Sustainable drainage systems (SuDS) address critical problems in society that traditional urban drainage methods couldn’t address. How it does this is simple – it controls peak runoff rates, increases attenuation capacity, and reduces pollutant loads reaching receiving waterbodies

As cities grow, so do their activities, and water demand would also increase. As you’d expect, there will be a need to create a more efficient drainage system to channel contaminated water away from the cities. This was a problem traditional drainage systems couldn’t solve, hence posing to the implementation of sustainable drainage systems. Fortunately, this guide aims to explain extensively the key principles and benefits of sustainable drainage systems.

Types of SuDS Systems

SuDS encompasses a range of individual components, each suited to specific site conditions, space constraints, and drainage objectives. Components are grouped into three categories — source control, site control, and regional control — reflecting where in the catchment they operate.

Source Control

Source control components manage rainfall at or close to the point where it falls. They reduce the volume and peak rate of runoff before it enters the drainage network.

Permeable Paving

Permeable paving replaces impermeable surfaces with porous materials that allow rainfall to pass through into a sub-base reservoir, from which it infiltrates to ground or is attenuated and discharged at a controlled rate. Types include porous asphalt, permeable block paving, and reinforced grass or gravel surfaces. Permeable paving is effective in car parks, driveways, and lightly trafficked roads.

Green Roofs

A green roof replaces a conventional roof covering with a layered system of drainage membrane, substrate, and vegetation. Rainfall is retained in the growing medium, reducing peak runoff and delaying discharge. Extensive green roofs (substrate depth 60–200 mm) are used where structural loading is limited and drainage performance is the primary objective. Intensive green roofs carry a deeper substrate, support a wider range of planting, and provide greater retention capacity but impose higher dead loads on the structure.

Site Control

Site control components convey and treat surface water across the development, managing flows between the point of generation and the site boundary.

Swales

A swale is a shallow, vegetated channel that conveys surface water runoff at a controlled rate. The vegetated base and sides treat runoff through sedimentation and biological uptake as flows pass through. Swales are designed to infiltrate where soils are permeable or to attenuate and discharge to a downstream watercourse where they are not.

A swale conveys and treats surface water runoff through vegetated channels, combining attenuation with pollutant removal.

Filter Strips

Filter strips are gently sloping, vegetated areas positioned at the edge of an impermeable surface. Sheet flow from roads, car parks, or roofs passes across the strip and is treated by sedimentation and filtration before entering the next component in the treatment train. Filter strips require low gradients (typically 1:100 to 1:33) and are most effective as a pre-treatment stage upstream of a swale or bioretention cell.

Bioretention Cells and Rain Gardens

Bioretention cells are shallow, vegetated depressions filled with an engineered growing medium. Runoff enters the cell, passes through the substrate — removing pollutants through filtration, adsorption, and plant uptake — and either infiltrates to ground or drains through an underdrain at a controlled rate. At a smaller, residential scale the same principle is applied as a rain garden. Bioretention cells perform well in constrained urban sites where the footprint available for conventional detention basins is insufficient.

Infiltration Trenches and Soakaways

Infiltration trenches are subsurface structures filled with permeable granular material or modular crates that receive runoff and allow it to infiltrate into the surrounding soil over time. Soakaways operate on the same principle in a more compact form. Both require a minimum 1 m clearance between the base of the feature and the seasonal high groundwater table, and are only viable where site permeability testing (BRE365 or falling head method) confirms an infiltration rate above 1 × 10⁻⁶ m/s.

Regional Control

Regional control components sit at the downstream end of a site and provide final attenuation and treatment before surface water discharges off-site.

Detention Basins

Detention basins are dry depressions designed to hold runoff temporarily during and after a storm event, releasing it at the permitted greenfield discharge rate through a flow control structure. Between storm events the basin is dry and can be designed as recreational green space or wildlife habitat. The basin volume is sized for the critical storm event — typically the 1-in-100-year, 6-hour storm with climate change applied.

