The lawful use of any building or land lawfully existing at the time of the enactment of this Part 4 or any amendment thereto may be continued although such use does not conform to the provisions of this Part 4 as a result of such enactment or amendment.

Any structure or portion thereof declared unsafe by a proper authority may be restored to a safe condition.

A nonconforming building may not be reconstructed or structurally altered during its life to an extent exceeding in aggregate cost 50% of the assessed value of the building, unless said building is changed to a conforming use.

A nonconforming use shall not be extended, but the extension of a lawful use to any portion of a nonconforming building which existed prior to the enactment of this Part 4 shall not be deemed the extension of such nonconforming use.

Nothing herein contained shall require any change in plans, construction or designated use of a building for which a building permit has heretofore been issued and the construction of which shall have been diligently prosecuted within three months of the date of such permit and the ground-story framework of which, including the second tier of beams, shall have been completed within six months of the date of the permit and which entire building shall be completed according to such plans as filed within a year from the date of this Part 4.

No building damaged by fire or other cause to the extent of more than 65% of its assessed value shall be repaired or rebuilt, except in conformity with the regulations of this Part 4. No lot shall be reduced in area that any required open space will be smaller than prescribed in the regulations for the district in which said lot is located. Whenever such reduction in lot area occurs, any building located on said lot shall not thereafter be used until such building is altered, reconstructed or relocated so as to comply with the area and yard requirements applicable thereto.

Any nonconforming building or use which has been changed to a conforming use shall not be changed back again to a nonconforming use.

A nonconforming use shall be considered to be abandoned if there occurs a cessation of the previous use or activity on the part of the owner or tenant for a period of six months from the date of cessation of use or activity.