Buildings shall be so grouped that each building shall face, for the full length of its front facade, either upon an existing street or upon an open space which, in its least dimensions, shall not be less than 75 feet. Where the rear elevations of the principal buildings are built parallel to each other or within 30° of parallel, the average distance between their long dimension shall be not less than 60 feet. Each building, whether principal or accessory, shall be at least 30 feet distant from any other building in the group, which distance shall be measured in a straight line between the points of the buildings which are nearest each other.