Before the demolition or razing of any building or structure has commenced the building or structure undergoing such demolition or razing and all debris on the property site shall be fenced and closed off to access by unauthorized persons with a fence not less than four feet in height. When such demolition and razing has reduced the building or structure to ground level, the resulting excavation consisting of a basement, cellar or other underground excavation shall be promptly filled to ground level not more than fifteen days after said building or structure has been lowered to ground level.