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What happens if a site fails a percolation test?

A failed percolation test does not automatically mean a site cannot be developed; the assessor first considers whether an alternative treatment system, a different location on the site, or an engineered solution such as a raised percolation area can meet the EPA Code of Practice, and only if no compliant solution exists is the site treated as unsuitable for on-site wastewater treatment.

A percolation test result outside the range suitable for a standard septic tank and percolation area is a setback, not necessarily the end of the road. The suitably qualified assessor carrying out the site suitability assessment will typically first ask whether a different, better-draining part of the same site could accommodate the percolation area, since T-values and soil conditions can vary noticeably across even a modest plot.

Where relocation is not possible or does not help, a range of engineered alternatives to a traditional percolation area may be considered, including a raised percolation area built up above natural ground level to overcome shallow soil or a high water table, or a proprietary secondary (or tertiary) treatment system that produces a higher-quality effluent requiring only a smaller polishing filter area rather than a full percolation area.

If neither relocation nor an alternative system can bring the site within the requirements of the EPA Code of Practice — for example because the soil is both very poor draining and there is insufficient land for any percolation area, or the site sits over an extreme groundwater vulnerability zone near wells — the assessor will conclude the site is unsuitable for on-site wastewater treatment, and a planning application relying on such a system is likely to be refused.

Options after a failed test

Depending on the specific reason for failure, options can include:

  • Testing an alternative location elsewhere on the same site
  • Specifying a raised or mound percolation area to overcome shallow soil or high water table
  • Installing a proprietary secondary or tertiary treatment system with a smaller polishing filter footprint
  • Reviewing whether a public sewer or group scheme connection is a realistic alternative
  • Reassessing house design or siting to free up land elsewhere on the plot for the treatment system
  • Commissioning additional trial holes to rule out a misleading initial result

The right option depends entirely on why the site failed — poor drainage, shallow soil, high water table and proximity to a well each point towards different solutions.

When to get further advice

Discuss the failed result and any recommended alternative in detail with the assessor before resubmitting or amending a planning application, and confirm with the local authority's planning or environmental section whether the proposed alternative system is acceptable for that specific site.

Sources

Last reviewed 20 August 2026. General information only — not legal or planning advice.

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