Retention Basins and Ponds

Retention basins maintain a permanent pool. The standing water provides settlement of fine suspended solids and contributes to the biodiversity and amenity objectives of the scheme. The permanent pool volume is calculated separately from the attenuation volume required to manage the design storm. Retention ponds are positioned downstream of pre-treatment components to prevent rapid siltation of the permanent pool.

Constructed Wetlands

Constructed wetlands are shallow, permanently wet features planted with emergent aquatic vegetation. They provide the highest water quality treatment of any SuDS component, removing suspended solids, nutrients, and hydrocarbons through sedimentation, plant uptake, and microbial action. They are positioned at the downstream end of the treatment train, upstream of the final discharge point.

Hybrid SuDS and the Treatment Train

Most SuDS schemes combine components from more than one category in sequence — a treatment train. Runoff passes through source control first, reducing the volume reaching site control features, which in turn reduce the load on regional control structures. Each stage targets a different pollutant fraction: coarse sediments settle in basins; fine particles and dissolved contaminants are removed in bioretention cells and constructed wetlands.

Hybrid SuDS schemes integrate conventional piped drainage with above-ground SuDS features where site constraints — restricted footprint, shallow utilities, or existing development — make a fully surface-based system impractical. The drainage hierarchy still applies: above-ground SuDS components are preferred at each stage, with below-ground attenuation used only where surface space is not available.

SuDS Design Principles

SuDS design is governed by four core objectives, as defined in the CIRIA SuDS Manual (C753). Every component selection, sizing calculation, and layout decision maps back to these objectives.

The Four Design Objectives

Water quantity controls peak runoff rates and total runoff volumes. The target is to match pre-development (greenfield) conditions for the 1-in-100-year storm event, with a minimum 40% climate change allowance applied to rainfall inputs.

Water quality treats surface water runoff before it reaches a watercourse or groundwater body. SuDS components remove pollutants through sedimentation, filtration, adsorption, and biological uptake — progressively finer treatment is achieved by combining components in series.

Amenity means SuDS features function as multifunctional spaces. A correctly specified swale or retention pond delivers the required attenuation volume and operates as accessible green infrastructure within the development.

Biodiversity is built into SuDS layout from the outset. Planted swales, wetlands, and open water features support invertebrate and bird populations and create habitat connectivity that conventional below-ground drainage does not provide.

The SuDS Design Process

Stage 1 — Site Assessment

The design process begins with a full baseline assessment covering:

• Soil type and permeability, tested using BRE365 or a falling head infiltration test — permeability class determines whether infiltration-based SuDS (soakaways, infiltration basins) are viable
• Topography and available gradient to the receiving watercourse or discharge point
• Groundwater depth — a minimum 1 m clearance between the base of any infiltration feature and the seasonal high groundwater table is required
• Flood risk designation and any discharge constraints imposed by the lead local flood authority (LLFA) or Environment Agency

Stage 2 — Drainage Hierarchy

Site assessment results determine which disposal route is technically feasible. The drainage hierarchy requires surface water to infiltrate at source before discharging to a watercourse, then a surface water sewer, and finally a combined sewer as a last resort. LLFAs require applicants to demonstrate why a preferred hierarchy option is not achievable before accepting a lower-preference route. The full four-level hierarchy — with technical thresholds, consent requirements, and how to apply it across a site — is set out in the SuDS Drainage Hierarchy section below.

Stage 3 — Design Calculations

Two outputs drive SuDS sizing:

• Greenfield runoff rate — the allowable discharge rate for the completed development, expressed in litres per second per hectare (l/s/ha), calculated from pre-development site characteristics
• Required attenuation volume — the storage volume needed to hold the difference between developed runoff and the greenfield allowable rate, sized for the critical storm duration

Calculations are run for the 1-in-1-year, 1-in-30-year, and 1-in-100-year return periods with climate change factors applied. Software commonly used for these calculations includes InfoDrainage, MicroDrainage, and WinDes.

