SuDS compliance guide

Drainage hierarchy, SuDS type selector, CIRIA C753 design standards, discharge rates, and maintenance requirements for UK construction projects.

When SuDS is required

Requirements differ by nation, development type, and planning authority. Check planning conditions and local SuDS guidance early, because drainage requirements are usually fixed before detailed construction design.

Wales

Mandatory since 7 January 2019 under Schedule 3 of the Flood and Water Management Act 2010.

Unless exempt, developments of more than one dwelling or with a construction area of 100m² or more need SAB approval from the local authority before construction.

England

NPPF paragraph 182 expects applications that could affect drainage to incorporate SuDS proportionate to the scheme.

Major developments must take LLFA advice, set operational standards, and secure maintenance arrangements. Schedule 3 has not been commenced in England, so planning conditions remain the main approval route.

Scotland

SEPA regulates water run-off from built development under EASR water activity authorisations.

Run-off from most developments built after 1 April 2007 is authorised by General Binding Rules where it is drained by a SUD system. Larger or higher-risk developments may need a different authorisation.

Practical rule: Treat SuDS as a baseline design requirement for any meaningful new impermeable area. Retrofitting a compliant drainage system after planning approval is usually more expensive than designing it in from the outset.

Drainage hierarchy

England's 2025 national standards and Approved Document H require evidence before moving down the discharge hierarchy. Local authorities may adapt the order in local guidance, but the principle is the same: keep runoff close to source and use the lowest-priority destination only where higher options are not practicable.

Collect for non-potable use Preferred

Rainwater harvesting for toilet flushing, irrigation, process water, or other non-potable demand. Particularly relevant for commercial, industrial, horticultural, and water-stressed sites.

Infiltration to ground

Soakaways, permeable paving, infiltration basins, and infiltration trenches. Requires suitable ground conditions confirmed by infiltration testing and groundwater assessment.

Discharge to surface water body

Direct to river, stream, ditch, or watercourse at a controlled rate. Requires ordinary or main watercourse consent. Preferred over sewer where a watercourse is accessible.

Discharge to surface water sewer

Adopted surface water sewer or other piped surface water drainage system. Discharge rate controlled by attenuation and agreed with the owner or relevant risk management authority.

Combined sewer (last resort)

Only where no higher-priority option is practicable. Discharge rate must be controlled and agreed with the receiving system owner. Surface water must not discharge to a foul-only system.

SuDS type selector

Select your site conditions to find suitable SuDS components. Most schemes use a treatment train - two or more types in series to achieve the required water quality and quantity control.

Soil type / infiltration potential

Site / land use

Design standards and discharge rates

Key parameters from CIRIA C753, England's 2025 national standards, LLFA guidance, and Water UK sewerage sector guidance. Always confirm local criteria before fixing storage volumes or flow controls.

Parameter	Requirement	Notes
Target discharge rate	Greenfield runoff rate or pre-development rate (whichever is lower)	Calculate site-specific rates. If the required orifice is below 50mm, blockage risk must be designed out and agreed with the approving body
Minor system design storm	1 in 30 year return period	No flooding or surcharging from inlet structures. Property protected to 1 in 100 year + climate change
Major system design storm	1 in 100 year + current climate change allowance	No internal flooding of buildings. Overland flow routes must be designed to pass this event safely
Climate change allowance	Use the current EA peak rainfall allowance for the site and design lifetime	England uses management-catchment allowances by epoch, not a fixed national percentage. Confirm with the LLFA where uncertain
Freeboard	Typically 300 mm above 100 year + CC flood level	Confirm local LLFA requirement. Emergency overflow must direct exceedance away from buildings
Everyday rainfall interception	At least the first 5 mm of rainfall should not run off to surface waters or piped drainage	England national standard 2. Water quality treatment must still match the land-use hazard and receiving environment
Sediment forebay	10-15% of total pond volume	Required at inflows to retention ponds and constructed wetlands to capture gross sediment and reduce maintenance frequency
Hydraulic design method	FEH rainfall-runoff or ReFH2 (QMED preferred for catchments >50 ha)	For small sites <5 ha, FSR/FEH rainfall depths with a simple reservoir routing model are generally accepted by LLFAs

