Whenever the Designated Official, in the enforcement of this chapter, finds in or about a dwellings conditions that pose an immediate and serious threat to the health, welfare or safety of the occupants or the general public, the Designated Official shall give to the owner and occupants of the premises a written order to vacate. This order shall be served personally upon the owner and tenant or by certified mail, restricted delivery, with return receipt requested to the owner and tenant. This notice shall explain each and every violation of this chapter that exists. The Designated Official shall post upon the dwelling a placard designating the dwelling has been determined unfit for human habitation. No dwelling which has been placarded shall be again used for human habitation until written approval is secured from the Designated Official and such placard has been removed by the Designated Official. The Designated Official shall remove such placard whenever the violation upon which the placarding act was based has been eliminated.