Stage 4 — Treatment Train

A single SuDS component rarely meets all four design objectives. The treatment train approach sequences multiple components across a site: source control measures (permeable paving, green roofs) intercept runoff at the point of generation; conveyance features (swales, filter strips) transport and treat flows across the site; site management features (detention basins, constructed wetlands) provide final attenuation and polishing before discharge. Each stage removes a different pollutant fraction and contributes to the overall water quality target.

The SuDS Drainage Hierarchy

The drainage hierarchy sets the order of preference for surface water disposal on any new development site in England and Wales. It is referenced in the National Planning Policy Framework (NPPF), the CIRIA SuDS Manual (C753), and lead local flood authority (LLFA) technical standards. LLFAs use the hierarchy to assess drainage strategies submitted with planning applications, and will reject a strategy that proposes a lower-preference disposal route without evidence ruling out the options above it.

The hierarchy has four levels, applied in order of preference from top to bottom.

Level 1 — Infiltration at Source

The preferred disposal route is infiltration of surface water into the ground at or near the point where it falls. Infiltration-based SuDS — soakaways, infiltration trenches, infiltration basins, permeable paving — achieve this without off-site discharge.

Infiltration at source is only viable where all three of the following conditions are met:

• Permeability testing (BRE365 or falling head method) confirms a soil infiltration rate at or above 1 × 10⁻⁶ m/s
• The seasonal high groundwater table sits at least 1 m below the base of the proposed infiltration feature
• The site does not fall within a Source Protection Zone (SPZ1 or SPZ2), where infiltration of untreated runoff poses a groundwater contamination risk

Where any of these conditions cannot be met, the drainage strategy documents this with supporting test data and evidence before proceeding to Level 2.

Level 2 — Attenuation and Discharge to a Surface Watercourse

Where infiltration is not feasible, the next preference is to attenuate runoff on site and discharge at the permitted greenfield rate to an adjacent ordinary watercourse — a ditch, stream, or non-main river. Discharge rates and volumes are agreed with the LLFA.

Where the receiving watercourse is a main river, the Environment Agency is an additional statutory consultee. A formal ordinary watercourse consent is required for any new outfall structure, and a flood risk activity permit is required if the works fall within 8 m of a main river.

Level 3 — Attenuation and Discharge to a Surface Water Sewer

Where a suitable watercourse connection is not available, attenuated discharge to an adoptable surface water sewer is the third preference. The sewerage undertaker (Thames Water, Severn Trent, etc.) sets the permitted discharge rate. Both the LLFA’s greenfield rate requirement and the sewerage undertaker’s network capacity constraint apply — the more restrictive of the two governs the design.

Pre-application engagement with the sewerage undertaker is required at this level to confirm that capacity exists in the receiving network before the design proceeds.

Level 4 — Discharge to a Combined Sewer

Discharge to a combined sewer — carrying foul and surface water in a single pipe — is the last resort. It is only accepted where Levels 1, 2, and 3 have each been demonstrated to be technically infeasible with supporting evidence. Combined sewer discharge increases the risk of combined sewer overflow (CSO) activation during storm events, releasing diluted sewage to watercourses. Sewerage undertakers and LLFAs require robust justification before accepting this route.

Applying the Hierarchy Across a Site

The hierarchy is not a single decision for the whole site — it is worked through for each surface catchment independently. A development may infiltrate roof runoff via soakaways (Level 1) and discharge car park runoff via a swale to a watercourse (Level 2). Each disposal route is justified separately in the drainage strategy report.

The drainage strategy submitted with a planning application sets out the hierarchy assessment for every catchment area on the site, supported by permeability test results, watercourse survey data, and written confirmation of sewer capacity from the sewerage undertaker. What that document contains in full is set out in the SuDS Drainage Strategy section below.

Benefits of Sustainable Drainage Systems

1. Flood Risk Reduction

The most obvious benefit of sustainable drainage systems is reduced flood risks in areas prone to flooding. For instance, in areas with heavy water usually, SuDS helps to reduce flooding and prevent basement flooding that could occur as a result of accumulated water.

