Septic tank drainage field in the UK: percolation test, size, layout and what is banned
A septic tank does only primary treatment, so its effluent must go to a drainage field or similar secondary treatment [1]. England allows a field where the percolation test gives an average Vp of 12–100 s/mm [2]; BS 6297 and Scotland use 15–100 [3]. The trench floor area is A = P × Vp × 0.25 m² [4]: for a four-bedroom house on soil with Vp 30 that is 45 m² (example calculation).
Updated 2026-09-29 · every regulation and price links to its source at the bottom of the page. A guide, not a construction design.
Why does a septic tank need a drainage field?
The tank only lets solids settle; the liquid leaving it still carries most of the pollution. The soil around the drainage field trenches finishes the job. That is why, in England, the only route to ground under the general binding rules is a drainage field after a septic tank or treatment plant: a well, a borehole or a rainwater soakaway installed after December 2007 does not qualify [5].
New fields are designed and built to BS 6297:2007 [6]. A site survey with percolation test typically costs £250–500 [7] and a new field £1,500–4,000 [8] (approximate, as of September 2026).
How do you check the groundwater level first?
Before the percolation test, dig a trial hole at least 1 m² in area and 2 m deep, or at least 1.5 m below the invert of the proposed distribution pipes. The groundwater table must not rise to within 1 m of that invert; if you dig in summer, allow for the higher winter level [9].
How do you do a percolation test?
The method in Approved Document H [10]:
- Dig a hole 300 × 300 mm, reaching 300 mm below the proposed invert of the distribution pipe.
- Fill it with at least 300 mm of water and let it soak away overnight.
- Next day, refill to 300 mm and time how many seconds the water takes to fall from 75% to 25% full, a depth of 150 mm.
- Divide that time by 150: the result is Vp, the average number of seconds for the level to drop 1 mm.
- Repeat at least three times in at least two holes and take the average. Do not test in heavy rain, frost or drought.
Which result allows a drainage field?
Approved Document H: “Drainage field disposal should only be used when percolation tests indicate average values of Vp of between 12 and 100” [2]. Faster soil lets poorly treated effluent reach groundwater; slower soil cannot absorb it.
| Vp, s/mm | England | Scotland |
|---|---|---|
| Below 12 | No drainage field; use another form of secondary treatment [2] | Up to 15: fast soil, subject to SEPA requirements [11] |
| 12–15 | Allowed by Approved Document H, but BS 6297:2007 treats normal soil as over 15 [3] | As above |
| 15–100 | Drainage field | Standard drainage field [11] |
| 100–140 | No drainage field [2] | Reed bed, constructed wetland or certified filter before the field [11] |
| Over 140 | No drainage field | No infiltration, or a mound filter system [11] |
How big should the drainage field be?
The trench floor area is A = P × Vp × 0.25 m², where P is the design population; after a sewage treatment plant use 0.20 instead of 0.25 [4]. Scotland uses the same formula [3]. P comes from British Water guidance: five people for a house with up to three bedrooms, plus one for each extra bedroom [13].
Example calculation, square metres of trench floor after a septic tank:
- Trenches are 300–900 mm wide [14], so area turns into length: 45 m² in 600 mm trenches is 75 m of trench (example calculation).
- Slow soil makes the field grow fast: the same house needs four times the area at Vp 80 as at Vp 20.
| House (design population) | Vp 20 | Vp 50 | Vp 80 |
|---|---|---|---|
| Up to 3 bedrooms (5) | 25 m² | 62.5 m² | 100 m² |
| 4 bedrooms (6) | 30 m² | 75 m² | 120 m² |
| 5 bedrooms (7) | 35 m² | 87.5 m² | 140 m² |
Size a septic system for your plot →
How is a drainage field laid out?
Distances in England: at least 15 m from any building, 10 m from a watercourse or permeable drain and 50 m from a groundwater abstraction point [15]. In Scotland: 5 m from a building and from the boundary, 10 m from watercourses, drains, roads and railways, and 50 m from any spring, well or borehole used for drinking water [16].
- Perforated pipe at a gradient no steeper than 1:200, on 300 mm of clean 20–50 mm stone, covered with stone and a geotextile [14].
- Pipe at least 500 mm below ground; trenches 300–900 mm wide with 2 m of undisturbed ground between parallel trenches [14].
- An inspection chamber between the tank and the field, and the field laid as a continuous loop [14].
- No water pipes or other services, no access roads, driveways or paving within the area; site it downslope of groundwater sources [17]. Natural Resources Wales adds: do not park cars on it [18].
What cannot replace a drainage field?
In Scotland greywater from baths, showers, basins, sinks and washing machines may go to a separate infiltration field of A = P × Vp × 0.2 m² [19].
