What is a site characterisation (percolation test) for a house?

How it works in Ireland

If your house will have no mains sewer, the planning application needs the site characterisation form. The EPA’s 2021 Code of Practice, which covers systems for up to 10 people, sets out the steps: a desk study of the area, a visual look at the site, a trial hole to examine the soil and the water table, and percolation tests that measure how fast water drains.

The old T-test and P-test are now the subsurface and surface percolation tests. The subsurface test is done at the depth of the proposed pipe. The surface test is done near the ground surface. Which one matters depends on the system the assessor expects to recommend.

The site passes only if five things line up: slope no steeper than 1 in 8, the required separation distances to wells, watercourses and the house, enough unsaturated soil above bedrock and the water table, a percolation value in range, and no other blocking issue. The assessor then recommends a system, such as a septic tank with percolation area or a packaged treatment plant, and the design follows from that. See domestic wastewater system registration for what happens once it is installed.

Do it before you buy. A failing plot cannot be fixed by a better design, and the cost of a test is small next to the cost of a site you cannot build on.

A common mistake

Booking the test for the week you want to submit planning. The trial hole must stay open at least 48 hours so the water table settles, slow clay can need a day or more for the percolation holes, and the report takes time. Also, open holes are a hazard, so agree who makes them safe, and tell your insurer.