What is the term for the substantial substructure that supports a newly constructed building?

The term that best describes the substantial substructure supporting a newly constructed building is "foundation." The foundation is a critical component of any structure, as it transfers the load of the building to the ground and stabilizes it against various forces, including gravity and environmental conditions. The foundation must be carefully designed and constructed based on soil conditions, local building codes, and the overall design of the building to ensure safety and longevity.

While the other terms may seem relevant at first glance, their meanings differ. The "framework" refers to the skeletal structure of a building that supports the walls and roof but does not directly convey the load to the ground. The term "understructure" is more general and can include any structural component beneath a building, including the foundation itself; however, it is not the specific term used in construction. "Base" could refer to the bottom portion of a structure but does not encompass the entire concept of load-bearing support provided by a foundation. Thus, the foundation is the most precise and commonly accepted term for this vital aspect of building construction.