SuDS Design Guide: Planning, Design, and Approval

What Are Sustainable Drainage Systems?

Sustainable Drainage Systems — universally known as SuDS — are an approach to managing surface water that mimics natural drainage processes. Rather than collecting rainfall in pipes and discharging it as quickly as possible to sewers and watercourses, SuDS use a combination of techniques to slow water down, store it temporarily, allow it to soak into the ground, and improve its quality before it leaves the site.

The concept is simple. Before development, a greenfield site managed rainfall naturally: water soaked into soil, was taken up by vegetation, evaporated, or flowed slowly overland to watercourses. Development replaces permeable surfaces with impermeable ones — roofs, roads, car parks — which dramatically increases the volume and speed of surface water runoff. SuDS are designed to offset this impact, managing the additional runoff so that the development does not increase flood risk downstream or degrade water quality.

SuDS are not a single technology. They are a philosophy of drainage design that uses a range of features — from permeable paving and rain gardens to swales, ponds, and underground storage — assembled into a coherent system that manages water as close to its source as possible.

The SuDS Management Train

The core design principle behind SuDS is the “management train” — a series of stages through which surface water passes as it moves from source to final discharge. Each stage provides additional attenuation, treatment, and amenity value.

Stage 1: Prevention

Reducing runoff at source through good site design. This includes minimising impermeable areas, directing roof drainage to gardens or permeable surfaces, and using water butts for rainwater harvesting. Prevention is the cheapest and most effective form of SuDS, but it is often overlooked.

Stage 2: Source Control

Managing runoff where it falls. Source control features include:

• Permeable paving — for driveways, car parks, and pedestrian areas
• Green roofs — for flat-roofed commercial and residential buildings
• Rain gardens — small planted depressions that collect runoff from adjacent hard surfaces
• Soakaways — underground chambers that allow water to infiltrate into the ground

Stage 3: Site Control

Collecting and managing runoff from multiple sources within the site. Features include:

• Swales — shallow grass-lined channels that convey and filter water
• Filter strips — gently sloping vegetated areas that receive sheet flow
• Detention basins — dry basins that temporarily store water during heavy rainfall
• Below-ground attenuation tanks — geocellular crates or oversized pipes that provide storage where space is constrained

Stage 4: Regional Control

Large-scale features that receive water from the wider site or from multiple sites. These include:

• Retention ponds — permanent water features with additional storage capacity
• Wetlands — shallow vegetated water bodies that provide treatment and biodiversity
• Strategic flood storage areas — large-scale attenuation serving entire developments or catchments

The management train is not a rigid sequence — not every stage is required on every site. The principle is that the more stages water passes through, the better the outcomes for flood risk, water quality, and amenity. A good SuDS design will use multiple features in combination, tailored to the site’s constraints and opportunities.

Planning Requirements

England

In England, the planning system requires SuDS through several policy mechanisms:

National Planning Policy Framework (NPPF): The NPPF states that major developments should incorporate SuDS unless there is clear evidence that this would be inappropriate. The aim is to ensure that surface water drainage systems are designed to manage flood risk and water pollution, considering climate change.

Planning Practice Guidance (PPG): The PPG provides detailed guidance on drainage requirements for planning applications. It establishes the drainage hierarchy — the preferred order of discharge:

1. Into the ground (infiltration)
2. To a surface water body (river, lake, canal)
3. To a surface water sewer or highway drain
4. To a combined sewer (last resort)

Lead Local Flood Authority (LLFA): The LLFA is a statutory consultee for surface water drainage on all major developments (10+ dwellings or sites over 1 hectare). Each LLFA publishes its own local standards and guidance, which supplement the national requirements. These can vary significantly — some LLFAs are far more prescriptive than others.

Key requirements in England:

• Greenfield runoff rates for the developed site (or as close to greenfield as practicable for brownfield sites)
• Attenuation for the 1 in 100 year storm plus climate change allowance
• Water quality treatment — typically assessed using the Simple Index Approach from the CIRIA SuDS Manual
• Exceedance flow management — demonstrating where water goes in a storm that exceeds the design capacity

Wales

Wales is ahead of England on SuDS legislation. Schedule 3 of the Flood and Water Management Act 2010 was commenced in Wales in January 2019, creating a mandatory SuDS Approval Body (SAB) process.

Key requirements in Wales:

• All new developments of more than one dwelling or over 100 square metres of construction area require SAB approval
• SAB approval is a separate consent process, independent of planning permission — both must be obtained
• The SAB sets standards aligned with Welsh Government statutory guidance
• SuDS must be designed to national standards covering water quantity, water quality, amenity, and biodiversity
• The SAB adopts and maintains approved SuDS features at the developer’s expense (through commuted sums)

Scotland

In Scotland, SEPA guidance and Scottish Planning Policy promote SuDS for all new developments. Scottish Water has published Sewers for Scotland standards that include SuDS requirements for adoptable drainage. The approach is broadly similar to England but with some differences in adoption procedures and design standards.

Designing SuDS: Practical Considerations

Infiltration Testing

If the site has permeable geology, infiltration-based SuDS (soakaways, permeable paving with infiltration, infiltration basins) are the preferred approach — they deal with water at source and reduce the volume of discharge to watercourses. However, infiltration must be verified through BRE 365 soakaway tests. These involve digging trial pits, filling them with water, and measuring the rate at which water drains away.

