BRE Digest 365 Soakaway Design Tool – Crate Quantities, Dimensions & Costs. Updated 2026.
Enter your drainage area, rainfall data, and soil infiltration rate to calculate the required soakaway volume using the BRE Digest 365 design method. Results include crate quantities, excavation dimensions, and a full cost estimate for 2026 UK prices.
A soakaway is a subsurface structure that receives surface water runoff from roofs, driveways, and other impermeable surfaces and allows it to infiltrate gradually into the surrounding soil. In the UK, soakaways are the standard method of managing surface water drainage from domestic properties where connection to a surface water sewer is not available or not permitted. They are specifically preferred over sewer connection by Building Regulations Approved Document H (Drainage and Waste Disposal) and by Sustainable Drainage Systems (SuDS) policy, which seeks to return surface water to the ground near where it falls rather than discharging it to combined sewers.
The definitive UK design guide for soakaways is BRE Digest 365: Soakaway Design (Building Research Establishment), which provides the standard methodology for sizing soakaways based on the catchment area, design rainfall, and the measured soil infiltration rate. The BRE 365 method requires a site-specific percolation test (soakage test) to determine the soil's actual infiltration rate — this is a simple on-site test involving filling a trial pit with water and measuring how quickly it drains. This measured infiltration rate is then used in the BRE 365 volume calculations to determine the minimum soakaway storage volume required.
In 2026, plastic infiltration crates (also called soakaway crates or storm cells) are the dominant construction method for domestic soakaways in the UK, having largely replaced the traditional rubble-filled pit. Crates offer approximately 95% void ratio (compared with 30–40% for rubble fill), meaning a much smaller excavation is needed for the same storage volume. They are lightweight, easy to handle, structurally robust under traffic loading, and can be configured in almost any shape to fit the available space. A single standard crate (typically 1,000mm × 500mm × 400mm) provides 0.2 m³ of storage volume.
The BRE Digest 365 soakaway design procedure is as follows:
Soakaway design resources UK 2026: GOV.UK – Approved Document H | Planning Portal | Environment Agency | CIRIA – SuDS Manual | Checkatrade – Find a Drainage Contractor
| Soil Type | Infiltration Rate f (m/s) | Approximate BRE 365 Percolation Test Time (300mm drop) | Soakaway Viability | Notes |
|---|---|---|---|---|
| Gravel / Clean Coarse Sand | 10⁻² (0.01) | <1 second | Excellent | Very large outflow; soakaway may be very small |
| Coarse Sand | 10⁻³ (0.001) | ~1–5 seconds | Very good | Fast drainage; small soakaway required |
| Medium Sand / Sandy Loam | 10⁻⁴ (0.0001) | ~30–120 seconds | Good | Most common UK soakaway-viable soil; standard design |
| Fine Sand / Silty Sand | 10⁻⁵ (0.00001) | ~200–600 seconds | Marginal | Larger soakaway needed; check drain-down time carefully |
| Sandy Clay / Silty Loam | 10⁻⁶ (0.000001) | ~1,000–3,600 seconds | Poor | Very large soakaway needed; may not be practical – consider alternatives |
| Clay / Heavy Clay | 10⁻⁷ or less | >1 hour | Not viable | Soakaway not recommended; use SuDS, detention, or sewer connection |
| Chalk / Limestone | 10⁻³ to 10⁻² (fissured) | Varies widely | Good to Excellent | Must test on site; fissured rock can have very high infiltration |
| UK Region | Typical 1-hour 10yr Return Period Rainfall (mm/hr) | Recommended Design Intensity (mm/hr) | Notes |
|---|---|---|---|
| Northern Scotland / Highlands | 20–30 | 25–30 | Lower intensity but high frequency |
| Northern Ireland | 25–35 | 30–40 | — |
| North England (upland) | 30–45 | 40–50 | Higher in Pennines and Lake District |
| North England (lowland) / Midlands | 35–50 | 50 | Standard UK design intensity |
| South England / South East | 45–60 | 50–60 | Higher convective storms; 50mm/hr is conservative minimum |
| East Anglia / East England | 40–55 | 50 | — |
| South West / Wales | 35–55 | 50 | Higher totals but often lower intensities than SE |
| Climate change allowance (+40%) | ×1.4 of above values | 70–85 | Future-proofing for UKCP18 climate projections; LLFA may require this |
| Crate Type | Standard Size (mm) | Volume per Crate (m³) | Void Ratio | Load Rating | Price per Crate (2026) | Notes |
|---|---|---|---|---|---|---|
| Standard soakaway crate | 1,000 × 500 × 400 | 0.20 | 95% | 12.5 tonne/m² | £18–£35 | Most common domestic crate; non-trafficked |
