(1) “Annual fair market rental amount” means the annualized sum of the rents collected for all rental units in the housing accommodation during the base calculation year, plus an amount equal to the percentage increase in the Consumer Price Index for Urban Wage Earners and Clerical Workers (CPI-W) for all items, in the Washington, D.C. Standard Metropolitan Statistical Area, during each calendar year; provided, however, that if no rents were collected in the base calculation year because the housing accommodation was then under construction, the annual fair market rental amount shall be a sum equal to the rents which would have been collected during the base calculation year had the housing accommodation been 100% occupied during the entire base calculation year, the sum to be determined by appraisal, as increased by the Consumer Price Index increase under this paragraph.