SiteRep

What is a site suitability assessment in Ireland?

A site suitability assessment is the site-specific investigation — including a site characterisation form and a percolation (T-test) — required under the Planning and Development Regulations 2001 and the EPA Code of Practice to establish whether a site can safely treat and dispose of domestic wastewater where there is no connection to a public sewer.

A site suitability assessment is the formal process used to decide whether a plot of land can support a domestic wastewater treatment system safely, without risk to groundwater, surface water, wells or neighbouring properties. It is required under Article 24 and the site suitability provisions of the Planning and Development Regulations 2001 for any house not connecting to a public sewer, and it must be carried out in line with the EPA's Code of Practice for Domestic Wastewater Treatment Systems.

The assessment is carried out by a suitably qualified person, typically a geologist or engineer with specific site assessment training, and covers desk-based research (geology, groundwater vulnerability, aquifer type, nearby wells) as well as fieldwork on site: trial holes to examine the subsoil, a percolation test to measure how quickly water drains, and an assessment of slope, flooding risk and separation distances to the house, boundary, wells and watercourses.

The output is a completed site characterisation form and percolation test report, together with a recommendation on the type and size of treatment system the site can support — for example, a traditional septic tank with a percolation area, or a proprietary secondary treatment unit with a smaller polishing filter, if the ground is less free-draining. This report forms part of the supporting documentation for the planning application.

What the assessment covers

A typical site suitability assessment addresses:

  • Site characterisation: topography, subsoil, geology and groundwater vulnerability
  • Trial hole excavation to examine the soil profile to at least 2.1 m depth
  • A percolation (T-test) to measure the soil's drainage rate
  • Identification of all wells within the zone of influence
  • Distance to watercourses, drains and flood-prone ground
  • Recommendation on treatment system type and percolation or polishing filter area size
  • A site layout plan showing the recommended system and all required setback distances

This report is normally requested by the planning authority as part of a valid planning application for a one-off house in an unsewered area.

What it costs and how long it takes

A site suitability assessment is typically arranged before or alongside lodging a planning application, and fieldwork can usually be completed in a single site visit, though timing depends on ground and weather conditions. Costs vary by consultant and site complexity, so it is worth getting quotes from a few suitably qualified assessors before committing.

Sources

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

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