--- title: "BRE 365 vs BS 6297 Drainage Field: UK Law & Sizing | SoakawayCalculator.co.uk" source_url: "https://soakawaycalculator.co.uk/comparisons/bre-digest-365-vs-bs-6297-drainage-field" retrieved_by: "AnswerRail Edge Engine v1.2.7-v27 (https://answerrail.com)" level_of_detail: "standard" token_payload_reduction: "90.2%" raw_html_bytes: 86031 compiled_body_bytes: 8432 entities_extracted: 8 currency_recall_static_html: "not_applicable" faq_answer_recall_static_html: "100%" monitored_faq_answers: 3 link_recall_static_html: "100%" monitored_content_links: 6 rfc_compliance: "RFC 9110" last_verified_at: "2026-11-07T17:47:51.934Z" cache_age_seconds: 0 origin_cache_control: "public, max-age=0, must-revalidate" --- > **Canonical Source:** [https://soakawaycalculator.co.uk/comparisons/bre-digest-365-vs-bs-6297-drainage-field](https://soakawaycalculator.co.uk/comparisons/bre-digest-365-vs-bs-6297-drainage-field) > > *Understand the statutory legal difference between BRE 365 stormwater soakaways (Part H3) and BS 6297 foul drainage fields (Part H2). Why crates are illegal.* ### Organization: SoakawayCalculator.co.uk (UK Drainage & Stormwater Advisory Desk) | Field | Value | | :--- | :--- | | **Organization** | SoakawayCalculator.co.uk (UK Drainage & Stormwater Advisory Desk) | | **Official Website** | https://soakawaycalculator.co.uk | | **Overview** | UK civil engineering & statutory building control advisory desk for stormwater soakaway attenuation and drainage fields. | ### Entity: Soakaway Calculator UK | Property | Detail | | :--- | :--- | | **Type** | WebSite | | **Name** | Soakaway Calculator UK | ### Entity: Soakaway Calculator UK | Property | Detail | | :--- | :--- | | **Type** | WebApplication | | **Name** | Soakaway Calculator UK | ### Entity: UK BRE Digest 365 & Approved Document H3 Soil Infiltration & Rainfall Rates | Property | Detail | | :--- | :--- | | **Type** | Dataset | | **Name** | UK BRE Digest 365 & Approved Document H3 Soil Infiltration & Rainfall Rates | | **Description** | Standardised geotechnical percolation parameters (f, Vp) and 10-year / 30-year design storm depths across UK local planning authorities and soil classifications.... | ### Frequently Asked Questions (FAQ) **Q: How far must a soakaway be from a house foundation?** > A: Under Building Regulations Approved Document Part H3 (paragraph 3.25) and BS EN 752-4, a soakaway must be installed at least 5.0 metres away from any building foundation or habitable structure, and at least 2.5 metres from property boundary lines, to prevent ground destabilisation and structural subsidence. **Q: What is the standard BRE Digest 365 percolation test procedure?** > A: BRE Digest 365 requires excavating a trial pit of representative depth (typically 1.5m to 2.5m) and measuring water fall from 75% full to 25% full depth (t_75-25). The test must be filled and observed three consecutive times to simulate pre-saturated soil conditions. The soil infiltration rate f is calculated as the storage volume V_75-25 divided by effective internal surface area a_p50 multiplied by t_75-25. **Q: What happens if soil percolation test fails due to heavy clay?** > A: If the soil infiltration rate f is less than 1 x 10^-6 m/s (or Vp > 100 s/mm in BS 6297), a conventional soakaway is infeasible and will trigger Building Control failure. Under the statutory Part H3 surface water hierarchy, you are legally entitled to bypass infiltration and apply for a Section 106 connection to a surface water or combined public sewer, or discharge to an attenuation tank with a hydro-brake flow restrictor. ### Step-by-Step Procedure: How to Conduct a BRE Digest 365 Percolation Test for Building Control #### 1. Excavate Trial Pit Excavate a trial pit to the