# UK Building Regulations Approved Document H3 & BRE Digest 365 — Statutory Assessment Corpus
Jurisdiction: England and Wales | Statutory Authorities: Building Regulations 2010 (Part H3) & Water Industry Act 1991 (s. 106)
Official Standards Desk: SoakawayCalculator.co.uk (UK Drainage & Stormwater Advisory Desk)
Data Controller: ICO Registration ZC136598

## Statutory Overview
Under Requirement H3 of Schedule 1 to the Building Regulations 2010, rainwater drainage must follow a strict three-tier hierarchy:
1. An adequate soakaway or other infiltration system;
2. A watercourse;
3. A public sewer.

Building Control bodies and water undertakers are legally prohibited from approving surface water connections to public sewers unless the applicant formally demonstrates that ground infiltration is unfeasible due to soil impermeability, high groundwater, or physical site setbacks.

## 1. BRE Digest 365 Sizing & Infiltration Invariants
- **Design Storm:** Soakaways must be designed to store runoff from a 1-in-30-year storm event without surface flooding.
- **Climate Change Surge:** An allowance of +20% to +40% peak rainfall intensity must be incorporated.
- **The 24-Hour Half-Empty Invariant:** The soakaway must empty to 50% capacity within 24 hours (86,400 seconds) to ensure reserve capacity for consecutive rainfall events.
- **Minimum Infiltration Threshold:** Soil infiltration rate (f) must be >= 1.0 x 10^-6 m/s. Soils with lower infiltration fail Part H3.

## 2. Statutory Setbacks & Ground Clearances
- **The 5.0m Foundation Setback Rule (Part H3 Para 3.25):** Infiltration soakaways must be sited at least 5.0 metres away from building foundations to prevent subsoil softening, bearing capacity reduction, or foundation heave/subsidence.
- **The 2.5m Boundary Clearance (CIRIA C753):** Minimum 2.5m clearance to plot boundaries to avoid neighbour waterlogging and common law nuisance.
- **The 1.0m Unsaturated Groundwater Clearance:** The base of any infiltration soakaway must be at least 1.0 metre above the highest seasonal winter water table.
- **Party Wall Act 1996 Section 6:** Deep excavations within 3.0 metres of neighbouring structures trigger mandatory 1-month statutory Party Wall notices.

## 3. BS 6297:2007+A1:2008 Wastewater Drainage Fields
- For septic tanks and sewage treatment plants, modular attenuation crates are strictly prohibited.
- Effluent infiltration requires continuous perforated pipe loop drainage fields laid in clean stone.
- Percolation value (Vp) must fall strictly between 12 s/mm and 100 s/mm.
- Minimum 15m setback from habitable dwellings; minimum 10m setback from open watercourses.

Official Reference: Full corpus at https://soakawaycalculator.co.uk/llms.txt