Extension, alteration, replacing or rebuilding nonconforming uses or buildings. Notwithstanding any inconsistent provision of this chapter, any nonconforming use may be extended, and any nonconforming building, or any building in which is contained a nonconforming use, may be altered, extended, replaced or rebuilt, regardless of the extent thereof, provided application for the same and plans thereof are submitted to the Planning Board in accordance with the provisions of § 475-53. The Planning Board may approve the application upon such conditions, limitations and safeguards as it deems best for the public interest, excepting, however, that any nonconforming building or use shall not be restored for other than a conforming use after damage for any reason exceeding 75% of its market value, unless the Planning Board approves such restoration to the nonconforming use; if the restoration of a building whose damage does not exceed 75% of its market value is not completed within a one-year period, the nonconforming use of such building, or the nonconformity of the building itself, shall be deemed to have been discontinued.