proposed soakaway invert depth (typically 0.3m wide by 1.0m to 2.0m long). Ensure the base is level and not smeared. #### 2. Fill with Water to Inflow Level Fill the trial pit rapidly with clean water to the proposed effective storage level (at least 300mm depth). #### 3. Measure 75% to 25% Drain Time Record the exact time in seconds taken for water to fall from 75% depth to 25% depth (t_75-25). #### 4. Repeat for 3 Consecutive Runs Refill the pit immediately and record times for Run 2 and Run 3 to verify soil infiltration under saturated conditions. ### Frequently Asked Questions (FAQ) **Q: Why are soakaway crates illegal for septic tank effluent?** > A: The Environment Agency General Binding Rules and BS 6297 prohibit crates because effluent requires aerobic subsoil treatment through shallow perforated pipes. Crates cause biomat choking and groundwater contamination. **Q: What percolation value (Vp) is legally required for BS 6297?** > A: Percolation value Vp must fall between 12 s/mm and 100 s/mm. A Vp under 12 drains too fast to treat effluent, while a Vp over 100 indicates impermeable ground that will pool at the surface. **Q: Can roof rainwater and septic effluent connect to the same trench?** > A: No. Building Regulations Part H strictly forbids connecting surface water to a foul drainage field, as storm surge flushes untreated solids directly into the subsoil. Statutory Distinction: Part H3 vs Part H2 # BRE Digest 365 vs BS 6297 Drainage Field: Stormwater vs Foul Effluent BRE Digest 365 governs clean stormwater attenuation crates under Building Regulations Part H3. BS 6297 governs foul septic tank and treatment plant drainage fields under Part H2. Installing crates for foul effluent is strictly illegal under Environment Agency General Binding Rules due to anaerobic bioclogging. Benchmark Focus**Statutory Distinction: Part H3 vs Part H2** Primary Standard**BRE 365 / Part H** Option A**BRE Digest 365 Stormwater Soakaway (Part H3)** Option B**BS 6297:2007+A1:2008 Drainage Field (Part H2)** FIG H3-01Statutory Rainwater Discharge Hierarchy Part H3 Para 3.2 Building Control inspectors legally reject public sewer connections unless Priority 1 ground infiltration is proven unfeasible via BRE 365 trial pit records. FIG BRE-02BRE Digest 365 Trial Pit Percolation Geometry Clause 3.1 (3 Runs) BRE Digest 365 excludes the pit floor from outflow area (a₅₀ = 2 × (L + W) × d/2) because fine roof sediment seals horizontal bases over time. ## Head-to-Head Specification Matrix | Evaluation Criterion | BRE Digest 365 Stormwater Soakaway (Part H3) | BS 6297:2007+A1:2008 Drainage Field (Part H2) | Statutory Verdict | | --- | --- | --- | --- | | Water Source & Legal Jurisdiction | Clean surface runoff from roofs, driveways, and patios (Part H3) | Treated foul sewage from septic tanks or treatment plants (Part H2) | Strictly segregated by UK law | | Permissible Hydraulic Geometry | Modular geocellular crates or clean 40/20mm gravel soakaway pits | Continuous perforated 110mm pipe loop bedded in 600mm pea gravel trench | Crates strictly illegal for foul wastewater | | Required Geotechnical Soil Test | 3-cycle trial pit infiltration test measuring soil velocity (f in m/s) | Floor percolation test measuring drop time in seconds per millimetre (Vp) | Different statutory test procedures | | Habitable Building Setback Distance | Minimum 5.0m setback from building foundations | Minimum 15.0m from habitable buildings (septic) or 10.0m (PTP) | BS 6297 requires substantial curtilage | | Distance to Watercourses & Wells | 2.5m from site boundaries; outfalls must avoid watercourse siltation | Minimum 10.0m from watercourses and 50.0m from drinking water boreholes | Part H2 enforces strict EA groundwater protection | **Building Control Rejection Alert & Common Failure Trap** CRITICAL STATUTORY WARNING: Never discharge foul septic or treatment plant effluent into stormwater crates. Suspended bio-solids create an impermeable anaerobic biomat within weeks, backing up raw sewage into your property and triggering Environment Agency enforcement. [Run Free Dual BRE 365 / BS 6297 Sizing Engine](/)[All Statutory Comparisons](/comparisons) ## Frequently Asked Legal & Technical Questions Why are soakaway crates illegal for septic tank effluent?