| Heavy duty crate (trafficked) | 1,000 × 500 × 400 | 0.20 | 90% | 25–40 tonne/m² | £30–£55 | For use under driveways and trafficked areas |
| Deep crate / storm cell | 1,000 × 500 × 600 | 0.30 | 95% | 12.5 tonne/m² | £25–£45 | Deeper configuration for space-constrained sites |
| Large storm cell | 1,200 × 600 × 600 | 0.432 | 95% | 12.5–25 tonne/m² | £40–£70 | Fewer units needed for large volumes |
| Modular tank crate | 800 × 800 × 400 | 0.256 | 96% | 17.5 tonne/m² | £35–£60 | Stormwater detention / infiltration combined |
| Perforated concrete ring (600mm dia.) | 600 dia. × 1,000 deep | 0.283 | ~85% | High | £45–£80 per ring | Traditional construction; durable; heavy |
| Project | Typical Volume (m³) | No. of Crates | Estimated Installed Cost (SE England) | Notes |
|---|---|---|---|---|
| Small extension / garage roof (30 m²) | 0.5–1.0 | 3–5 | £800–£1,500 | Simple dig, 3–5 crates, basic installation |
| Average house roof (80 m²) | 1.5–3.0 | 8–15 | £1,500–£3,000 | Standard domestic soakaway |
| Large house roof + driveway (150 m²) | 3.0–5.0 | 15–25 | £2,500–£4,500 | Combined roof and driveway drainage |
| Commercial / industrial roof (500 m²) | 8–20 | 40–100 | £6,000–£18,000 | Multiple soakaways or large crate array; engineer design required |
| Rubble-filled pit (small domestic) | 2.0 m³ excavation | — | £600–£1,200 | Traditional method; larger excavation for same storage |
All UK soakaway designs for domestic and light commercial properties should follow BRE Digest 365: Soakaway Design. This methodology requires a site-specific percolation test to measure the soil infiltration rate (f-value), which is then used to calculate the required soakaway volume based on design rainfall and catchment area. Building Regulations Approved Document H references the BRE 365 method as the standard design approach.
A BRE 365 percolation test involves excavating a 300mm square test pit to the proposed soakaway depth, filling it with water, and measuring the time for the water to drop from 75% to 25% of the fill depth. This is repeated at least three times and in at least three locations. The infiltration rate f = 0.3m / (measured time in seconds × (225-75)/300). Never design a soakaway without carrying out this test — assuming the infiltration rate without testing is the most common cause of soakaway failure.
Building Regulations Approved Document H requires that soakaways are positioned: at least 5 metres from any building or structure (to prevent foundation undermining); at least 2.5 metres from any property boundary; and not under driveways unless specifically designed for trafficked loading. For contamination risk, soakaways should be at least 50 metres from any borehole or abstraction point used for drinking water supply.
Plastic infiltration crates (95% void ratio) are approximately 3× more volume-efficient than rubble fill (30–35% void ratio) — meaning a much smaller excavation is needed. A 1.0 m³ storage requirement needs only 1.05 m³ of crate excavation but 3.0+ m³ of rubble-filled pit. Crates are also easier to install, maintain, and inspect. The only advantage of rubble fill is lower material cost — but this is usually offset by the larger excavation cost.
Since 2015, Schedule 3 of the Flood and Water Management Act 2010 has been in effect in Wales, making SuDS approval mandatory for most new developments. In England, planning policy (NPPF) strongly encourages SuDS — the Local Lead Flood Authority (LLFA) is a statutory consultee for major developments. Soakaways are one of the simplest SuDS techniques. For new residential developments (10+ units) in England, SuDS compliance is effectively mandatory as part of the planning process.
LLFA and Environment Agency guidance increasingly requires that soakaway designs include a climate change allowance of +20–40% on design rainfall intensity to account for projected increases in rainfall intensity due to climate change (per UKCP18 projections). Many local authorities require this for all new drainage designs. Adding a 40% climate change uplift to a 50mm/hr design storm gives an effective design intensity of 70mm/hr — significantly increasing the required soakaway volume.
Soakaway crate arrays must be completely wrapped in a permeable geotextile membrane to prevent soil particles from migrating into the crates and reducing the void space and infiltration surface over time. The geotextile allows water to pass freely but retains soil fines. A minimum 150gsm non-woven geotextile is typically specified. The geotextile wrap must be continuous with no gaps — any soil ingress will progressively block the soakaway. Inspection chambers with sump chambers help trap sediment before it enters the crate array.
