At all street intersections, no obstruction exceeding 30 inches in height above the established grade of the street at the property line, other than an existing building, post, column, hedge or tree, shall be erected or maintained on any lot within the area bounded by the line drawn between points along such street lot lines 25 feet distant from their intersection. The determination of the front yard of a corner lot shall be at the option of the owner or developer and shall be so designated on all maps and official records.