What is BS 6297 infiltration testing for foul drainage fields?

BS 6297 infiltration testing measures whether the ground on a site can absorb and treat effluent safely, and it is what decides whether a drainage field is a workable way to deal with foul drainage where there is no mains sewer. It is the British Standard test behind almost every off-mains foul drainage proposal, and getting it right — or wrong — can be the difference between a planning approval and a refusal.

The standard is BS 6297:2007+A1:2008, the code of practice for designing and installing drainage fields for small sewage treatment systems. If a development is not served by a mains foul or combined sewer, the planning authority and the Environment Agency will want evidence that the soil can take the effluent before they accept a septic tank or package treatment plant discharging to ground.

This article explains what BS 6297 infiltration testing is, when you need it, how the percolation test is carried out, how to read the percolation value, how it sizes the drainage field, and what your options are if the ground does not pass.

BS 6297:2007+A1:2008 is the code of practice for drainage fields — and a drainage field only works where a percolation test proves the soil drains within an acceptable range.

BS 6297 vs BRE 365: what is the difference?

BS 6297 is for foul drainage; BRE 365 is for surface water. They are different tests, for different liquids, judged against different acceptable results, and confusing the two is one of the easier ways to have a planning application held up.

BS 6297 infiltration testing assesses whether soil can absorb and treat effluent from a septic tank or package treatment plant through a drainage field. BRE 365 infiltration testing assesses whether soil can absorb rainwater through a soakaway as part of a surface water drainage strategy. A soakaway designed for rainwater cannot be used to dispose of sewage effluent, and the two are assessed against separate criteria. If you are unsure which test your site needs, it is worth settling before any digging starts.

Submitting BRE 365 results where BS 6297 is required — or the reverse — is one of the most common reasons an off-mains drainage application is sent back for further information.

When do you need BS 6297 infiltration testing?

You need BS 6297 infiltration testing when a development cannot connect to a mains foul sewer and you intend to discharge treated effluent to ground through a drainage field. Building Regulations set a clear hierarchy for foul drainage, and infiltration to ground only comes into play once a mains connection has been ruled out.

The order of preference set out in Approved Document H is:

1. Connection to a public sewer. If a public foul or combined sewer lies within roughly 30 metres of the development, a connection should be made where it is feasible. This is always the first option.
2. Package treatment plant. This treats sewage to a high standard using mechanical and biological processes, and its treated effluent can discharge to a watercourse or to ground via a drainage field, depending on site conditions and regulatory approval.
3. Septic tank discharging to a drainage field. A septic tank gives only primary treatment by settlement, so the effluent still contains pollutants and must pass through a drainage field, where the soil provides secondary treatment as the liquid percolates away.
4. Cesspool. Where nothing else is feasible, a sealed cesspool stores sewage with no outlet until it is pumped out and tanked away at intervals.

Both a package treatment plant and a septic tank can rely on a drainage field, and that is where infiltration testing becomes essential: it determines whether the ground can take the effluent at all.

The wider regulatory picture has tightened in recent years, which is partly why this question comes up so often. Since January 2020 it has been an offence in England for a septic tank to discharge directly to a watercourse, so affected systems have had to be upgraded, replaced, or redirected to a compliant drainage field. Under the Environment Agency's general binding rules a discharge to ground is limited to 2 cubic metres per day, and any drainage field must be designed to BS 6297. Where you cannot meet the binding rules, you need a permit — we cover the detail in our guide to the general binding rules for small sewage discharges.

What is a drainage field?

A drainage field — also called an infiltration system or leach field — is a network of perforated pipes laid in gravel-filled trenches that lets treated effluent soak into the surrounding soil, where biological and physical processes complete the treatment. Its size and layout depend on the soil's permeability, which is exactly what BS 6297 infiltration testing measures.

Where the field can go is governed by siting rules that protect groundwater, watercourses and buildings. A drainage field should keep to these minimum separations:

The field must also sit clear of, or downslope of, any groundwater source, and be kept away from roads, paved areas and underground services that are not part of the drainage system. Drainage fields are not permitted within a Zone 1 Groundwater Source Protection Zone. If a site cannot satisfy these distances, a drainage field may not be possible even where the soil itself drains well.

What is a percolation test?

A percolation test is the practical measurement at the heart of BS 6297: it times how quickly water drains out of a trial hole dug at the proposed drainage field, and reduces that to a single figure — the percolation value, or Vp. It is the number the whole drainage field design is built on.

