Any lawful nonconforming use existing at the date of the adoption of this chapter may be continued, and any existing building designed, arranged, intended or devoted to a nonconforming use may be reconstructed or structurally altered, subject to the following regulations:

No nonconforming building damaged by fire or other causes shall be repaired or rebuilt so as to extend or enlarge either the nonconformity of the building or the nonconforming use to which it was devoted at passage of this chapter. Buildings totally destroyed by fire or other causes, or demolished by the owner, shall not be rebuilt except in conformity with the regulations of this chapter. Nothing in this chapter shall prevent the strengthening or restoring to a safe condition of any wall, floor or roof which has been declared unsafe by the Construction Official.

Whenever the boundaries of a district shall be changed so as to transfer an area from one district to another of a different classification, the foregoing provisions shall also apply to any nonconforming uses existing therein or created thereby.

Nothing herein contained shall require any change in plans, construction or designated use of a building, structure or sign 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 plan as filed within one year from the date of the adoption of this chapter.

Nothing in this chapter shall be interpreted as authorization for or approval of the continuance of the use of a structure or premises in violation of zoning regulations in effect at the time of the effective date of this chapter.