There is no single depth that applies to every house. Foundations must reach ground capable of carrying the load, be below the depth affected by frost and seasonal moisture movement, and be deepened further where trees, slopes, drains or made ground are present. The depth is set by the engineer using site investigation data.
Foundation depth has to satisfy several conditions at once: the founding level must be on ground with adequate bearing capacity, it must be deep enough to avoid frost action and seasonal shrink-swell movement near the surface, and it must not be undermined by nearby excavations, drains or slopes.
Clay soils and nearby trees are the two factors that most often push foundations deeper in practice, because clay changes volume as its moisture content changes and tree roots draw moisture from the soil. On sites with made ground or peat, foundations may need to go through the poor material entirely, or a raft or piles may be a better solution.