- A soakaway pit, a well or a borehole: not allowed for sewage [5].
- Concrete rings or shafts: to be avoided [20]; if effluent reaches groundwater directly through them, no permit will be granted [21].
- A pipe from a septic tank to a ditch or stream: banned; only a treatment plant may discharge to flowing surface water [22], and a partial drainage field near the bank also works only with a plant [23].
Frequently asked questions
What is a good percolation test result?
An average Vp between 12 and 100 s/mm allows a drainage field in England [2]; BS 6297 and Scotland use 15–100 [3].
How big a drainage field does a 3-bedroom house need?
Five people × Vp × 0.25. With Vp 40 that is 5 × 40 × 0.25 = 50 m² of trench floor (example calculation) [4].
Can I use an old soakaway for septic tank effluent?
No. A rainwater soakaway installed after December 2007, a well or a borehole does not meet the general binding rules [5].
Can I park or lay a patio over a drainage field?
No: no access roads, driveways or paving within the disposal area [17].
How far from a borehole must a drainage field be?
Sources
- Approved Document H (Building Regulations 2010, England), 2015 edition — H2, H2 para 1.2, 1.15 (UK) · regulation · checked 2026-09-29 · source
- Approved Document H (Building Regulations 2010, England), 2015 edition — H2, H2 para 1.38 (UK) · regulation · checked 2026-09-29 · source
- gov.scot (UK) · regulation · checked 2026-09-29 · source
- Approved Document H (Building Regulations), H2, H2 1.34-1.37 (UK) · regulation · checked 2026-09-29 · source
- Environment Agency General Binding Rules: small sewage discharges (England), Rules that apply to all discharges — If you use a non-standard system (such as a well, borehole and soakaway) (UK) · prohibition · checked 2026-09-29 · source
- Environment Agency General Binding Rules: small sewage discharges (England), Make sure your treatment system meets the relevant British Standard (UK) · regulation · checked 2026-09-29 · source
- MyJobQuote — Septic Tank Installation Cost Guide (UK Prices 2026), Average cost; cost breakdown (UK) · price · checked 2026-09-29 · ✓ · source
- MyJobQuote — Septic Tank Installation Cost Guide (UK Prices 2026), Drainage field costs (UK) · price · checked 2026-09-29 · ✓ · source
- Approved Document H (Building Regulations 2010, England), 2015 edition — H2, H2 para 1.33 (UK) · regulation · checked 2026-09-29 · source
- Approved Document H (Building Regulations 2010, England), 2015 edition — H2, H2 para 1.34–1.37 (UK) · regulation · checked 2026-09-29 · source
- Scottish Building Standards Technical Handbook — Domestic (April 2026), 3.9.2 Design of infiltration fields (UK) · regulation · checked 2026-09-29 · ? · source
- Environment Agency General Binding Rules: small sewage discharges (England) (UK) · regulation · checked 2026-09-29 · source
- British Water Code of Practice — Flows and Loads 4 (2013) (UK) · recommendation · checked 2026-09-29 · source
- Approved Document H (Building Regulations 2010, England), 2015 edition — H2, H2 para 1.40–1.44 (UK) · regulation · checked 2026-09-29 · source
- Approved Document H (Building Regulations), H2, H2 (UK) · regulation · checked 2026-09-29 · source
- Scottish Building Standards Technical Handbook — Domestic (April 2026), 3.9.4–3.9.5 Location of infiltration fields (UK) · regulation · checked 2026-09-29 · ? · source
- Approved Document H (Building Regulations 2010, England), 2015 edition — H2, H2 para 1.28–1.30 (UK) · prohibition · checked 2026-09-29 · source
- Natural Resources Wales — Running and maintaining a septic tank or small sewage plant, Maintaining a septic tank; Maintenance records (UK) · recommendation · checked 2026-09-29 · ? · source
- Scottish Building Standards Technical Handbook — Domestic (April 2026), 3.9.3 Greywater disposal (UK) · regulation · checked 2026-09-29 · ? · source
- Environment Agency General Binding Rules: small sewage discharges (England), If you use a non-standard system (such as a well, borehole and soakaway) (UK) · recommendation · checked 2026-09-29 · source
- Environment Agency General Binding Rules: small sewage discharges (England), If you use a non-standard system — section G of the EA approach to groundwater protection (UK) · prohibition · checked 2026-09-29 · source
- Environment Agency General Binding Rules: small sewage discharges (England), Surface water — Use the correct treatment system (UK) · prohibition · checked 2026-09-29 · source
- Environment Agency General Binding Rules: small sewage discharges (England), Surface water — Make sure any partial drainage field is close enough to the watercourse (UK) · regulation · checked 2026-09-29 · source
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