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Percolation tests: what the result means for a drainage field

By the Rowanspring team · Updated 10 November 2026Rules checked 10 November 2026, England

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On this page
  1. Short answer
  2. The evidence comes as a set
  3. Why the trial hole comes first
  4. What can make a result misleading?
  5. From test result to field design
  6. Questions to ask before paying for a test
  7. Sources
A downpipe running into a stone rill beside a garden path, with a rowan tree and hills behind
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Short answer

A percolation test measures how quickly water moves through the soil where a drainage field is proposed. It helps a competent designer decide whether infiltration is suitable and calculate the field's floor area. It does not, by itself, approve a site or prove that an existing system complies.

The Environment Agency says a trial hole is needed first to understand the water table, followed by tests carried out in line with BS 6297 and the Building Regulations. Its groundwater risk-assessment guidance explains the sequence and calculation. Approved Document H gives the test method for England.

The evidence comes as a set

Ask for a report that records:

  • the proposed field location and levels;
  • trial-hole dimensions, soil observations and groundwater findings;
  • test-hole locations, dates, weather and preparation;
  • every timed result, not only the average;
  • the calculated percolation value and design population;
  • the formula and resulting floor area;
  • the drawing, trench layout and assumptions; and
  • the designer's conclusion, limitations and next steps.

A single unlabelled number is hard to audit. Photographs can help identify locations, but they do not replace measurements and calculations.

Why the trial hole comes first

Soil may drain at the surface while groundwater sits too close to the proposed pipe level. The Environment Agency guidance says the trial hole is used to find the minimum depth to the water table before the percolation work. Seasonal conditions matter, so ask how the designer accounted for the likely worst groundwater level.

Do not enter a trial hole. Excavations can collapse and may need controls chosen by the contractor.

What can make a result misleading?

Weather outside normal conditions, a hole at the wrong depth, inadequate pre-soaking, too few repetitions or testing in a convenient patch rather than the proposed field can all make a result less useful. Ask the designer to state which published method was followed and explain any departure.

The test is for foul drainage-field design. A rainwater soakaway test or surface-water drainage calculation does not automatically answer the same question.

From test result to field design

The result feeds a formula; it is not the finished design. Occupancy, the type of treatment, available undisturbed land, pipe levels, boundaries, water sources, protected sites, access and future maintenance all remain relevant. Keep the calculation with the plan so that a later owner can see why the field has its particular area and layout.

If the result falls outside the range accepted by the applicable guidance, do not average it into a preferred answer or move the field on paper. Ask the designer what other discharge routes are realistically available.

Questions to ask before paying for a test

  1. Which national guidance and standard will you follow?
  2. Who chooses the locations and proposed invert level?
  3. Is the trial hole, groundwater assessment and reinstatement included?
  4. Will I receive raw timings, calculations and a scaled plan?
  5. What happens if the ground is unsuitable?
  6. Can the report be used by building control, the regulator and the drainage designer, or is further work expected?

Sources

Checked 10 November 2026.