S1: Runoff Destinations

S1

S1: Runoff Destinations

Source: Defra Non-Statutory Technical Standards for Sustainable Drainage Systems (refreshed 19 June 2025), Standard 1. Implements the surface-water hierarchy that has been UK government policy since the 2010 Flood and Water Management Act and is reproduced in CIRIA C753 §3, BS 8582 and the Sewerage Sector Guidance (SSG) for adoption.

S1 establishes a hierarchy of preferred destinations for site runoff, in order of environmental preference. Designers must follow this hierarchy, selecting the higher-priority option unless evidence demonstrates that it is not reasonably practicable or technically feasible. The hierarchy applies to every drainage area on the site, not just the site as a whole, so a single scheme may use different destinations for different sub-catchments.

The five-step hierarchy (H1 → H5)

1. H1: Collected for non-potable use (Rainwater Harvesting). Water collected from roofs or paved surfaces and stored for non-potable demand: WC flushing, irrigation, vehicle washing, cleaning. Sized to BS EN 16941-1:2024 (formerly BS 8515). Treated as a Tier-0 source-control measure. Highest priority because it offsets mains-water demand AND attenuates peak runoff. Tools: Rainwater Harvesting Calculator.
2. H2: Infiltrated to ground. Water soaked into the soil on-site via permeable paving, soakaways, bioretention, swales with infiltration, or filter drains. Requires soil with adequate infiltration rate (typically > 1×10⁻⁵ m/s, sandy silt or coarser per BRE Digest 365), groundwater clearance ≥ 1.0 m below the soakaway invert (per Environment Agency guidance), and no contamination risk (Source Protection Zone considerations). Tools: BRE Soakaway, Permeable Paving, Bioretention, Filter Drain, Soil Classification, Groundwater Impact Screening.
3. H3: Discharged to surface water body. Watercourse, pond, lake, ditch or estuary on or adjacent to the site. Requires Land Drainage Consent (under the Land Drainage Act 1991) and may need an Environmental Permit under the Water Resources Act 1991 if the discharge is to a Main River. Discharge rate normally restricted to greenfield runoff rates per IH124 / FEH. Water quality must meet the receiving water's WFD classification. See the Water Framework Directive Screening Module. Tools: Greenfield Runoff, Detention Basin, Retention Pond, WFD Screening, Treatment Train.
4. H4: Discharged to surface water sewer. Discharged to a dedicated surface-water sewer that is the responsibility of the water and sewerage company (WaSC) or local authority. Subject to the WaSC's restricted-discharge consent (typically 5 l/s/ha for adoptable schemes per SSG / SSG/DCG) and sufficient downstream capacity. Acceptable where H1–H3 are not feasible but less preferred because it transfers stormwater into the sewer network. Tools: Pipe Sizing, Hydraulic Grade Line, Manhole Sizing.
5. H5: Discharged to combined sewer. Discharged to a combined sewer carrying both foul and surface water. Lowest priority because it increases combined sewer overflow (CSO) frequency, downstream flooding risk, and treatment plant load. Generally only acceptable for retrofit constraints with no alternative, and even then only with the WaSC's express consent and a detailed engineering justification. Tools: the same as H4 plus Foul Drainage for combined-flow sizing.

Evidence requirement

Designers cannot select a lower-priority destination simply because it is cheaper, more familiar, or already on a CAD template. Justification for skipping a higher-priority option must be based on documented technical or environmental constraints. For example, infiltration may not be feasible if soils have very low permeability (BRE 365 test fails), groundwater is shallow and contaminated, or the site lies in a Source Protection Zone where discharge to ground is restricted. The justification must be evidenced through:

• Site investigation results: boreholes, BRE 365 trial pits, contamination screening
• Consultation with the LLFA, water company, EA / NRW / SEPA / NIEA as relevant
• Specialist assessment: hydrogeologist, contaminated-land specialist, ecological surveyor
• Cost-benefit comparison only as a tie-breaker between feasible higher-priority options, never to demote them

Common rejections at planning

The most common reasons LLFAs reject SuDS strategies on S1 grounds:

• Defaulting to H4 / H5 without evidence that H2 / H3 were properly investigated
• "Infiltration not feasible" stated without a BRE 365 test result or hydrogeologist note
• Single-destination scheme covering the whole site when sub-catchments could use different destinations
• Rainwater harvesting (H1) dismissed on capex grounds without yield analysis (BS EN 16941-1:2024)

Pin the S1 hierarchy decision into the FRA / SuDS strategy document submitted with the planning application. Include the evidence chain for every step downgrade. The S1–S7 Compliance Report tool generates a defensible evidence pack.