Typical greenfield runoff rates by UK rainfall zone

Region / rainfall zone	Typical greenfield rate	Notes
South-east England (low rainfall)	2.0-2.5 l/s/ha	Confirm with ICP SuDS or local LLFA drainage SPD
Midlands / East Anglia	2.5-3.0 l/s/ha	Run QBAR calculation using FEH Web Service for site-specific value
Northern England / Yorkshire	3.0-4.0 l/s/ha	Higher rainfall but similar control philosophy; confirm with LLFA
Wales / West Country	3.5-5.0 l/s/ha	SAB approval required in Wales; greenfield rate from FEH or agreed method
Scotland (upland / west)	4.0-8.0 l/s/ha	High variability; SEPA and SUDS for Roads guidance apply

Greenfield runoff rates are indicative only. The authoritative method is to calculate QBAR for the specific catchment using FEH catchment descriptors from the NRFA web service.

The four SuDS pillars

CIRIA C753 defines SuDS performance across four equal pillars. Most planning refusals and SAB objections arise from schemes that address water quantity but neglect the other three.

Water quantity

Control peak runoff rates and total volumes so the developed site discharges no more than the equivalent greenfield catchment. Achieved through attenuation storage with a controlled outlet.

Water quality

Remove suspended solids, hydrocarbons, and heavy metals from surface water before discharge. Achieved through treatment components in a treatment train - filter strips, swales, filter drains, wetlands.

Amenity

Create usable space that also manages water. Grassed basins that double as recreation areas, swales that form green corridors, ponds with footpaths. LPAs increasingly require this to be demonstrated in the design.

Biodiversity

Support and enhance local habitats. Native marginal planting in ponds, wildflower seeding in basins, and habitat connectivity through planting areas. In England, Biodiversity Net Gain requirements make this pillar commercially important from 2024.

Maintenance requirements

Inadequate maintenance is the most common reason SuDS fail inspection or lose planning approval. Maintenance obligations must be secured by planning condition or s106 agreement before adoption. The responsible body (management company, highways authority, water company) must be confirmed at design stage.

SuDS component	Frequency	Key tasks	Typical failure mode
Permeable paving	Inspect 2x/year; jet every 5-10 years	Clear surface fines with vacuum sweeper, check joints, inspect edges and kerbs for rutting	Silt clogging of joints and sub-base, loss of infiltration capacity
Swales	2-4x/year; desilting every 5-10 years	Mow vegetation to 50-150mm height, remove litter and debris, check check dams and inlet structures	Vegetation over-growth blocking flow; erosion at inlets; check dam silting
Filter strips	2-4x/year	Mow to 50-100mm, rake and remove cuttings, reseed bare patches, inspect kerb/edge drainage	Channelling (preferential flow paths), bare patches reducing treatment
Detention basins (dry)	2x/year; desilting every 5-10 years	Mow, inspect outlet structure and trash screens, clear blockages, check embankment for erosion	Blocked outlet causing waterlogging; embankment erosion; litter accumulation
Retention ponds (wet)	Quarterly inspection; desilting every 10-15 years	Inspect inlet, outlet, and overflow structures; manage marginal planting; monitor water quality; dredge forebay	Blocked outlet; algal bloom; invasive species; embankment instability
Infiltration trenches / soakaways	Annual inspection; CCTV every 5 years	Check inspection chambers, clear geotextile inlets if accessible, confirm soakage by timed emptying	Silt clogging of geotextile, loss of infiltration capacity, root ingress
Underground attenuation tanks	Annual inspection; clean every 5 years	CCTV or confined space inspection, jet and vacuum, inspect flow control device and bypass	Sediment accumulation reducing volume; flow control blockage; joint failure
Green roofs	2x/year	Remove weeds, replace dead planting, inspect drainage outlets and upstand flashings, check substrate depth	Weed establishment, blocked outlets, membrane damage at upstands

Frequencies are general guidance from CIRIA C753 chapter 33. Site-specific maintenance schedules must be agreed with the adopting body and secured by planning condition.

