--- title: "Roof & Driveway Catchment Soakaway Sizing Index [2026/27] | BRE 365 | SoakawayCalculator.co.uk" source_url: "https://soakawaycalculator.co.uk/catchment" retrieved_by: "AnswerRail Edge Engine v1.1.0-parity-v20 (https://answerrail.com)" level_of_detail: "standard" token_payload_reduction: "89.1%" raw_html_bytes: 125434 compiled_body_bytes: 13730 entities_extracted: 7 currency_recall_static_html: "100%" monitored_currency_tokens: 26 faq_answer_recall_static_html: "50%" monitored_faq_answers: 2 link_recall_static_html: "100%" monitored_content_links: 24 rfc_compliance: "RFC 9110" last_verified_at: "2026-10-17T12:55:45.339Z" cache_age_seconds: 0 origin_cache_control: "public, max-age=0, must-revalidate" --- > **Canonical Source:** [https://soakawaycalculator.co.uk/catchment](https://soakawaycalculator.co.uk/catchment) > > *Compare soakaway crate requirements, attenuation volumes, and groundwork costs across 14 UK roof extensions, driveways, and commercial footprints.* ### 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. ### Entity: UK Roof & Impermeable Surface Catchment Sizing Index | Property | Detail | | :--- | :--- | | **Type** | CollectionPage | | **Name** | UK Roof & Impermeable Surface Catchment Sizing Index | | **Description** | Statutory BRE Digest 365 sizing directory for 14 residential and commercial roof and paving footprints.... | BRE DIGEST 365 RUNOFF FOOTPRINTS (14 HUBS) # Roof, Extension & Driveway Soakaway Sizing Index Standardised BRE Digest 365 stormwater crate sizing schedules for typical UK domestic extensions, garage roofs, permeable block paving, and commercial developments. Catchment Benchmark Sizing Table (BRE 365 / Sandy Loam Subsoil) | Catchment Profile | Category | Effective Area | Typical Crates | Est. Groundwork Cost | Calculator | | --- | --- | --- | --- | --- | --- | | [40m² Single-Storey House Extension Soakaway Calculator](/catchment/house-extension-40m2) | Residential | 8m × 5m (40m²) | 6 Crates | £1,400 - £2,200 | [Size Soakaway →](/catchment/house-extension-40m2) | | [30m² Detached Garage Pitched Roof Soakaway Sizing](/catchment/detached-garage-30m2) | Outbuildings & Commercial | 6m × 5m (30m²) | 4 Crates | £1,100 - £1,800 | [Size Soakaway →](/catchment/detached-garage-30m2) | | [25m² Garden Room & Timber Pod Soakaway Sizing](/catchment/garden-room-annexe-25m2) | Outbuildings & Commercial | 5m × 5m (25m²) | 4 Crates | £950 - £1,600 | [Size Soakaway →](/catchment/garden-room-annexe-25m2) | | [60m² Block Paved Driveway SuDS Infiltration Sizing](/catchment/block-paved-driveway-60m2) | Paving & Surfacing | 10m × 6m (60m²) | 8 Crates | £1,800 - £2,800 | [Size Soakaway →](/catchment/block-paved-driveway-60m2) | | [35m² Patio & Permeable Resin Bound Soakaway](/catchment/permeable-resin-patio-35m2) | Paving & Surfacing | 7m × 5m (35m²) | 5 Crates | £1,250 - £1,950 | [Size Soakaway →](/catchment/permeable-resin-patio-35m2) | | [80m² Standard 3-Bedroom House Full Roof Replacement](/catchment/standard-3bed-house-80m2) | Residential | 10m × 8m (80m²) | 12 Crates | £2,400 - £3,600 | [Size Soakaway →](/catchment/standard-3bed-house-80m2) | | [120m² Large 4-Bedroom Detached House Drainage Design](/catchment/large-4bed-detached-120m2) | Residential | 12m × 10m (120m²) | 18 Crates | £3,200 - £4,800 | [Size Soakaway →](/catchment/large-4bed-detached-120m2) | | [200m² Agricultural Barn Conversion Class Q Drainage](/catchment/commercial-barn-conversion-200m2) | Outbuildings & Commercial | 20m × 10m (200m²) | 28 Crates | £4,800 - £7,500 | [Size Soakaway →](/catchment/commercial-barn-conversion-200m2) | | [20m² Conservatory & Glazed Orangery Soakaway](/catchment/conservatory-orangery-20m2) | Residential | 5m × 4m (20m²) | 3 Crates | £850 - £1,400 | [Size Soakaway →](/catchment/conservatory-orangery-20m2) | | [300m² Agricultural Shed & Steel Portal Frame Roof](/catchment/agricultural-shed-300m2) | Outbuildings & Commercial | 25m × 12m (300m²) | 42 Crates | £6,800 - £10,500 | [Size Soakaway →](/catchment/agricultural-shed-300m2) | | [150m² Commercial Tarmac & Hardstanding Attenuation](/catchment/tarmac-car-park-150m2) | Paving & Surfacing | 15m × 10m (150m²) | 22 Crates | £3,800 - £5,900 | [Size Soakaway →](/catchment/tarmac-car-park-150m2) | | [10m² Front Porch & Entrance Canopy Micro-Soakaway](/catchment/porch-canopy-10m2) | Residential | 3.5m × 2.8m (10m²) | 2 Crates | £650 - £1,100 | [Size Soakaway →](/catchment/porch-canopy-10m2) | | [250m² Residential Apartment Flat Roof Storm Attenuation](/catchment/flats-roof-catchment-250m2) | Residential | 18m × 14m (252m²) | 36 Crates | £6,200 - £9,800 | [Size Soakaway →](/catchment/flats-roof-catchment-250m2) | | [50m² Swimming Pool Surround & Terrace Drainage](/catchment/swimming-pool-surround-50m2) | Paving & Surfacing | 10m × 5m (50m²) | 7 Crates | £1,600 - £2,400 | [Size Soakaway →](/catchment/swimming-pool-surround-50m2) | ## Detailed Catchment Hubs - **[40m² Single-Storey House Extension Soakaway Calculator](/catchment/house-extension-40m2)** — Residential40m² planStandard calculation for rear kitchen, side return, and wraparound extensions under UK Building Regulations Part H3.Typical Crates: 6£1,400 - £2,200 - **[30m² Detached Garage Pitched Roof Soakaway Sizing](/catchment/detached-garage-30m2)** — Outbuildings & Commercial30m² planSizing modular infiltration units for domestic garages, workshops, and double cart-lodges.Typical Crates: 4£1,100 - £1,800 - **[25m² Garden Room & Timber Pod Soakaway Sizing](/catchment/garden-room-annexe-25m2)** — Outbuildings & Commercial25m² planCalculation for contemporary flat-roof insulated garden offices, granny annexes, and timber studios.Typical Crates: 4£950 - £1,600 - **[60m² Block Paved Driveway SuDS Infiltration Sizing](/catchment/block-paved-driveway-60m2)** — Paving & Surfacing60m² planSuDS statutory compliance for paved front driveway conversions under UK Town and Country Planning Order 2008.Typical Crates: 8£1,800 - £2,800 - **[35m² Patio & Permeable Resin Bound Soakaway](/catchment/permeable-resin-patio-35m2)** — Paving & Surfacing35m² planDrainage calculation for rear porcelain tile, natural sandstone, and resin bound patio areas.Typical Crates: 5£1,250 - £1,950 - **[80m² Standard 3-Bedroom House Full Roof Replacement](/catchment/standard-3bed-house-80m2)** — Residential80m² planComplete roof catchment drainage calculation for semi-detached and detached 3-bedroom UK dwellings.Typical Crates: 12£2,400 - £3,600 - **[120m² Large 4-Bedroom Detached House Drainage Design](/catchment/large-4bed-detached-120m2)** — Residential120m² planFull residential stormwater soakaway calculation for executive detached houses and chalet bungalows.Typical Crates: 18£3,200 - £4,800 - **[200m² Agricultural Barn Conversion Class Q Drainage](/catchment/commercial-barn-conversion-200m2)** — Outbuildings & Commercial200m² planStormwater management dossier for Class Q agricultural-to-residential conversions and rural commercial units.Typical Crates: 28£4,800 - £7,500 - **[20m² Conservatory & Glazed Orangery Soakaway](/catchment/conservatory-orangery-20m2)** — Residential20m² planCompact soakaway sizing for glazed extensions, lantern orangeries, and uPVC conservatories.Typical Crates: 3£850 - £1,400 - **[300m² Agricultural Shed & Steel Portal Frame Roof](/catchment/agricultural-shed-300m2)** — Outbuildings & Commercial300m² planLarge volume surface water attenuation for farm buildings, equestrian arenas, and machinery storage.Typical Crates: 42£6,800 - £10,500 - **[150m² Commercial Tarmac & Hardstanding