A site investigation is the process of finding out what is under and around a site before design. For a house it usually means trial holes or boreholes, records of soil layers and groundwater, simple strength testing, and a report from an engineer setting out recommended foundation types and depths.
A typical domestic site investigation combines a desk study — historic maps, geological mapping, flood and radon data, previous uses — with intrusive work on the ground. Trial pits are dug at the proposed building positions, the soil profile is logged, groundwater is recorded and samples or in-situ tests are taken where needed.
The output is a report the designer can build from: soil descriptions, groundwater levels, recommended founding depth and allowable bearing pressure, comments on excavation and any special measures such as radon protection, contamination or gas protection.