Key points on infiltration:

• Test at the depth and location where SuDS features are proposed
• Multiple tests are needed across the site — infiltration rates can vary significantly
• Clay sites may have negligible infiltration, ruling out soakaways entirely
• High groundwater levels can also preclude infiltration
• Contaminated sites may require lined systems to prevent pollutant mobilisation

Hydraulic Design

SuDS features must be sized to manage the required design storms. In England, this is typically the 1 in 100 year event plus a climate change uplift of 20-45% depending on the epoch and scenario (see our climate change allowances guide). The hydraulic design involves:

• Calculating greenfield runoff rates using methods such as the IH124 or FEH statistical method
• Modelling the drainage network using industry-standard software (such as MicroDrainage or InfoDrainage)
• Demonstrating that the system does not flood for the 1 in 30 year event and manages the 1 in 100 year plus climate change event within acceptable limits
• Sizing attenuation storage to restrict discharge to the agreed rate
• Demonstrating safe exceedance routes for storms beyond the design capacity

Water Quality

SuDS are not only about flow management — they also improve water quality by trapping sediments, absorbing pollutants, and promoting biological treatment. The CIRIA SuDS Manual provides the Simple Index Approach for assessing water quality treatment, which assigns hazard levels to different pollution sources and mitigation indices to different SuDS features.

A typical requirement is to provide sufficient treatment stages to mitigate the pollution hazard of the development. For example:

• Residential roofs have a low pollution hazard and may need only one treatment stage
• Car parks have a medium pollution hazard and typically need two treatment stages
• Commercial yards and loading areas may have a high pollution hazard and need three treatment stages

Amenity and Biodiversity

Modern SuDS design goes beyond engineering. The best schemes create genuine amenity and ecological value — public open spaces with wetland features, tree-lined swales that provide shade and habitat, rain gardens that add visual interest to streetscapes.

Planning authorities increasingly expect SuDS to deliver multiple benefits, not just flow management. Biodiversity net gain requirements further reinforce this, as well-designed SuDS features can contribute to habitat creation and species diversity.

Common SuDS Components

Feature	Typical Application	Key Benefit	Adoption Route
Permeable paving	Driveways, car parks, pedestrian areas	Source control, infiltration, reduces pipe sizes	Management company or highway authority
Swales	Road edges, open spaces, parkland	Conveyance, treatment, amenity	Management company, highway authority, or open space maintainer
Rain gardens	Front gardens, verges, public realm	Source control, treatment, visual amenity	Management company or homeowner
Detention basins	Open spaces, parks, playing fields	Large volume attenuation	Management company or local authority
Attenuation tanks	Below car parks, roads, buildings	Volume storage where space is constrained	Water company (if S104 adopted) or management company
Ponds	Site edges, open spaces	Attenuation, treatment, biodiversity	Management company or local authority
Green roofs	Flat-roofed buildings	Source control, insulation, biodiversity	Building owner

Adoption and Maintenance

Adoption — the question of who maintains SuDS features in perpetuity — remains one of the biggest challenges in SuDS design. Poorly maintained SuDS features can fail, leading to flooding and water quality issues.

In Wales, the SAB adopts and maintains SuDS features that have been approved through the SAB process. Developers pay a commuted sum to cover future maintenance costs. This provides long-term certainty.

In England, adoption routes are more fragmented:

• Water company adoption (Section 104): Pipes, tanks, and some SuDS features can be adopted by the water company under a Section 104 agreement. However, many water companies are reluctant to adopt above-ground SuDS features like swales and ponds.
• Highway authority adoption (Section 38): Drainage within the adoptable highway can be adopted by the highway authority. This typically covers road gullies, pipes, and some highway swales.
• Management company: For features that cannot be adopted by a public body, a private management company funded by a service charge is the default route. This is common for SuDS features within open spaces, communal areas, and private drives.
• Local authority adoption: Some local authorities adopt SuDS features within public open space, but this is not universal and depends on the maintenance agreement.

The key message for developers is to plan the adoption strategy from the outset. Designing SuDS features that no one wants to maintain is a recipe for long-term problems. Discuss adoption with the water company, LLFA, and highway authority during the design stage.

Common SuDS Design Mistakes

1. Designing SuDS as an afterthought. SuDS should inform the masterplan, not be fitted into leftover spaces. A well-integrated SuDS scheme reduces costs and improves outcomes.
2. Ignoring ground conditions. Designing infiltration-based SuDS without infiltration test data leads to redesign during construction.
3. Underestimating maintenance needs. SuDS features require ongoing maintenance. Failing to plan for this leads to neglected, non-functional systems.
4. Over-relying on below-ground storage. Geocellular tanks provide storage but no water quality treatment, amenity, or biodiversity value. They should be a last resort, not the default.
5. Not engaging the LLFA early. Each LLFA has its own preferences and standards. Understanding these before starting detailed design avoids rework.
6. Ignoring exceedance. Designing for the 1 in 100 year event is necessary, but you must also show what happens when that storm is exceeded. Safe exceedance routes are a non-negotiable requirement.

Costs

Contrary to popular belief, SuDS do not necessarily add cost to a development. When designed early, SuDS can reduce overall drainage costs by:

• Reducing pipe sizes and trench lengths
• Eliminating or reducing the need for large underground attenuation tanks
• Reducing the need for pumping stations
• Providing dual-purpose open space that serves both amenity and drainage functions

A 2019 CIRIA study found that SuDS schemes cost, on average, 10-20% less than equivalent conventional drainage solutions. The savings come from replacing expensive below-ground infrastructure with simpler above-ground features.

Where SuDS do add cost, it is typically because they are designed late (requiring site redesign) or because site constraints limit options to expensive below-ground solutions. The message is clear: engage your drainage engineer early and design the site layout around the drainage strategy, not the other way round.

For expert SuDS design and approval support, explore our surface water drainage service or contact our team to discuss your project.