A drainage field (or "soakaway") is the buried network of perforated pipes that finishes the treatment of septic-tank effluent — releasing it slowly into the soil where bacteria complete the breakdown. Get the design right and it works invisibly for 20-30 years. Get it wrong and you have repeated saturation, premature failure, and a regulatory non-compliance you can't legally ignore. Cornwall's mixed soils, high rainfall, and high-occupancy holiday lets all make field design harder here than the textbook average. This guide explains what BS 6297 actually requires, what costs look like in 2026, and the Cornwall-specific things to watch for.
The standard you must comply with: BS 6297
UK drainage field design is governed by BS 6297:2007+A1:2008 — the British Standard "Code of practice for design and installation of drainage fields for use in wastewater treatment". Anything installed after 2006 must conform to it. The standard sets out percolation testing methodology, field sizing formula (based on Vp and design population), minimum separation distances, depth and groundwater clearance requirements, and pipe spacing / trench size / aggregate specification. Building Regulations Part H requires drainage fields to comply with BS 6297, and the Environment Agency's General Binding Rules 2020 reference it. Skipping percolation testing or eyeballing the design isn't optional — a non-compliant install can't be retrospectively certified.
The percolation test (covered separately in detail)
Before designing the field, you must measure how quickly the soil absorbs water. The result — Vp, in seconds per millimetre of water drop — determines whether a standard drainage field is even viable. See our full percolation test guide for the BS 6297 method.
Quick reference:
- Vp < 12 sec/mm: Too fast (sandy/gravel). Treatment plant required instead.
- Vp 12–100 sec/mm: Drainage field viable. Size accordingly.
- Vp > 100 sec/mm: Too slow (heavy clay, peat). Treatment plant + surface water discharge, or mound soakaway.
The size formula
Field area (m²) = Vp × P × 0.25
Where P = design population (number of bedrooms × 2 minimum, or actual occupancy if higher). A 4-bedroom Cornwall property with Vp = 30 needs 30 × 8 × 0.25 = 60m² of drainage field. Real-world footprints typically end up 50-80m² for 4-bed properties, 80-120m² for 6-bed holiday lets, and 120m²+ for larger commercial.
The minimum separation distances (non-negotiable)
| From what | Minimum distance |
|---|---|
| Watercourse, stream, ditch | 10m |
| Drinking water supply (borehole, well, spring) | 50m |
| Habitable building | 15m (recommended) |
| Boundary fence | 2m |
| Roads, paths, hardstanding | 5m (avoid compaction) |
| Trees with invasive roots (willow, sycamore, leylandii) | 5m minimum |
Plus: the base of the drainage trenches must be at least 1.2m above the seasonally highest groundwater level to prevent direct entry to groundwater. On low-lying coastal Cornwall properties this often makes a buried field unviable — see "mound soakaway" below.
What an installation involves
- Site survey + percolation test — 2 days minimum (overnight saturation), typically £200-£500
- Design and quote — drawings showing field layout, trench plan, separation distances, agreed with you in writing
- Permits (if needed) — most domestic fields don't need a permit if they comply with GBR 2020; properties in Source Protection Zones may need one (~£141 EA fee)
- Excavation — typically a mini-digger digs parallel trenches 600mm wide × 600-900mm deep, spaced at 2m centres minimum, total length per the design
- Gravel base — 300mm of 20-40mm clean gravel below the pipe
- Perforated pipe — 100mm perforated drainage pipe laid level along each trench, connected via a distribution chamber from the tank outlet
- Gravel surround + geotextile — 50mm gravel cover above pipe, geotextile membrane over the top to prevent soil ingress
- Topsoil reinstatement — minimum 300mm of topsoil over the geotextile
- Commissioning — connection tested, paperwork including BS 6297 design rationale provided
Typical Cornwall job: 3-5 days on site for a standard 60m² field, longer for difficult access or larger holiday-let designs.
