Knowledge Base
Compliance Walkthrough
S1–S7 standard by standard: what to prove, which calculator to use, and the common mistakes to avoid
Runoff Destinations
What it Requires
Follow the discharge hierarchy:
- Harvest (rainwater use)
- Infiltrate (ground)
- Watercourse (surface water body)
- Surface sewer
- Combined sewer (last resort)
No defaulting to sewer without evidence.
Evidence You Need
Site investigation showing why higher-priority options were ruled out:
- Soil infiltration testing (BRE 365)
- Groundwater assessment
- Watercourse/sewer availability check
- Technical feasibility justification
Calculators
Common Mistake
Don't default to sewer: Local Lead Flood Authorities will push back unless you've evidenced why infiltration isn't viable. Keep your soil testing and investigation records.
Interception
What it Requires
First 5 mm of rainfall managed on-site with no runoff:
- Summer (May–Oct): 80% interception
- Winter (Nov–Apr): 50% interception
Not just storage: storage with evaporation or infiltration.
Evidence You Need
Interception storage calculations:
- Volume required for 5 mm rainfall
- Infiltration rate or evapotranspiration rate
- Storage depth verification (80% summer / 50% winter)
- Treatment feature sizing
Calculators
Common Mistake
Water butts alone don't comply. You need sized rainwater harvesting, permeable surfaces with adequate base, or vegetated features with proven infiltration/evaporation. A small water butt fills up and provides no further retention.
Flood Risk
What it Requires
Peak discharge limited to greenfield QBAR rate:
- 50% AEP event: greenfield rate or 3 l/s/ha (whichever greater)
- 1% AEP event: climate change uplift applied
- No flooding within development
- Exceedance routes designed
Evidence You Need
Complete flood mitigation case:
- Greenfield runoff (QBAR) calculation
- Climate change uplift (EA allowances)
- Storage volume (depth × area)
- Orifice sizing (flow restriction)
- Exceedance routing plan
Critical Point
The greenfield baseline is NOT climate-changed: only the developed site's rainfall is uplifted. This is the most commonly misunderstood point in S3. Your discharge limit is set relative to what natural land would produce today, not a hypothetical future baseline.
Water Quality
What it Requires
Treatment train achieving adequate pollutant removal:
- Sequential features (settlement → filtration → infiltration/discharge)
- Simple Index Approach score ≥ 14
- Removal of TSS, metals, hydrocarbons, nutrients
Evidence You Need
Water quality assessment per CIRIA C753:
- Site hazard classification (land use)
- Treatment feature efficiency scores
- Simple Index calculation (≥ 14)
- Treatment train sequence validated
Calculators
Key Principle
Match your treatment indices to the CIRIA C753 hazard tables for your land use. Residential is low-hazard (mainly sediment); commercial/industrial is high-hazard (metals, hydrocarbons). A simple swale works for residential; busy car parks need bioretention or infiltration trenches with soil media.
Amenity
What it Requires
SuDS features that enhance the public realm:
- Visual amenity integration
- Community use / health benefit
- Landscape design quality
- Safe access and sightlines
Evidence You Need
Design statements and landscape documentation:
- Site plans showing integration
- Landscape strategy & planting plans
- Design appraisal narrative
- Accessibility & safety design
Calculators
Assessment Type
S5 is qualitative: you're showing how SuDS features integrate with your landscape design. Focus on demonstrating community benefit, access, visual quality, and how the design contributes to place, not just drainage.
Biodiversity
What it Requires
SuDS contributing to 10% BNG target:
- Native planting and diverse habitats
- Aquatic habitat creation (retention ponds, wetlands)
- Pollinator support
- Long-term management commitment
Evidence You Need
Biodiversity net gain assessment:
- BNG calculation (baseline + uplift)
- Habitat creation plan
- Native species lists & sourcing
- Long-term maintenance specification
Calculators
High-Value Features
Retention ponds and wetlands score highest for habitat creation. Native planting, marginal vegetation, and submerged aquatics support invertebrates, fish, and amphibians. Commit to seasonal management (cutting, invasive species control, monitoring). Biodiversity is only realized with proper long-term care.
Maintenance & Delivery
What it Requires
Buildable, maintainable design with lifecycle plan:
- Structural integrity for loads
- Safe access for maintenance
- Clear responsibility assignment
- Adaptive management triggers
- Embodied carbon minimization
Evidence You Need
Complete lifecycle documentation:
- Management & Maintenance Plan (MMP)
- Construction Method Statement
- Adoption / Ownership Register
- Structural calculations
- Embodied carbon assessment
Calculators
SAB Adoption Critical
SABs (Sustainable Drainage Approving Bodies) won't adopt SuDS they can't maintain. Design for safe access (1–2 m minimum width), stable maintenance surfaces, and clear inspection/sediment removal procedures. Hard-to-reach features create long-term liability and will be rejected.
Pull it All Together
Once you've addressed S1–S7 across your scheme, use this tool to generate your complete compliance evidence pack:
This exports all your calculations, site evidence, and compliance statements into a single PDF ready for submission to planners and SABs.
Last reviewed: 2026-05-10 · Sources cross-checked against the canonical registry at the time of publication. Standards may be updated; verify currency against the publisher before relying on any citation.