The Enforcement Agency shall commence proceedings to cause the repair, rehabilitation, vacation or demolition of unsafe buildings by having served upon the owner and parties in interest in any such building, a complaint stating the conditions found making the building an unsafe building, together with a notice that a hearing will be held before the Enforcement Agency at a place therein designated not less than ten days and not more than thirty days after serving the complaint and notice. The owner and parties in interest shall have the right to file an answer to such complaint.