Traditional drainage focuses on getting rainwater off site as quickly as possible through pipes and sewers. This approach increases flood risk downstream and does nothing for water quality or the environment.

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Sustainable drainage systems take a different approach—managing rainfall close to where it lands, mimicking natural drainage processes. SuDS slow runoff down, provide treatment, and often store water temporarily before controlled release. Along the way, they deliver benefits you don't get from conventional pipework: improved water quality, biodiversity gains, and amenity space people actually want to use.

Pretty much, yes. The NPPF requires major developments to incorporate sustainable drainage unless there's a clear reason not to. Many local planning authorities now require SuDS for smaller schemes too, particularly in areas with known drainage issues or high groundwater.
 

The key test is whether you're creating new impermeable area that will generate surface water runoff. If you are, you'll need to demonstrate how you're managing that water sustainably. Even single dwelling extensions or changes of use can trigger SuDS requirements depending on local policy.

The most frequently specified SuDS components include:

• Permeable paving – block paving or porous asphalt that lets water soak through into sub-base storage rather than running off

• Attenuation tanks or crates – underground storage that holds water temporarily and releases it slowly at controlled rates

• Swales – shallow vegetated channels that convey and treat surface water whilst adding green infrastructure

• Rain gardens and bioretention – planted depressions that store, treat and infiltrate runoff whilst looking good

• Green roofs – vegetation on roof surfaces that reduces runoff volumes and provides insulation benefits

• Filter drains – gravel-filled trenches that collect and convey water whilst providing treatment

The right solution depends on your site's ground conditions, available space, adoption requirements, and maintenance arrangements.

Yes, but the design approach changes. Sites with clay soils or shallow groundwater can't rely on infiltration, so we look at alternatives:

• Above-ground attenuation in landscaped basins or ponds

• Underground storage tanks with controlled discharge to sewers or watercourses

• Blue-green features like swales that provide temporary storage and slow conveyance

• Green roofs and rainwater harvesting to reduce runoff volumes at source

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Clay soils actually aren't always a problem—they can support excellent swales and wetland features precisely because they hold water. The key is designing for your site's actual conditions rather than forcing infiltration where it won't work.

Maintenance responsibility depends on the adoption route:

• Water company adoption (Section 104) – For residential developments, sewerage undertakers increasingly adopt SuDS under Section 104 of the Water Industry Act, taking on long-term maintenance responsibility. This requires designs that meet their technical standards.

• Highway adoption (Section 38) – Highways authorities can adopt some SuDS that form part of highway drainage, such as swales or filter drains alongside roads.

• Private management companies – Many commercial developments and some residential schemes establish management companies funded through service charges.

• Individual property owners – Permeable paving on private driveways, for instance, remains the homeowner's responsibility.

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We specify maintenance requirements clearly in our strategies, including access needs, inspection frequencies, and typical maintenance tasks. Good design minimises maintenance burden from the outset.

It varies significantly depending on site constraints, but SuDS aren't necessarily more expensive than conventional systems. In many cases, they actually cost less.

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Above-ground features like swales or basins can be cheaper than deep pipe networks, especially on sloping sites.

Permeable paving costs more per square metre than standard paving, but you save on gullies, pipes, and manholes.

Underground storage tanks cost more upfront but take up less space.

 

The real saving often comes from reduced sewer connection fees. Water companies charge substantially for new connections to combined sewers. Reducing or eliminating that connection through on-site infiltration or attenuation can more than offset SuDS costs.

 

Long-term maintenance costs tend to be similar overall—SuDS swap reactive blockage clearing for planned vegetation management and periodic sediment removal.

Not necessarily. Space requirements depend on the design approach and site characteristics.

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Underground storage (tanks or crates beneath car parks or landscaping) takes minimal surface space. Permeable paving serves dual purposes—vehicle access or pedestrian areas plus drainage—so needs no additional space at all.

 

Above-ground features like basins or ponds do need surface area, but they're increasingly designed as attractive landscape features that enhance rather than detract from developments. A well-designed attenuation basin doubles as a meadow, wildflower area, or informal play space for much of the year.