Attenuation](/catchment/tarmac-car-park-150m2)** — Paving & Surfacing150m² planParking bay and vehicular circulation stormwater calculation with CIRIA C753 pollution control.Typical Crates: 22£3,800 - £5,900 - **[10m² Front Porch & Entrance Canopy Micro-Soakaway](/catchment/porch-canopy-10m2)** — Residential10m² planSmall-scale infiltration sizing for domestic front porches, carports, and over-door pitched canopies.Typical Crates: 2£650 - £1,100 - **[250m² Residential Apartment Flat Roof Storm Attenuation](/catchment/flats-roof-catchment-250m2)** — Residential252m² planMulti-family residential development roof drainage sizing for Building Control and LLFA sign-off.Typical Crates: 36£6,200 - £9,800 - **[50m² Swimming Pool Surround & Terrace Drainage](/catchment/swimming-pool-surround-50m2)** — Paving & Surfacing50m² planControlled rainwater drainage around private pool coping stones and outdoor entertainment patios.Typical Crates: 7£1,600 - £2,400 NAVRE NEXT-ACTION WALL • BAYESIAN ROUTER ## Recommended Next Statutory Steps & Sister Approvals Automatically ranked for your extension, driveway, or drainage project across the UK statutory compliance network. navre\_bayesian UK Extension & Structural Compliance Suite (NAVRE Network): [RSJCalculator.co.uk ↗](https://rsjcalculator.co.uk "BS EN 1993 Steel Beam & Padstone Sizer")[BuildOverAgreement.co.uk ↗](https://buildoveragreement.co.uk "Part H4 Build-Over Sewer Checker")[PartyWallChecker.co.uk ↗](https://partywallchecker.co.uk "Party Wall Act 1996 3m/6m Notice Checker")[DroppedKerbCalculator.co.uk ↗](https://droppedkerbcalculator.co.uk "Section 184 Vehicle Crossover Estimator")[GardenOfficePower.co.uk ↗](https://gardenofficepower.co.uk "BS 7671 SWA Armour Cable Calculator")[HedgeHeight.co.uk ↗](https://hedgeheight.co.uk "BRE Part 8 Action Hedge Height Calculator") OFFICIAL BILL OF QUANTITIES (BoQ) ## Trade Materials & Merchant Supply Links Direct trade merchant pricing for BRE 365 attenuation crates, 150g/m² non-woven geotextile wrap, and 110mm silt traps. Awin Publisher ID: 3035391 Essential Structural CoreScrewfix / Toolstation ### Polystorm-R / AquaCell Modular Soakaway Crate 200L capacity (1000x500x400mm), 95% void ratio, 61 tonnes/m² vertical load rating Req: **8 units**£34.99 / crate [Order at Trade Counter (Screwfix)](https://www.awin1.com/cread.php?awinmid=1294&awinaffid=3035391&clickref=boq-0-soakaway-hub&ued=https%3A%2F%2Fwww.screwfix.com%2Fc%2Fbuilding-joinery%2Fdrainage%2Fcat840480) Mandatory Part H3 WrapTravis Perkins ### 150g/m² Non-Woven Geotextile Membrane Roll Needle-punched polypropylene filter fabric, prevents fines & silt migration into crate voids Req: **25m² roll (2.25m x 25m)**£42.00 / roll [Order at Trade Counter (Travis Perkins)](https://www.awin1.com/cread.php?awinmid=2024&awinaffid=3035391&clickref=boq-1-soakaway-hub&ued=https%3A%2F%2Fwww.travisperkins.co.uk%2Fgeotextiles%2Fc%2F1500096%2F) Part H3 Maintenance SumpToolstation ### 110mm Inline Silt Trap / Leaf Interceptor Chamber BS EN 1401-1 compliant, removable sediment basket required by BRE Digest 365 Req: **1–2 units**£58.50 [Order at Trade Counter (Toolstation)](https://www.awin1.com/cread.php?awinmid=16582&awinaffid=3035391&clickref=boq-2-soakaway-hub&ued=https%3A%2F%2Fwww.toolstation.com%2Funderground-drainage%2Fc608) Base Bedding & SurroundTravis Perkins ### 20–40mm Clean Washed Shingle / Pea Gravel (Bulk Bag) BS EN 12620 washed granular sub-base (100mm blinding bed & pipe surround) Req: **1–2 bulk bags (850kg each)**£65.00 / bulk bag [Order at Trade Counter (Travis Perkins)](https://www.awin1.com/cread.php?awinmid=2024&awinaffid=3035391&clickref=boq-3-soakaway-hub&ued=https%3A%2F%2Fwww.travisperkins.co.uk%2Faggregates%2Fc%2F1500057%2F) ## 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