The lawful use of a building or premises existing on April 17, 2001, may be continued although such use does not conform with the provisions of this chapter. The total structural repairs or alterations on such a nonconforming building shall not, during its life, exceed 50% of the fair market value of the building unless permanently changed to a conforming use. If such nonconforming existing use is discontinued for a period of 12 months, any future use of the building and premises shall conform to this chapter.