Treas. Reg. § 1.48-1(e)(1) provides a detailed explanation of buildings and their structural components for ITC purposes and has been the primary source for guidance, both with respect to component depreciation and cost segregation studies. The term "building" is described as

…any structure or edifice enclosing a space within its walls and usually covered by a roof whereby the structure improves the land, and provides shelter or housing for work, office, display, or sales space. The term includes, for example, structures such as apartment houses, factory and office buildings, warehouses, barns, garages, railway or bus stations, and stores. Such term includes any such structure constructed by, or for, a lessee even if such structure must be removed, or ownership of such structure reverts to the lessor, at the termination of the lease.

Specifically excluded from the definition of the term "building" are the following:

1. a structure which is essentially an item of machinery or equipment, or
2. a structure which houses property used as an integral part of an activity specified in section 1.48(a)(1)(B)(i) if the use of the structure is so closely related to the use of such property that the structure clearly can be expected to be replaced when the property it initially houses is replaced. Factors which indicate that a structure is closely related to the use of the property it houses include the fact that the structure is specifically designated to provide for the stress and other demands of such property and the fact that the structure could not be economically used for other purposes.

The term "structural components" is defined in § 1.48-1(e)(2) of the Regulations as

…includes such parts of a building as walls, partitions, floors, and ceilings, as well as any permanent coverings therefor such as paneling or tiling; windows and doors; all components (whether in, on, or adjacent to the building) of a central air condition or heating system, including motors, compressors, pipes and ducts; plumbing and plumbing fixtures, such as sinks and bathtubs; electric wiring and lighting fixtures; chimneys; stairs, escalators, and elevators, including all components thereof; sprinkler systems; fire escapes; and other components relating to the operation or maintenance of a building.

However, the term "structural components" does not include machinery the sole justification for the installation of which is the fact that such machinery is required to meet temperature or humidity requirements which are essential for the operation of other machinery or the processing of materials or foodstuffs. Machinery may meet the "sole justification" test provided by the preceding sentence even though it incidentally provides for the comfort of employees, or serves, to an insubstantial degree, areas where such temperature or humidity requirements are not essential. For example, an air conditioning and humidification system installed in a textile plant in order to maintain the temperature or humidity within a narrow optimum range which is critical in processing particular types of yarn or cloth is not included within the term "structural components".