(A)   Regulations not retroactive. This chapter shall not apply to existing structures or objects of natural growth so as to require the removal, lowering or other changes or alteration of any such structure or object of natural growth. However, no nonconforming structure shall be made higher nor shall any nonconforming object of natural growth be allowed to become a greater hazard than it is at the time these regulations are adopted. If any nonconforming structure or object of natural growth which is hereafter damaged by any means to an extent exceeding 50% of its then reproduction value may not be restored or reconstructed to a height greater than permitted under the provisions of these regulations.
   (B)   Marking and lighting. Notwithstanding division (A) hereof, the owner of any nonconforming structure or object of natural growth is hereby required to permit the installation, operation and maintenance thereon of such markers and lights as shall be deemed necessary by the Federal Aviation Agency to indicate to the operators of aircraft in the vicinity of the airport the presence of such airport hazards. Such markers and lights shall be installed, operated and maintained at the expense of the city.
('65 Code, § 1347.09)