▼ The Environment Agency General Binding Rules and BS 6297 prohibit crates because effluent requires aerobic subsoil treatment through shallow perforated pipes. Crates cause biomat choking and groundwater contamination. What percolation value (Vp) is legally required for BS 6297?▼ Percolation value Vp must fall between 12 s/mm and 100 s/mm. A Vp under 12 drains too fast to treat effluent, while a Vp over 100 indicates impermeable ground that will pool at the surface. Can roof rainwater and septic effluent connect to the same trench?▼ No. Building Regulations Part H strictly forbids connecting surface water to a foul drainage field, as storm surge flushes untreated solids directly into the subsoil. ### Related Statutory Comparisons [All Comparisons →](/comparisons) - [Soakaway Crates vs Gravel Trench: 95% Void vs 30% Aggregate Pit](/comparisons/soakaway-crates-vs-gravel-trench)BRE 365 Hydraulic Efficiency - [Soakaway vs Section 106 Sewer Connection: Clay Soil Exemption](/comparisons/soakaway-vs-section-106-sewer-connection)Part H3 Hierarchy & WIA 1991 s.106 - [Surface Water vs Highway Drainage Rebate: B2B Tariff Comparison](/comparisons/surface-water-vs-highway-drainage-rebate)Form H/01 & CMOS Wholesale Market - [Direct Vetted Groundworkers vs Multi-Resold Lead Directories](/comparisons/vetted-groundworkers-vs-lead-directories)Engineering Dispatch vs Aggregator Directories ## Related UK Statutory, Planning & Building Regulations Calculators (Verified Edge Mesh) - **[RSJCalculator.co.uk](https://rsjcalculator.co.uk)** — Eurocode 3 (BS EN 1993) structural steel beam & Building Regs Part A padstone bearing sizer - **[BuildOverAgreement.co.uk](https://buildoveragreement.co.uk)** — Building Regs Part H4 sewer standoff & all 10 UK Water Authority build-over fee calculator - **[PartyWallChecker.co.uk](https://partywallchecker.co.uk)** — Party Wall etc. Act 1996 s.1/s.2/s.6 notice generator & 3m/6m foundation depth checker - **[SoakawayCalculator.co.uk](https://soakawaycalculator.co.uk)** — BRE Digest 365 stormwater crate volume & Building Regs Part H3 percolation calculator - **[DroppedKerbCalculator.co.uk](https://droppedkerbcalculator.co.uk)** — Highways Act 1980 s.184 vehicle crossing council fees & 4.8m driveway depth checker - **[GardenOfficePower.co.uk](https://gardenofficepower.co.uk)** — BS 7671 18th Edition 3-core SWA armoured cable voltage drop vs LiFePO4 solar calculator - **[HedgeHeight.co.uk](https://hedgeheight.co.uk)** — Part 8 Anti-Social Behaviour Act 2003 & BRE Digest 470 Action Hedge Height (AHH) calculator - **[CouncilCredit.co.uk](https://councilcredit.co.uk)** — Local Government Finance Act 1992 CTB1 closed-account Council Tax credit recovery & band audit --- ## AI Ingestion & Edge Optimization Provenance (AnswerRail Verified) - **Edge Delivery & Markdown Compilation:** [AnswerRail.com](https://answerrail.com) — Layer-7 AI Crawler Content Negotiation, `/llms.txt` & Byte-0 Schema Compiler - **Dedicated AI Markdown Subdomain:** https://ai.soakawaycalculator.co.uk/ - **Cryptographic Ingestion Certificate:** https://answerrail.com/verify/soakawaycalculator.co.uk - **Live Crawler & Referral Telemetry:** https://answerrail.com/stats/soakawaycalculator.co.uk