A nonconforming use shall not be enlarged, unless Board of Adjustment approval is obtained, unless the use is changed to a conforming use; provided, however, that where a building meets the use requirements of this chapter and is nonconforming because of heights, area and/or yard regulations, said use may be enlarged, upon receipt of the appropriate Board approval provided that the height, area, volume or yard regulations are not further violated. The cost of structural alterations made in such building shall in no cases exceed 50% of its assessed value.