Realistic Cornwall costs (2026)
| Job | Typical cost |
|---|---|
| Percolation test + design | £200-£500 |
| Standard new drainage field (60m², 4-bed) | £3,500-£6,500 |
| Larger field (100m²+, 6-bed holiday let) | £5,500-£9,500 |
| Drainage field rebuild (replacing failed field) | £2,000-£5,000 |
| Partial repair (one trench/section) | £800-£1,800 |
| Mound soakaway (high water tables) | £4,500-£8,500 |
| Environment Agency permit (if required) | £141.43 application + costs |
When a drainage field isn't viable — alternatives
- Vp too slow (clay, peat): Sewage treatment plant with surface-water discharge to a watercourse (requires EA permit), or in rare cases a cesspit
- Vp too fast (sandy/gravel): Treatment plant required (cleaner effluent before reaching groundwater)
- High water table (1.2m clearance impossible): Mound soakaway built above ground level, or treatment plant with surface water discharge
- Site too small / wrong shape: Cluster of dispersal trenches, treatment plant, or in extreme cases a sealed cesspit
A reputable installer assesses the percolation result before quoting — if they bid for a drainage field on a property where the soil won't take it, walk away.
Cornwall soil-type guide
- Mid-Cornwall clay: Often borderline Vp. Larger fields, sometimes mound soakaways. Field lifespan typically 15-25 years.
- Granite/shillet (Penwith, parts of Bodmin): Variable; often too slow for standard fields. Treatment plants common.
- Sandy/loamy coastal: Often too fast; treatment plants recommended over septic tanks.
- Peat (Bodmin Moor): Wet, acidic, slow percolation. Mound soakaways or treatment plants standard.
- Mining-impacted ground (Camborne-Redruth): Test carefully — old workings can be unpredictable for groundwater and stability.
DIY drainage field — why it usually goes wrong
- No percolation test. Field sized by eye, often undersized for the soil — fails in 3-7 years.
- Trenches too close together. BS 6297 specifies minimum 2m spacing; many DIY jobs are 1m or less.
- Wrong gravel grade. Builders' gravel mixed with fines clogs the perforations within months.
- No geotextile membrane. Soil washes into the gravel; effective field area shrinks rapidly.
- Building Regulations / GBR non-compliance. Without a documented design rationale, the install fails any compliance check during a future sale.
DIY rebuilds account for a significant portion of the "premature soakaway failure" jobs Cornwall drainage specialists handle. Hire a competent installer.
Need a drainage field design, install, or rebuild on a Cornwall property? Submit your postcode and we'll match you with a specialist who does BS 6297-compliant work and provides full documentation.
Frequently asked questions
What is BS 6297?
BS 6297:2007+A1:2008 is the British Standard "Code of practice for design and installation of drainage fields for use in wastewater treatment". It sets out percolation testing methodology, sizing formulas, minimum separation distances, depth requirements, and aggregate/pipe specifications. Building Regulations Part H requires drainage fields to comply with it.
How big should my drainage field be?
Use the BS 6297 formula: field area (m²) = Vp × P × 0.25, where Vp is your soil's percolation value (seconds per mm) and P is design population (typically bedrooms × 2). A 4-bed Cornwall property with Vp = 30 needs roughly 60m² of drainage field.
How much does a drainage field cost to install in Cornwall?
Standard 60m² new field for a 4-bed property: £3,500-£6,500 fitted. Larger 100m² fields for holiday lets: £5,500-£9,500. Drainage field rebuild after failure: £2,000-£5,000. Mound soakaways (for high water tables): £4,500-£8,500.
How long does drainage field installation take?
3-5 days on site for a standard Cornwall installation. Add a few weeks for the design/quote/permit process beforehand. Excavation, gravel base, perforated pipe, geotextile, topsoil reinstatement. Full BS 6297 documentation provided on commissioning.
What if my soil percolation is too slow or too fast?
Too slow (Vp > 100 sec/mm — heavy clay, peat): standard drainage field not viable. Options: sewage treatment plant with surface water discharge (requires EA permit), or in rare cases a cesspit. Too fast (Vp < 12 — sandy/gravel): treatment plant needed for cleaner effluent before groundwater contact. Mound soakaways are an option for high water tables.