In any case where it appears that a steeper driveway grade than that specified in this section is necessary or desirable because of existing terrain conditions, a driveway may be constructed (in any place where no sidewalk exists) at such steeper grade. It shall be a condition to the granting of any permit to construct a driveway at a grade other than that specified that all rights acquired by virtue of such permit shall be subject to the right of the City to construct sidewalks at the established grade. In any place where sidewalks have, prior to July 28, 1958, been constructed at a different grade than that specified in this section, driveways may conform to the grade of the existing sidewalks. Nothing herein shall permit the construction of any driveway so as to divert or obstruct the normal flow of drainage in the adjoining street.