Soakaways should include an inspection chamber (rodding eye or access manhole) at the inlet to allow periodic inspection, rodding, and cleaning. Over time, silt accumulates in the soakaway and reduces its effectiveness. A pre-treatment chamber (silt trap) at the inlet significantly extends the soakaway's service life by capturing sediment before it enters the crate array. Annual inspection of the inspection chamber and silt trap is recommended; the soakaway itself should be inspected every 5–10 years.
Soakaways cannot be used in: areas with heavy clay soil (infiltration rate below ~10⁻⁶ m/s); areas where the seasonal high groundwater table is within 1 metre of the proposed soakaway base; Source Protection Zones (SPZ) for groundwater abstractions; areas of known contaminated land; or where slopes or unstable ground create a risk of slope instability. In these situations, alternative SuDS (detention basins, permeable paving, green roofs) or connection to a surface water sewer or watercourse must be considered.
Soakaway installation for drainage from a new dwelling or extension is covered by Building Regulations Approved Document H. Building Control must be notified and will inspect the soakaway installation. A drainage plan showing the soakaway location, size, and connection to the property must be submitted. Planning permission is not generally required for a soakaway as permitted development, but the LLFA should be consulted for larger or more complex drainage schemes.
As a rough guide for planning purposes: a standard UK detached house with approximately 80 m² of roof area in medium sand soil (f = 10⁻⁴ m/s) at 50mm/hr design rainfall typically requires a soakaway of approximately 1.5–3.0 m³ storage volume, achieved with 8–15 standard crates. A single car driveway (20 m²) requires approximately 0.5–1.0 m³ (3–5 crates). Always calculate specifically for your site conditions.
The soakaway is typically connected to roof downpipes via an underground drainage run in 110mm perforated or solid UPVC pipe (to BS EN 1401). A trapped gully at the downpipe base prevents odours and captures leaves before they enter the drainage system. The pipe should fall at a minimum gradient of 1:40 (25mm fall per metre) towards the soakaway. Where the pipe run exceeds 6m, an inspection chamber or rodding eye should be included at the midpoint for maintenance access.
Soakaway size depends on three key factors: your roof / catchment area, the design rainfall intensity for your location, and the soil infiltration rate from a percolation test. As a rough guide for a typical UK detached house with 80 m² of roof in medium sand soil (f = 1 × 10⁻⁴ m/s) at 50mm/hr design rainfall and a 2-hour storm duration, the required storage volume is approximately 2.5–4.0 m³ — equivalent to 13–20 standard soakaway crates. Use the soakaway calculator above to calculate the exact size for your specific site conditions. Never size a soakaway without first carrying out a percolation test to determine the actual soil infiltration rate.
The BRE Digest 365 percolation test procedure is: (1) Excavate a trial pit approximately 300mm square to the proposed soakaway base depth (typically 1.0–1.5m below ground level). (2) Fill the pit with water and allow to drain completely. Repeat twice more to saturate the surrounding soil. (3) On the fourth fill, measure the time in seconds for the water level to fall from 225mm (75% of 300mm) to 75mm (25% of 300mm). (4) Calculate the infiltration rate: f = 0.15 / (measured time in seconds) m/s (where 0.15m = the 150mm drop). (5) Repeat the test in at least two more locations and use the geometric mean of the results. If f < approximately 1 × 10⁻⁶ m/s, a soakaway is unlikely to be viable and alternative drainage options should be considered.
To calculate the number of soakaway crates needed: (1) Calculate the required storage volume using the BRE 365 method (use the calculator above). (2) Divide the storage volume by the void volume per crate. For a standard crate (1,000 × 500 × 400mm = 0.2 m³ total volume × 95% void ratio = 0.19 m³ effective storage per crate): Number of crates = storage volume (m³) ÷ 0.19. Round up to a whole number. For example, a 2.5 m³ storage requirement: 2.5 ÷ 0.19 = 13.2 → 14 crates. Always add a margin — if the calculation gives a non-round number, go to the next complete layer arrangement rather than cutting crates.
Yes — soakaway crates can be installed under driveways, provided you use heavy-duty trafficked crates with a load rating appropriate for the expected vehicle loading. Standard domestic soakaway crates (12.5 tonne/m² rating) are suitable for cars and light vans. Heavy-duty crates (25–40 tonne/m²) are required under driveways with HGV access. The crates must be installed at a minimum depth of 300mm (measured from the underside of the paving to the top of the crate) to provide adequate load distribution, and the paving must be designed to spread loads appropriately. Always use purpose-designed trafficked soakaway crates — standard domestic crates will collapse under vehicle loading.
