Side yards (two), width: 20 feet each, on interior lot. On a corner lot, the side yard abutting the street shall be not less than 35 feet in width. No side yard shall be required where adjoining property owners shall mutually agree in writing. However, in no case shall party walls be permitted between properties or lots of separate ownership. In the case of a series of abutting structures paralleling and abutting a public right-of-way, an open and unobstructed passage for vehicles and pedestrians, of at least 20 feet in width shall be provided at grade level at intervals of not more than 400 feet.