Nothing, herein contained shall require any change in the plans, construction, or designated use of a building for which a building permit has been heretofore issued and plans for which are on file with the city clerk at the time of passage of this ordinance and the construction of which in either case shall have been diligently executed within one year of the date of such permit and the ground-story framework of which, including the second tier of beams, shall have been completed within such year and which entire building shall be completed, according to such plans as filed within two years from the date of the passage of this ordinance.

Nothing in this ordinance shall be taken to prevent the restoration within 12 months of a legal nonconforming building destroyed or partially destroyed by fire, explosion, or other casualty or act of God or the public enemy, provided that when such restoration becomes involved in litigation the time required for such litigation shall not be counted as a part of the 12 months allowed for construction and further provided that upon restoration of a legal nonconforming use of the building may not be changed unless changed to a conforming use, and nothing in this ordinance shall be taken to prevent the continued occupancy or use of such building or part thereof which existed at the time of such partial destruction, but any building so damaged more than 50 percent of its reasonable value may not be rebuilt, repaired, or used unless it is made to conform to all regulations for all buildings in the district in which it is located.