II. That whenever a petition is filed with the public agency by at least 10 residents of the municipality charging that any dwelling is unfit for human habitation or whenever it appears to the public agency by inspection that any dwelling is unfit for human habitation, it shall, if preliminary investigation discloses a basis for such charges, issue and cause to be served upon the owner, every mortgagee of record and all parties in interest in such dwelling (including persons in possession) a complaint stating the charges in that respect. If the person to be served resides outside the state, service may be made upon him by registered mail; and if there are any unascertained persons having an interest in said dwelling, notice may be given them by publication in a newspaper having general circulation in the municipality, such publication to be at least 10 days before the date set for the hearing. Such complaint shall contain a notice that a hearing will be held before the public agency at a place therein fixed not less than 10 days nor more than 30 days after the serving of said complaint; that the owner, mortgagee and parties in interest shall be given the right to file an answer to the complaint and to appear in person, or otherwise, and give testimony at the place and time fixed in the complaint; and that the rules of evidence prevailing in courts of law or equity shall not be controlling in hearings before such public agency.