In most cases, installing a soakaway as part of the drainage from a new dwelling extension or to replace an existing drainage arrangement is permitted development — no planning permission is required. However, Building Regulations approval is required — you must notify Building Control before installation, and the soakaway design must comply with Approved Document H. For new dwellings and major developments, the Local Lead Flood Authority (LLFA) is a statutory consultee for drainage. In some special areas (Source Protection Zones, areas of high flood risk, or sites with environmental designations), additional consents from the Environment Agency may be needed. Always confirm with your local authority before commencing drainage works.
There is no single mandatory depth for a soakaway — the depth is determined by the required storage volume, the available plan area, the soil infiltration rate at depth, and the minimum setback from groundwater. However, practical guidelines are: minimum depth 1.0 metre below ground level to avoid frost effects on soil infiltration; maximum depth governed by groundwater — the base of the soakaway must be at least 1 metre above the seasonally highest groundwater table; and optimum depth for UK domestic soakaways is typically 1.0–1.5 metres to the base of the crate array. Deeper soakaways can achieve the same volume in a smaller plan area, but excavation costs increase and access becomes more difficult. For crate-based soakaways, a practical maximum depth is approximately 2.5–3.0m without specialist excavation equipment.
Building Regulations Approved Document H requires soakaways to be located at least 5 metres from any building or structure. This setback is required to prevent the saturated zone created around the soakaway from undermining building foundations — particularly important in cohesive soils (clay, silt) that lose strength when wetted. Additional setback requirements include: at least 2.5 metres from any property boundary (to prevent saturating a neighbour's ground); at least 5 metres from any watercourse or highway; and at least 50 metres from any borehole or water abstraction point used for drinking water. Check these minimum distances carefully against your site plan before committing to a soakaway location.
The most common causes of soakaway failure are: incorrect soil type — a soakaway installed in clay soil (without a percolation test) will never drain adequately; insufficient size — many older soakaways were sized by rule of thumb rather than calculation, resulting in overflow during heavy rain; silt ingress — soil particles have entered the crates or rubble fill over time, reducing the void space and blocking the infiltration surfaces; root ingress — tree or shrub roots growing into the soakaway; high groundwater table — seasonal groundwater rise filling the soakaway from below; and no geotextile protection — older rubble-filled soakaways without geotextile wrapping invariably silt up over 10–20 years. A blocked or undersized soakaway can sometimes be rehabilitated by jetting and vacuuming the inlet and chambers; severe siltation of the crates or rubble fill typically requires replacement.
Soakaway installation costs in the UK in 2026 depend heavily on the size required, the soil conditions (which affect excavation difficulty), and the depth. As a guide for South East England: a small soakaway for a garage or extension roof (0.5–1.0 m³, 3–5 crates) costs approximately £800–£1,500 installed; a standard domestic house soakaway (1.5–3.0 m³, 8–15 crates) costs approximately £1,500–£3,000; and a larger combined roof and driveway soakaway (3–5 m³) costs approximately £2,500–£4,500. These prices include excavation, crates, geotextile, pipe inlet connection, and reinstatement of the surface. Excavation in difficult ground (rock, tree roots, services) adds significantly to cost. Deduct 10–18% for North England; add 15–25% for London. Always get at least three quotes from drainage contractors with soakaway experience.
Yes — soakaways can receive runoff from driveways and hardstandings, but with an important caveat for areas where vehicles are regularly washed, refuelled, or serviced. Clean driveway runoff (rain draining from a normal residential driveway used only for vehicle parking) can drain directly to a soakaway. However, if the driveway is regularly used for vehicle washing with detergents, or if oil or fuel spillages are possible (e.g. a petrol forecourt, commercial vehicle park, or workshop yard), the runoff must be passed through a Class 1 petrol/oil interceptor (to BS EN 858) before reaching the soakaway, to prevent hydrocarbon contamination of groundwater. The Environment Agency has guidance on drainage from vehicle washing and refuelling areas.
The choice between a soakaway and sewer connection is governed by Building Regulations Approved Document H, which establishes a preference hierarchy: (1) soakaway / infiltration (preferred — returns water to ground); (2) discharge to watercourse (second preference); (3) discharge to surface water sewer (third preference); (4) discharge to combined sewer (last resort — only with water company consent). You should not connect to the combined sewer if a soakaway is viable on your site, as this contributes to sewer flooding and combined sewage overflows. Water companies increasingly refuse new surface water connections to combined sewers. However, if your soil is not suitable for a soakaway (heavy clay, high groundwater), sewer connection or an alternative SuDS solution is necessary. Always check with your local water company about the available drainage options for your specific site.
Additional soakaway and drainage resources UK 2026: GOV.UK – Building Regulations Approved Document H | Environment Agency | CIRIA – SuDS Manual C753 | Planning Portal | Checkatrade – Drainage Contractors | TrustATrader