Bearing, not just depth

Foundations are not deep for the sake of it. They are taken down to soil that is strong enough and stable enough to carry the load without moving.

Why clay sites go deep

Clay shrinks and swells with moisture. Trees make it worse. To avoid that movement, foundations are taken below the seasonal active zone — often 1.5m to 3m or more near mature trees.

If yours look shallow

Compared to similar nearby projects, a shallow foundation should prompt questions. Ask the engineer to confirm the design assumption against the bearing actually exposed.