A building or structure used by a nonconforming use shall not be reconstructed, structurally altered, restored or repaired to an extent exceeding 50% of the replacement cost of such building or structure, exclusive of foundations, unless the use of such building or structure is changed to a conforming use, provided that the reconstruction, restoration or repair of a structure partially destroyed by fire or similar accidental cause shall commence within six months of the date of the partial destruction and be completed within 12 months of the date of destruction.