Nonconforming uses. The continued lawful use of a building, premises, structure, or fixture existing at the time of the adoption or amendment of this chapter may not be prohibited although the use does not conform with the provisions of this chapter. The nonconforming use may not be extended. The total structural repairs or alterations in such a nonconforming building, premises, structure, or fixture shall not during its life exceed 50% of the assessed value of the building, premises, structure, or fixture unless permanently changed to a conforming use. If the nonconforming use is discontinued for a period of 12 months, any future use of the building, premises, structure, or fixture shall conform to this chapter.