No building in the rear of a principal building on the same lot shall be used for residential purposes unless it conforms to all the yard and other open space and off-street parking requirements. For the purpose of determining the front yard in such cases, the rear line of the required rear yard for the principal building in front shall be considered the front lot line for the building in the rear. In addition, there must be provided for any such rear dwelling, an unoccupied and unobstructed accessway not less than twenty feet wide, to a public street for each dwelling unit in such dwelling, or one not less than fifty feet wide for three or more dwelling units. (Ord. 21-67. Passed 12-4-67.)