 

For tight urban sites, we focus on multi-functional solutions: green roofs, permeable surfaces, tree pits, and bioretention planters integrated into the streetscape. Even brownfield sites with contaminated ground can incorporate SuDS through above-ground or lined systems.

Good SuDS design addresses four objectives simultaneously:

1. Water quantity – Controlling runoff volumes and peak flow rates to prevent flooding

2. Water quality – Treating pollutants before they reach watercourses or groundwater

3. Amenity – Creating attractive, useable spaces that enhance the public realm

4. Biodiversity – Providing habitat for wildlife and supporting ecological networks

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Meeting all four pillars means SuDS deliver multiple benefits rather than just shifting water around. That's why planning authorities increasingly expect designs that demonstrably address each pillar, not just flood risk reduction in isolation.

The drainage hierarchy sets out the preferred order for disposing of surface water:

1. Infiltration to ground – The most sustainable option where ground conditions allow

2. Discharge to watercourse – Second preference, subject to appropriate treatment and controlled flow rates

3. Discharge to surface water sewer – Third option if above aren't feasible

4. Discharge to combined sewer – Least preferred, only acceptable when other options are demonstrably not viable

 

Planning authorities expect drainage strategies to work down this hierarchy systematically, with clear evidence why higher options can't be used. You can't just default to a sewer connection because it's easier.

Good SuDS design considers a range of storm events from frequent small rainfalls up to extreme events. The approach works on two levels:

• Primary drainage – Sized to handle typical design storms (usually 1 in 30 or 1 in 100 year events plus climate change allowance) with controlled discharge or infiltration

• Exceedance flow routes – For events that exceed the primary system's capacity, safe overland flow paths direct water away from buildings and critical infrastructure

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We model multiple scenarios to ensure the development remains safe even when design storms are exceeded. That means identifying exceedance routes across the site and to receiving watercourses or highways, ensuring flood depths and velocities remain acceptable.

• Yes, though it's more challenging than incorporating them into new builds. Retrofit opportunities include:

• Converting areas of car parking to permeable surfaces

• Creating rain gardens or bioretention areas in existing landscaping

• Disconnecting roof drainage from sewers and directing it to planters or soakaways

• Installing green roofs during re-roofing works

• Repurposing underused amenity space as multi-functional attenuation features

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• Retrofit SuDS are increasingly popular with local authorities looking to reduce sewer flooding, improve water quality, or deliver biodiversity net gain across their estates. The key is identifying opportunities where drainage improvements align with other planned improvements—resurfacing, landscaping upgrades, car park reconfigurations.

To prepare a drainage strategy, we typically need:

• Site layout plans showing buildings, access roads, landscaping, and levels

• Topographical survey with spot levels or contours

• Ground investigation data, particularly infiltration test results if available

• Details of any watercourses or drainage features on or adjacent to the site

• Information on existing drainage infrastructure and available connection points

 

Any discharge rate restrictions or requirements from the local authority or water company

If you don't have all of this yet, that's fine—we can advise what's needed at different project stages and work with outline information initially, refining the strategy as more detail becomes available.

For straightforward sites with good information, we typically deliver drainage strategies within 10-15 working days. More complex sites—brownfield locations, contaminated land, or schemes with multiple discharge points—may take 3-4 weeks.

 

The critical path is usually infiltration testing. If ground investigations haven't been done yet, that adds time. We can prepare outline strategies based on BGS borehole data and desk study information, then refine once actual site investigation results are available.

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If you're working to tight planning submission deadlines, let us know upfront. We can often fast-track reports or work in parallel with other consultants to keep programmes on track.

If you've got questions we haven't covered here, or you're ready to discuss your site's specific drainage requirements, get in touch. We're happy to have an initial conversation about your project at no charge—it helps us give you an accurate quote and means you get advice tailored to your circumstances rather than generic guidance.

 

Contact us to request a quote or discuss your development's sustainable drainage strategy.