drainage systems, however, there are limited exemptions.

A common exemption would be an extension atop the existing built footprint. If it doesn’t exceed the existing building footprint and will connect into former/existing roof drainage, it can be exempt from needing a new SuDS strategy.

Each application is unique and although this is a common exemption, we would strongly recommend liaison with your Local Planning Authority and Lead Local Flood Authority before proceeding with similar projects.

The main difference between outline planning and full planning is the level of detail that’s required. You need significantly more data and information in a full planning application than an outline plan. Outline planning could also be viewed as a well thought out conceptual plan.

Outline Planning is focused to the concept and principles of drainage. How will the runoff be mitigated post development? Where could the sustainable urban drainage systems be located on the site? How will water be discharged from the site? Planning Authorities will expect these questions to be answered, with surface water drainage calculations and a plan demonstrating the potential future arrangement.

Outline planning may not be as detailed as full planning or pre-construction, but it still has to be implementable and detailed enough to justify the selection and specific sizing of the chosen SuDS (such as tank size, etc).

Due to it being more conceptual, it can lead to more information being requested as planning consent is being issued before permission to start the works can go forward.

Full Planning has much greater detail allows for a works to go forward (provided no conditions have been imposed).

Full planning, when focused to a surface water drainage strategy, evolves the conceptual elements to a level that is near to pre-construction detail. This includes things like pipe sizes, diameters, lengths, rain water pipes, manhole covers, invert levels, discharge points, flow rates, and agreements with LLFAs/water companies. It further details the composition and maintenance of each drainage device or SuDs and creates a fully detailed plan that can be evolved by a contractor once the project progresses to construction.

The National Planning Policy Framework (NPPF) defines a planning condition as, ‘A condition imposed on a grant of planning permission (in accordance with the Town and Country Planning Act 1990) or a condition included in a Local Development Order or Neighbourhood Development Order.’

Conditions are imposed on some applications as an alternative to issuing an objection or refusal of a planning application. These conditions might require additional information or approvals for specific aspects of the development. On occasions there are opportunities to engage with the Local Planning Authority before the application is submitted to broadly agree the conditions that could be imposed and recommended. This saves you and them time and cost as you won’t have to resubmit anything.

Our team are able to progress a planning applications that has been imposed with planning conditions. If a condition has been issued for a drainage strategy it is likely that more information is needed. More information will likely require more surveys to be conducted in order to source the data needed to inform the strategy and discharge the condition.

Our team are able to review your project and the condition imposed to advise whether further surveys could be required whilst working with you on how we can overcome discharging of the conditions.

Each Surface Water Drainage Strategy is unique, however, typically our expert team can produce a surface water drainage strategy within 10 to 20 working days from receiving all necessary data. Much of this time-frame depends greatly on the size of the project, complexity of the site (brownfield, existing connections), environmental constraints (geology, topography, ground water etc.) and the amount of data that is already available. Review of all of these elements is required to allow our experts to cross review and interrogate each element and piece of data in order to provide you with a national planning policy compliant report for planning.

It is the responsibility of the client/applicant to source the majority of the datasets and necessary pieces of information. However, our experts are able to support in sourcing information and supporting you through the project. We have a network of specialists who can provide additional services as and when required.

If you’re looking for a much shorter time frame, our team can provide reports in an accelerated time-frame at an additional cost to meet your requirements. We’re able to provide this rapid service due to our teams experience of supporting our clients and the tried and tested methodologies and approaches.

Seven non-statutory standards published by Defra on 19 June 2025, covering runoff destinations, everyday rainfall, extreme rainfall and flooding, water quality, amenity, biodiversity, and construction, operation and maintenance. Since 17 August 2026, NPPF Policy F8 says SuDS for development that could affect drainage should be designed in accordance with them.

Standard 2 of the National Standards for SuDS (2025) requires that at least the first 5 mm of rainfall for the majority of rainfall events does not run off the site to surface waters or piped drainage. It is usually met through features such as green roofs, permeable paving, rain gardens or rainwater harvesting.

Under Standard 3 of the National Standards for SuDS, the peak discharge for the 50% annual exceedance probability event (roughly a 1-in-2-year storm) should be limited to the 50% AEP greenfield rate or 3 l/s/ha, whichever is greater. For the 1% AEP (1-in-100) event the limit is the 1% AEP greenfield rate or 3 l/s/ha; if the development increases the runoff volume above greenfield, the 50% AEP greenfield rate or 3 l/s/ha applies instead. On previously developed land, a relaxation factor of up to five times the greenfield rate can apply where it is justified and agreed with the approving body.

Often, in a proportionate form. In England, Policy F8 applies to proposals that could affect drainage, and many councils add a drainage condition to a new dwelling. In Wales, a single dwelling needs SAB approval if the construction area is 100 m² or more. In Scotland, a single dwelling and its curtilage are outside SEPA’s SUDS rule, but NPF4 Policy 22(c) still applies at planning.

Usually, where infiltration might work. Infiltration is the second priority in the discharge hierarchy, so a strategy needs evidence on whether it is feasible before moving down the hierarchy. Soakaway testing is normally carried out to BRE Digest 365 (2016 edition).

Only as a last resort. Discharge to a combined sewer is the lowest priority in the English National Standards and the Welsh Statutory Standards. In Scotland, NPF4 presumes no surface water connection to the combined sewer, and Sewers for Scotland states that surface water “will not be permitted” into combined or foul sewers.

Yes, where the construction work covers 100 m² or more, which includes almost all developments of more than one dwelling, and permitted development that builds a structure of that size. SAB approval has been required since 7 January 2019, is separate from planning permission, and must be in place before construction begins.