A house needs piles when competent load-bearing ground is too deep to reach economically with shallow foundations, for example where there is deep peat, alluvium or made ground. The decision follows a geotechnical site investigation; it should never be assumed either way without ground data.
Piling becomes the sensible option when the depth of poor ground exceeds what can be excavated and replaced or bridged with a raft. Deep peat deposits, soft estuarine silts and thick layers of uncontrolled fill are the usual triggers in Ireland, and these are often flagged by Geological Survey Ireland mapping before they are confirmed by boreholes on site.
Because piling can change a project budget significantly, it is worth carrying out a site investigation before purchasing a site or fixing a construction budget, particularly on low-lying, reclaimed or previously developed land.