The effective date of this chapter shall be 10 days after the adoption of this chapter as required by law, and the use of all land and every building or portion of a building or structure erected, altered with respect to height and area, added to, or relocated and every use within a building or use accessory thereto, in any district, shall be in conformity with the provisions of this chapter. Provided, however, that additions, and/or extensions to buildings, at the time of passage of this chapter, shall be permitted without restriction as to height, area, or setback, in all cases where it is shown that the plans under which the existing buildings were erected definitely provided for such future additions and/or extensions.