(a) In the Residence District, for every main dwelling, including detached or attached garages, there shall be a rear yard extending the full width of the lot, the least dimension of which shall be twenty feet. The rear yards herein provided for shall be open for their full required dimensions from the ground to the sky in the same manner and under the same restrictions as side yards.
(b) It shall be lawful to continue the use, occupancy, maintenance and repair of any structure in existence at the time that this section becomes law, even though in violation of this section.