(b) Such local law may also provide that the eligible property shall be exempt upon the conclusion of the exemption period authorized by paragraph (a) of this subdivision, for as long as construction or rehabilitation continues and, thereafter, for so long as such mortgage is outstanding and the housing development, as defined in section forty-two of the private housing finance law, is used for residential unit purposes; provided, that the exemption authorized by this subdivision shall be for a period not to exceed fifteen years in the aggregate after the conclusion of the exemption authorized by paragraph (a) of this subdivision, and shall not exceed the following limitations: three years of full exemption, followed by three years of exemption from eighty percent of the assessed value of such property, followed by three years of exemption from sixty percent of the assessed value of such property, followed by three years of exemption from forty percent of the assessed value of such property, followed by three years of exemption from twenty percent of the assessed value of such property; and provided that taxes shall be paid during any such period after the taxable status date immediately following the completion of construction or rehabilitation at least in the amount of the taxes paid on such land and improvements thereon during the fiscal year preceding the commencement of such construction or rehabilitation and that the exemption from taxes shall not be availed of concurrently under any other law. * NB Repealed July 23, 2021