A percolation test for a septic tank drainage field follows BS 6297 and is judged against a foul-drainage range. A percolation test for a surface water soakaway follows BRE 365 and is judged differently. The idea is the same; the standard, and the acceptable result, are not — so always be clear which one your site needs before you start.

A drainage field is only acceptable when the average percolation value falls between 15 and 100 seconds per millimetre.

How is BS 6297 infiltration testing carried out?

The test measures how quickly water drains out of a trial hole and reduces that to the percolation value, Vp, in seconds per millimetre. The method is deliberately simple, but the conditions and the repeats matter, because the result feeds directly into the drainage field design.

Excavating the trial hole

The trial hole is a 300 mm square pit, excavated to 300 mm below the proposed invert level of the effluent distribution pipe. The base must keep its 300 mm square shape; if the hole needs to go deeper to be excavated safely, it can be widened at the top, but the tested section at the bottom stays 300 mm square.

The percolation test method

1. Fill the 300 mm square section with water to a depth of 300 mm and leave it to drain overnight. This pre-wets the soil so the reading reflects normal wet-season behaviour rather than dry ground.
2. The next day, refill the hole to 300 mm.
3. Time, in seconds, how long the water takes to fall from 75% full to 25% full.
4. Divide that time by 150 to get the average time, in seconds, for the water to drop by 1 mm. That figure is the percolation value (Vp), expressed in seconds per millimetre.

How many trial holes and test runs are needed?

BS 6297 requires at least three test runs in each trial hole, and at least two separate trial holes across the proposed drainage field. The average Vp from all the valid results is the figure used in the design. Testing should not be done in abnormal weather, such as heavy rain, frost or drought, because those conditions distort the soil's behaviour and can produce a Vp the field will not actually deliver in service.

How to read the results: the acceptable Vp range

A drainage field is only acceptable when the average percolation value falls between 15 and 100 s/mm. The Vp tells you how quickly effluent moves through the soil, and both ends of the range exist to protect the environment.

If the value is too low, the effluent passes through so quickly that the soil cannot treat it, raising the risk of polluting groundwater. If it is too high, the effluent has nowhere to go and the system backs up, with waterlogging and odour the usual symptoms.

How the percolation value sizes the drainage field

Once you have a valid average Vp, BS 6297 and Approved Document H turn it into a floor area for the drainage field with a straightforward calculation. The slower the soil, the larger the field has to be.

For a septic tank, the floor area of the drainage field is A = p × Vp × 0.25, where A is the trench floor area in square metres, p is the number of people the system serves, and Vp is the percolation value. For fully treated effluent from a package treatment plant the factor drops to 0.20, because the effluent arriving at the field is cleaner and the soil has less work to do.

Take a four-bedroom house, counted as five people, on ground with a Vp of 50. A septic tank drainage field would need roughly 5 × 50 × 0.25 = 62.5 square metres of trench floor area. Double the Vp to 100 and the same house needs 125 square metres — which is often the point at which a field stops fitting the available plot.

The slower the soil drains, the more trench you need — so a borderline Vp can turn a workable field into one that no longer fits the site.

What if your site fails the BS 6297 test?

If the average Vp falls outside the 15 to 100 s/mm range, a discharge to ground via a standard drainage field is not permitted under the general binding rules, and you need an alternative. The right alternative depends on which way the result failed.

• Infiltration too fast (Vp below 15). Engineered solutions such as a drainage mound or a sand filter can slow the effluent down and add treatment before it reaches the ground.
• Infiltration too slow (Vp above 100). Discharge to ground is unlikely to work at all; the realistic options are a package treatment plant discharging to a watercourse, or, as a last resort, a cesspool.

A failed percolation test is not the end of the scheme — it just changes the drainage solution, and identifying it early avoids redesigning after permission.

Frequently asked questions

How Unda can help with BS 6297 infiltration testing

Unda specialises in foul and surface water drainage strategies for planning applications. We work with a trusted network of geotechnical specialists who carry out accurate, standard-compliant in-situ testing, and we turn the results into a drainage design that satisfies planning and Building Regulations requirements. If you are preparing an application that relies on a drainage field, or you are not sure whether your site needs BS 6297 or BRE 365 testing, find out more about our preliminary drainage design for planning, or contact us for an initial discussion.