Reports or notices issued by the Building Inspector pursuant to this chapter shall be served upon persons involved either personally or by registered mail, but if the whereabouts of any person is unknown and the same cannot be ascertained by the Building Inspector in the exercise of reasonable diligence, and the Building Inspector shall make an affidavit to that effect, then the serving of a complaint or order upon the person may be made by publishing the same once a week for two successive weeks in the official newspaper of the city and service shall be deemed complete upon the data of the last publication.  A copy of the complaint or order shall be posted in a conspicuous place on premises affected by the complaint or order and shall be recorded in the office of the Circuit Clerk of the county wherein the dwelling or building is located
('76 Code, § 210.05)