Separate facilities. The determination of what constitutes a separate facility must be made on the facts and circumstances of each individual case. Generally, each facility with respect to which a separate election is in effect under section 169 shall be treated as a separate facility for purposes of this paragraph. However, if the depreciation or amortization which would have been allowable without regard to section 169 with respect to any part of a facility is based on a different useful life, date placed in service, or method of depreciation or amortization from the other part or parts of such facility, such part is considered a separate facility for purposes of this paragraph. For example, if a building constitutes a certified pollution control facility and various component parts of the building have different useful lives, each group of component parts with the same useful life would be treated as a separate facility for purposes of this paragraph. Two or more facilities may be treated as one facility for purposes of this paragraph where, with respect to each such facility: (i) The initial amortization under section 169 commences on the same date, (ii) the facility is placed in service on the same date, (iii) the estimated useful life which would be the basis for depreciation or amortization other than under section 169 has continually been the same, and (iv) the method of depreciation or amortization which could have been used without regard to section 169 could have continually been the same.