4. Depreciation shall be subtracted in order to take into account, where applicable, the significant difference existing between:(1) the interior space that would be available in a structure having exactly the same outside dimensions than the structure the value of which is to be established, as they exist on the date applicable under the first or second paragraph of section 46 of the Act, as the case may be, if the materials and techniques currently used on that date had been used for building such a structure;
(2) the interior space actually available on the same date, whether used or not, in the structure the value of which is to be established.