Except as provided in § 62.23(7), (hb) and (hc), Wis. Stats., the total structural repairs, alterations or improvements to 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. For the purpose of determining whether the 50% value limit has been exceeded, the assessed value, if known, in the tax year the nonconformity began shall be used as the base value. If such assessed value is unknown, the assessed value in the earliest year after the nonconformity began for which assessment records are available shall be used as the base value.