What is the lowest part of a building?

There are two ways to look at this question. What is the lowest part of the structure, or what is the lowest human occupied part of the building.

First question. The answer is the foundation. this is the part of the building which interfaces with the building and ground it sits on.

Second question. There is no special or specific answer for this. Typically, it would be the ground or first floor as it sits directly on the ground. Rooms that go below the ground (at least by a significant portion) are called basements. Most of these rooms are named by use rather than by location.