In the case of a nonconforming use, such enlargement or extension shall not exceed, in all, 50% of the floor area of such building or structure actually devoted to such nonconforming use on the effective date of this use having first become nonconforming. In the case of a nonconforming building or structure, such enlargement or extension shall not exceed, in all, 50% of the usable floor area of such building or structure existing on the effective date of the building or structure having become nonconforming.