2. Reduce Waterborne Diseases

SuDS also reduces waterborne diseases because it practically removes stagnant water from the environment. Remember that stagnant water can breed the larva of disease-carrying insects? Yes, the SuDS cuts their life cycle.

3. Improved Water Quality

SuDS facilitates natural filtration processes, removing pollutants from runoff before they enter water bodies. This system leads to improved water quality in rivers, lakes, and other receiving waters, benefitting both the environment and public health.

4. Climate Change Adaptation

SuDS is a primary tool for adapting to climate change impacts on the built environment. The NPPF requires a minimum 40% uplift on rainfall inputs for the 1-in-100-year design event, and SuDS attenuation storage is sized to accommodate this additional volume — giving developments a drainage system that performs across the range of rainfall intensities projected for the current century. Unlike conventional piped systems, SuDS attenuate, infiltrate, and store runoff in ways that remain functional as rainfall patterns intensify. 

SuDS in the UK: Regulations and Requirements

SuDS requirements in the UK differ between England and Wales. Wales operates a mandatory SuDS approval process for all new development; England applies SuDS requirements through national planning policy without a standalone statutory approval regime. Both jurisdictions reference the CIRIA SuDS Manual (C753) as the primary technical standard.

England

In England, SuDS requirements are set by the National Planning Policy Framework (NPPF) and its associated Planning Practice Guidance (PPG). The NPPF requires that SuDS be provided for all new major developments unless demonstrated to be inappropriate, and that drainage systems be designed to manage the 1-in-100-year storm event with climate change allowances applied.

Major development is defined as:

• 10 or more dwellings (residential)
• Sites of 0.5 hectares or more where the number of dwellings is not specified
• Non-residential development with a gross floor area of 1,000 m² or more, or a site area of 1 hectare or more

Lead local flood authorities (LLFAs) — typically county councils or unitary authorities — are statutory consultees on surface water drainage for all major planning applications. The LLFA reviews the drainage strategy against the drainage hierarchy and any local technical standards before the local planning authority (LPA) grants permission. A drainage strategy that does not follow the hierarchy or fails to meet the LLFA’s discharge rate requirements will trigger an objection that blocks planning consent.

Schedule 3 of the Flood and Water Management Act 2010 has not been commenced in England as of 2026. If commenced, it would make SuDS approval a formal pre-construction consent separate from planning permission, with LLFAs granted statutory powers to approve or reject drainage proposals. The government has confirmed its intent to commence Schedule 3 in England. Drainage schemes designed now should be built to a standard that satisfies both current NPPF requirements and the more formal approval process that Schedule 3 will introduce.

Wales

In Wales, Schedule 3 of the Flood and Water Management Act 2010 has been in force since 7 January 2019. All new developments that require building regulations approval and drain to public sewers or controlled waters must obtain SuDS approval before construction starts.

Approval is granted by Sustainable Drainage Approval Bodies (SABs), administered by county and county borough councils. SABs assess drainage proposals against the National Standards for Sustainable Drainage Systems — statutory standards published by the Welsh Government covering runoff quantity, water quality, amenity, and biodiversity.

Key requirements under the Welsh National Standards:

• Surface water runoff is managed as close to the source as possible, following the drainage hierarchy
• Drainage systems are sized for the critical 1% annual exceedance probability (AEP) event, with climate change allowances
• SuDS features contribute to biodiversity and water quality improvement where site conditions permit
• A long-term maintenance plan is required as part of the SAB submission

SAB approval is a separate consent from planning permission. Building regulations approval cannot be granted until SAB approval is in place.

Technical Standards and Guidance

CIRIA SuDS Manual (C753)

The CIRIA SuDS Manual, published in 2015 and referenced as C753, is the primary technical design reference for SuDS in England and Wales. It covers the four design objectives, component design guidance, hydraulic calculation methods, construction standards, and long-term maintenance requirements. LLFAs in England and SABs in Wales expect drainage strategies to demonstrate compliance with C753. Where a local authority has published its own drainage guidance, C753 sets the national baseline from which local requirements build.

