Where a building is situated on a corner lot, in no case shall the building be closer than 20 feet to the curb line as provided in subsection 26-23.6a (Fences, Walls and Sight Triangles) Should the prevailing front yard setback of existing buildings along the street serving as a side yard of the corner lot be less than 20 feet, the building on the corner lot shall still be set back at least 20 feet from the street serving as the side yard and the other side yard may be reduced to five feet. On a corner lot, the narrowest width of the lot shall be determined to be the front yard for meeting the front, side, and rear yard requirements of this Chapter regardless of how the owner faces the building. All front and side yards shall be specifically designated as same on the plans submitted at the time a construction permit is applied for and shall not be altered or changed at a later date unless authorized by variance.