The Township Council hereby determines that an additional improvement, conversion or construction completed on a property already granted a previous exemption pursuant to this article during the period in which the previous exemption is in effect shall qualify for an additional exemption under the standards identified in this article. The additional improvement, conversion or construction shall be considered as separate for purposes of calculating the exemption, except that the assessed value of any previous improvement, conversion or construction shall be added to the assessed valuation as it was prior to that improvement, conversion or construction for the purpose of determining the assessed value of the property for which any additional exemption is to be subtracted.