New single and multiple dwellings constructed for residential purposes and improvements to existing unoccupied dwellings or improvements to existing structures for purposes of conversion to dwellings shall not be valued or assessed for purposes of real property taxes until occupied, conveyed to a bona fide purchaser or 30 months from the first day of the month after which the building permit was issued or, if no building permit or other notification of improvement was required, then from the date construction commenced. The assessment of any multiple dwelling because of occupancy shall be upon the proportion which the value of the occupied portion bears to the value of the entire multiple dwelling. As used in this section, the term "dwellings" means buildings or portions thereof intended for permanent use as homes or residences.