Pollution hazard and treatment train

CIRIA C753 Table 26.1 defines three pollution hazard levels by land use. Match your treatment train components to the hazard level - schemes that under-specify treatment are increasingly rejected by LLFAs and the EA.

Hazard class	Typical land uses	Treatment train example
Low	Roofs, residential roads, footpaths, amenity grassland	Filter strip or swale + infiltration or attenuation
Medium	Car parks (<50 spaces), commercial roofs, light industrial yards	Filter strip + bioretention or swale + pond or wetland
High	HGV/vehicle storage yards, petrol forecourts, container storage, highway runoff with de-icing salts	Interceptor + filter drain + constructed wetland. No direct infiltration to ground without EA consent

Vehicle storage yards and logistics facilities: HGV and car storage yards are usually medium to high pollution hazard because of fuel, oil, metals, and tyre particle run-off. Proprietary treatment, separators, or environmental permitting may be needed in addition to SuDS, and direct infiltration should not be assumed acceptable. Check groundwater sensitivity and consult the regulator before relying on infiltration.

Common design pitfalls on site

Percolation test before drainage design, not after

The most common programme risk in SuDS design: assuming infiltration will work, then finding clay at 0.5m depth. BRE Digest 365 testing costs little compared with redesign. Carry it out during ground investigation, not when the drainage scheme is already drawn. If the measured infiltration rate is materially below about 1e-5 m/s, plan for attenuation and controlled discharge to a watercourse or sewer unless the drainage engineer agrees otherwise.

Check EA groundwater source protection zones

Infiltration SuDS in Source Protection Zone 1 or 2 will require careful groundwater risk assessment and may need Environment Agency agreement or permitting. This applies even where infiltration tests are favourable. The Environment Agency publishes SPZs through Defra's Magic map and GIS data services, so check the zone before committing to an infiltration-based scheme. In SPZ1, assume only clean roof water infiltration may be acceptable unless the regulator confirms otherwise.

The 2 l/s minimum discharge rate

For sites calculating a theoretical greenfield rate below 2 l/s, most LLFAs and sewerage undertakers will accept 2 l/s as a practical minimum - hydrobrake and penstock flow control devices are unreliable at lower rates. If your site catchment is very small (under 0.5 ha) and the greenfield rate calculates at 1 l/s, discuss with the approving authority before designing a flow restrictor that will block at the first autumn leaf fall.

Get the adoption route agreed before SAB or LPA submission

The biggest single cause of SuDS scheme delay is submitting a drainage strategy without a confirmed adopting body for each component. Swales and basins in public open space are usually adopted by the local authority parks department or a management company. Highway drainage goes to the highways authority. Private drainage serving a single commercial occupier stays private. Resolving this at planning stage avoids expensive redesigns when the funder or purchaser raises it during due diligence.

Sources

CIRIA C753: The SuDS Manual (2015) - primary UK design reference. Available through CIRIA
Flood and Water Management Act 2010, Schedule 3 - statutory basis for SuDS approval in Wales
Statutory national standards for Sustainable Drainage Systems (Wales) - the standards SABs apply when approving SuDS in Wales
National Planning Policy Framework (NPPF), chapter 14 - flood risk and sustainable drainage policy for England
National standards for sustainable drainage systems - England's current SuDS design standards
Flood risk assessments: climate change allowances - current EA rainfall allowances for drainage design in England
Building Regulations Approved Document H - drainage hierarchy and soakaway sizing
BRE Digest 365: Soakaway design - infiltration test methodology and soakaway sizing
Water UK Design and Construction Guidance - adoption standards for drainage infrastructure
SEPA water run-off authorisation rules - Scottish EASR requirements for run-off from built development
Groundwater source protection zones - Environment Agency SPZ mapping and risk context for infiltration

Related resources

Drainage gradients & pipe falls Percolation test & soakaway sizer Temporary works compliance checklist Automotive & logistics facilities