A nonconforming building or structure having been damaged or partially destroyed by fire or other calamity to an extent not exceeding 60 percent of the assessed valuation, exclusive of foundations, at that time, may be restored and its immediately previous occupancy or use, existing at the time of such partial destruction, may be continued or resumed; provided the work of restoration is commenced within one year of the date of such partial destruction and is diligently carried on to completion. Whenever a nonconforming building or structure is damaged in excess of 60 percent of its assessed valuation, exclusive of foundations, at that time, the repair or reconstruction of such building shall conform to all of the regulations of the district in which it is located and it shall be treated as a new building.