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Sustainable Drainage Systems
Sustainable drainage systems (SuDS) manage rain close to where it falls, slowing runoff and cleaning it before it leaves your site. We design SuDS for new development in England and Wales, and we prepare drainage strategies for the planning officers and lead local flood authorities who review them. Here, we set out what the 2025 national standards ask of a site and how we design schemes that pass review and keep working once they’re built.
Speak to our sustainable drainage systems team
What sets us apart in sustainable drainage systems
Ground conditions decide what a drainage scheme can do, so we assess them early. On Jaguar Land Rover’s Vehicle Operations facility, early involvement let our engineers identify poor ground and historic tree roots, and our analysis of geo-environmental data produced a lean substructure with no piled foundations or ground slabs. SuDS design needs that scrutiny, because infiltration rates and groundwater set its limits.
Ground conditions decide what a drainage scheme can do, so we assess them early. On Jaguar Land Rover’s Vehicle Operations facility, early involvement let our engineers identify poor ground and historic tree roots, and our analysis of geo-environmental data produced a lean substructure with no piled foundations or ground slabs. SuDS design needs that scrutiny, because infiltration rates and groundwater set its limits.
How SuDS work
What are sustainable drainage systems?
Sustainable drainage systems copy the way undeveloped land deals with rain. A traditional network pipes runoff away as fast as it can, whereas SuDS hold it at or near the surface and release it slowly, letting some soak into the ground on the way.
The four pillars of SuDS are water quantity, water quality, amenity and biodiversity. The national standards expect your design to deliver across all four.
Most schemes link several features in what the industry calls a management train. It’s a bit like a series of sieves, with green roofs, permeable paving, swales, rain gardens and detention basins each taking out some of the flow and some of the pollution, so no single element carries the whole load.
Defra’s national standards favour features that sit on the surface and carry planting. They also ask for attenuation (the temporary storage that slows the flow) at several points across a site rather than in a single feature just before the outfall, unless there’s a justified reason.
England and Wales
Are SuDS a legal requirement in the UK?
It depends which side of the Welsh border your site is on. England hasn’t brought Schedule 3 of the Flood and Water Management Act 2010 into force, so local planning authorities assess drainage as part of the planning application.
Defra published the national standards for sustainable drainage systems on 19 June 2025, and some lead local flood authorities now ask applicants to show how a design meets each of the seven standards.
Wales made SuDS approval a legal requirement on 7 January 2019. Since then, developments of more than one dwelling, or with a construction area of 100 square metres or more, have needed approval from the local authority acting as SuDS Approving Body before work starts. The Welsh Government’s statutory SuDS guidance treats SAB approval as independent of planning permission, so a scheme can hold one without the other and your programme has to allow for both.
The standards in practice
What does good SuDS design involve?
For sites in England, the national standards set most of the design numbers. They start with a five-step hierarchy for where your runoff goes, with reuse on site at the top and a combined sewer as the last resort. They’re also clear that higher cost alone isn’t a reason to drop a step.
The first 5mm of most rainfall events shouldn’t run off your site at all, with a target of 80% of events in summer and 50% in winter. Garden water butts only count towards that target if someone has designed and calculated them as storage.
Where your site discharges to a watercourse or sewer, the peak rate for a storm with a 50% chance of happening in any year is capped at the greenfield runoff rate or 3 litres per second per hectare, whichever is greater. The 1 in 30 and 1 in 100 year design storms also carry the Environment Agency’s upper end climate change allowance, which adds a margin for the heavier rainfall expected in future.
Infiltration depends on the ground. Soakaways can’t take all your runoff where soils drain slower than 1×10⁻⁶ metres per second (roughly 3.6mm an hour), and the base of each infiltration feature needs at least 1m of clearance above the highest likely groundwater level. The standards ask for groundwater levels to be confirmed during high groundwater periods, because readings taken in drier months can sit below the winter peak.
Beyond handover
How do you keep a SuDS scheme working?
Swales silt up and flow controls block without regular upkeep. The standards require the designer to hand over a management and maintenance plan. It’s written for the owners and operators of your scheme, setting out who’s responsible for each feature and how often it needs inspecting, and we produce it as part of our design work.
Adoption shapes the design as well. Since 1 April 2020, sewerage companies have been able to adopt SuDS that meet the definition of a sewer under Ofwat’s Code for Adoption Agreements, and we’ll design with that route in mind where it suits you.
The plan also has to explain what owners should expect to see in different rainfall conditions, including areas designed to hold water for a while, so nobody reports a full detention basin as a fault when it’s doing its job. We also offer training for the teams who’ll run the system.
Planning ahead
Why bring drainage engineers in early?
Drainage shapes the layout of a site, which is why Defra’s standards ask for surface water to be considered at the earliest stages of site appraisal, and for applicants to show compliance from the conceptual stage of a planning application. The Welsh Government’s guidance makes a similar point, saying the best results come when multi-benefit SuDS are first considered at the master planning stage.
Habitat created through SuDS may also count towards biodiversity net gain, subject to the BNG planning requirements.
That said, your site may not be able to meet every standard. Where the type or size of a development makes one impossible, the standards allow a departure agreed with the approving body, as long as the opportunity to meet the standard has been maximised. They also note that large car parks may need manufactured treatment units such as oil separators alongside SuDS, and they aren’t written for retrofit schemes, although they’re a sensible starting point for existing places.
If you’re not sure how the 2025 standards apply to your site, get in touch with our team and we’ll talk it through.
Sustainable drainage systems work in practice
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Here’s the thing about sustainable design: it’s not just about ticking environmental boxes. It’s about creating something brilliant that works for everyone – your budget, your timeline, and yes, our planet too.
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Sustainable Materials
Building tomorrow’s world shouldn’t cost the earth – literally. At Morson Praxis, we’re not just talking about sustainable construction; we’re engineering it, one clever material choice at a time.
At Morson Praxis, we’re the bridge between water’s unpredictable nature and your need for reliable, sustainable solutions. We don’t just manage water – we orchestrate it, turning complex hydrology into practical engineering that actually works in the real world.
Understanding sustainable drainage systems
Quick answers to common questions about sustainable drainage systems and SuDS design.
SuDS and SUDS are two names for the same approach. SUDS stands for sustainable urban drainage systems, and SuDS stands for sustainable drainage systems, which is the term used by Defra’s national standards and CIRIA’s SuDS Manual (C753).
A lead local flood authority is the county or unitary council responsible for managing local flood risk from surface water and groundwater. In England, it’s been a statutory consultee on surface water drainage for major planning applications since 15 April 2015, so on major schemes it’s usually the body that reviews your drainage strategy.
A greenfield runoff rate is the rate at which rain would run off a site in its undeveloped state. Designers estimate it using the Environment Agency’s flood estimation guidelines and soil data confirmed by ground investigation, and discharge from new development is measured against it.
SuDS ponds and basins are designed to manage public safety risk. The national standards call for shallow slopes and stepped margins, known as benching, to allow safe access, and for signs in community spaces that explain each feature’s flood control role and any safety risks.
SuDS design is still workable on clay soils. Under the national standards, permeable paving that drains only its own area and flat unlined detention basins draining up to five times their own area both count towards everyday rainfall interception on any soil type, with the remaining runoff released at a controlled rate.
SuDS design costs are shaped mainly by how much investigation and analysis a site needs. The national standards require geotechnical investigation before infiltration is relied on, and phased developments need drainage designed for each phase, so larger or more complex sites take more design work.
