It's never 7 year property, even if practitioners are using that shorter life incorrectly. First, the definition of seven year property includes property with a class life of 10 years or more but less than 16 years (Code sec 168(e)(1)). This includes office furniture, equipment and fixtures that are not structural components.
Further, Code sec 1.48-1(e)(2) actually defines the structural components of a building -
(e) Definition of building and structural components. (1) Generally, buildings and structural components thereof do not qualify as section 38 property. See, however, section 48(a)(1)(E) and (g), and §1.48–11 (relating to investment credit for qualified rehabilitated building). The term “building” generally means any structure or edifice enclosing a space within its walls, and usually covered by a roof, the purpose of which is, for example, to provide shelter or housing, or to provide working, office, parking, 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. Such term does not include (i) a structure which is essentially an item of machinery or equipment, or (ii) a structure which houses property used as an integral part of an activity specified in section 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 designed 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. Thus, the term “building” does not include such structures as oil and gas storage tanks, grain storage bins, silos, fractionating towers, blast furnaces, basic oxygen furnaces, coke ovens, brick kilns, and coal tipples.
(2) The term “structural components” 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 conditioning 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”. For special rules with respect to an elevator or escalator, the construction, reconstruction, or erection of which is completed by the taxpayer after June 30, 1963, or which is acquired after June 30, 1963, and the original use of which commences with the taxpayer and commences after such date, see section 48(a)(1)© and paragraph (m) of this section.