Whenever any grade is established in any street, alley, avenue or public place or ground, no person shall lay, build or construct any such sidewalk therein except in conformity therewith. Such sidewalk shall be so constructed that the outer edge of the top surface thereof at the curb line shall coincide with such established grade. In all cases where there is no established grade, any such sidewalk may be constructed wholly in accordance with the directions of and subject to the approval of the board of trustees. (1999 Code § 7.15)