A geotechnical survey is not a universal legal requirement for a house, but foundations must be designed for the ground they sit on, so in practice some level of ground investigation is needed on every project. Full borehole investigation is normally reserved for larger, more complex or poor-ground sites.
For a straightforward house on good ground, trial pits logged by an engineer are often enough to design the foundations with confidence. A fuller geotechnical investigation using boreholes, in-situ testing and laboratory analysis becomes appropriate where the ground is known to be poor, where the building is large or heavily loaded, or where basements, retaining structures or slopes are involved.
Whatever the scale, the investigation exists to support a designed and certifiable foundation. Under building control, the designer and certifier are relying on ground information, so it should be recorded properly rather than assessed informally on site.