The building inspector and plans examiner are authorized and empowered to enter any building or premises within the city for purposes of inspection or to prevent a violation of this chapter, upon presentation of proper credentials. Whenever in their opinion any provision of this chapter is being violated by the work, they or either of them may order such work to forthwith cease. Whenever in the opinion of the building inspector any building or portion thereof is dangerous or unsafe to persons or property, he may condemn the same and order its repair or removal. Written notice of such condition shall be served upon the owner, reputed owner or person in charge of such building and no building so condemned shall thereafter be used or occupied for any purpose after written notice as above provided has been served until the same shall have been repaired and approved by the building inspector. (1961 code § 6.04.020.)