Building Regulations Part H

Part H of the Building Regulations 2010 covers drainage and waste disposal for all development subject to building regulations — not only major planning applications. Schedule 1, Paragraph H3 requires surface water from roofs and paved areas to be discharged in the following order of preference:

1. To a soakaway or infiltration system
2. To a watercourse
3. To a surface water sewer
4. To a combined sewer

This mirrors the SuDS drainage hierarchy and applies independently of whether a planning-level drainage strategy is required. Non-compliance with Part H is grounds for rejection at building control stage.

Local Authority Technical Standards

Many LLFAs publish local drainage guidance that supplements the NPPF and C753. These documents set authority-specific permitted discharge rates, required storm return periods, freeboard requirements for attenuation features, and conditions for adoption of SuDS by the LLFA or a management company. Drainage strategies must be checked against the relevant LLFA’s published standards before submission — national policy sets the minimum, and local standards frequently exceed it.

What is a SuDS Drainage Strategy?

A SuDS drainage strategy is a technical document submitted to the local planning authority (LPA) and lead local flood authority (LLFA) as part of a planning application for major development. It sets out how surface water runoff from the proposed development will be managed, demonstrates compliance with the drainage hierarchy, and specifies the SuDS components, hydraulic calculations, and long-term maintenance arrangements for the scheme.

In Wales, the drainage strategy forms the core of the SAB submission and must meet the requirements of the National Standards for Sustainable Drainage Systems before SAB approval is granted.

When a Drainage Strategy is Required

In England, a drainage strategy is required for all major planning applications:

• Residential developments of 10 or more dwellings
• Non-residential development with a gross floor area of 1,000 m² or more
• Sites of 1 hectare or more, regardless of use

Many LLFAs set lower thresholds in their local drainage guidance, requiring a drainage strategy for minor development in areas at elevated surface water flood risk. The LPA’s validation checklist confirms the requirement for a specific application before submission.

In Wales, all new development subject to building regulations that drains to a public sewer or controlled water requires a SAB submission, with the drainage strategy as a statutory component.

What a Drainage Strategy Contains

Site Description and Baseline Assessment

The document opens with a description of the proposed development — site area, total impermeable area, land use — and an assessment of existing drainage conditions: current runoff rates, receiving watercourse details, soil permeability test results (BRE365 or falling head), and groundwater depth. This baseline establishes the greenfield runoff rate that the developed site must match at the point of discharge.

Hierarchy Assessment

The drainage hierarchy is assessed for each surface catchment on the site. Each level is either adopted as the proposed disposal route or ruled out with supporting technical evidence. Permeability test data, groundwater monitoring records, and written confirmation from the sewerage undertaker and Environment Agency are attached as appendices. LLFAs will not accept a hierarchy assessment that skips a level without documented evidence of infeasibility.

Proposed SuDS Design

This section sets out the selected SuDS components, their layout across the site, and how they combine into a treatment train from source to discharge. It includes:

• SuDS component specifications — type, dimensions, substrate specification, inlet and outlet details
• Drainage layout drawing — a plan showing all SuDS features, pipe connections, catchment boundaries, and flow routes
• Exceedance pathways — the routes that surface water takes across the site during events that exceed the design standard, confirming that flood water is directed away from buildings and does not leave the site boundary uncontrolled

Hydraulic Calculations

The calculations section demonstrates that the proposed scheme meets the permitted discharge rate for all design storm events. Outputs include:

• Greenfield runoff rate (l/s/ha), calculated from pre-development site characteristics using the FEH statistical method or IH124
• Required attenuation volume (m³), sized for the critical storm duration across the 1-in-1-year, 1-in-30-year, and 1-in-100-year return periods with climate change factors applied
• Flow control structure sizing — orifice plate, vortex control, or weir — set to restrict discharge to the permitted greenfield rate

Water Quality Assessment

The water quality section confirms that runoff from higher-risk surfaces — roads, car parks, commercial yards, petrol forecourts — passes through an adequate number of treatment stages before reaching a watercourse or groundwater. The CIRIA Simple Index Approach or a site-specific assessment confirms that the treatment train achieves the required pollution removal for the development type and receiving water sensitivity.

