Nonconforming uses. The nonconforming use of a nonconforming structure, building, premises, or fixture existing at the time of the adoption or any amendment of this chapter may be continued although it does not conform to the provisions of this chapter or any amendments thereto, but the alteration of, or addition to, or repair shall not exceed 50% of its assessed value of any nonshoreland existing building, premises, structure, or fixture for the purpose of carrying on a nonconforming use. The continuance of the nonconforming use of a temporary structure is prohibited.