A foundation is the lowest part of a structure that transfers all the building's loads — its own weight plus occupants, furniture and wind — safely into the ground. It is the most critical part of any building: a failure here can affect the entire structure, and unlike walls or finishes it cannot easily be repaired later.
Foundations are broadly shallow or deep. Shallow foundations — isolated footings, combined footings, strip footings and raft (mat) foundations — spread the load near the surface and suit most houses on reasonable soil. Deep foundations — piles — carry the load down to firm strata when the surface soil is weak or the loads are very heavy.
The right foundation depends on the building loads and, crucially, the soil's safe bearing capacity, which should come from a soil investigation. Designing a foundation — its type, size, depth and reinforcement — is a structural-engineering task, but estimating the concrete and steel once sized is straightforward element-by-element work.