Management and Maintenance Plan

The maintenance plan sets out the long-term inspection and maintenance regime for every SuDS component on the site. It specifies:

• The responsible party — a management company, residents’ management company, or the LLFA where adoption is agreed
• Inspection frequencies and maintenance activities for each component type (vegetation management, sediment clearance, inlet and outlet inspection, structural inspection)
• Funding arrangements and long-term financial provision

Planning conditions and Section 106 agreements typically require the maintenance plan to be approved and legally secured before first occupation of the development. In Wales, SAB-approved drainage systems may be adopted by the SAB for long-term maintenance under an adoption agreement.

Who Reviews the Drainage Strategy

A drainage strategy submitted with a major application is reviewed by:

• LLFA — assesses the hierarchy assessment, hydraulic calculations, SuDS layout, and maintenance arrangements for all surface water drainage
• Environment Agency — consulted where the site is in Flood Zone 2 or 3, where the discharge point is to a main river, or where a flood risk activity permit is required for the outfall structure
• Sewerage undertaker — consulted where any level of the hierarchy involves discharge to a public sewer; confirms available network capacity at the point of connection
• Internal Drainage Board (IDB) — consulted where the site falls within an IDB district; sets permitted discharge rates that may differ from the LLFA’s requirements

Each consultee issues a formal response to the LPA. An objection from any statutory consultee prevents planning consent from being granted. Pre-application engagement with the LLFA and sewerage undertaker before the formal submission identifies technical constraints and discharge rate requirements early enough to shape the scheme design, not revise it after submission.

Urban Water prepares SuDS drainage strategies, surface water management reports, and SAB submissions for residential and commercial developments across England and Wales. Contact us for a free quote.

Implementing SuDS: Challenges and Solutions

SuDS must be integrated into the site layout from the earliest design stage. A drainage strategy prepared at pre-application stage — before the site layout is fixed — gives engineers the flexibility to position SuDS components where ground conditions and topography favour them, rather than retrofitting drainage around a completed layout.

Four challenges arise consistently in SuDS implementation:

High water tables and low-permeability soils. Infiltration-based SuDS are not viable where the seasonal high groundwater table sits within 1 m of the proposed feature base, or where permeability testing returns rates below 1 × 10⁻⁶ m/s. The response is to move to Level 2 of the drainage hierarchy — attenuation and discharge to a watercourse — with above-ground SuDS features sized for the permitted discharge rate.

Constrained urban sites. Retrofit SuDS in existing urban areas face competition for space with utilities, structures, and hard surfaces. Linear components — swales, filter strips, permeable paving — deliver attenuation and treatment within the available footprint. Green roofs address runoff from roofscapes where ground-level space is limited.

Long-term maintenance. SuDS require active maintenance: vegetation management, sediment clearance from inlets and basins, and regular inspection of flow control structures. Drainage strategies submitted with planning applications must include a funded maintenance plan with a named responsible party secured by Section 106 agreement or planning condition before occupation.

LLFA adoption and statutory consultee requirements. Planning policy in England requires SuDS for all major developments under the NPPF, with LLFAs as statutory consultees on drainage submissions. In Wales, Schedule 3 of the Flood and Water Management Act 2010 gives SABs formal powers to approve or reject drainage schemes before construction starts. Drainage strategies that do not follow the hierarchy or fail to meet the LLFA’s discharge rate requirements trigger a statutory objection that blocks planning consent. The regulatory framework is set out in full in the SuDS in the UK section above.

To Conclude

Cities experiencing rapid urban movements know better to incorporate SuDS projects in their countries. Sustainable drainage systems not only address immediate challenges but control long-term challenges like water pollution and flooding. Plus, they prevent the occurrence of diseases like typhoid and cholera. 

 

Urban Water delivers flood risk assessments, SuDS drainage strategies, and surface water management reports for developers, architects, and planning consultants across England and Wales. Get in touch for a free quote.