It shall be the duty of the owner of abutting property to keep the sidewalk in repair, but the city may, after giving five days' notice to the owner or his agent, if known, of the necessity for making repairs, and without notice if the lot or piece of land is unoccupied, make all necessary repairs at any time. The same shall be done and the cost thereof assessed against the lot or piece of land abutting on the sidewalk so repaired as may be provided by law. (Prior code § 14-108)