If any nonconformity is destroyed by reason of windstorm, fire, explosion or other act of God or a public enemy to an extent of more than 75% of the market value as appraised for the tax assessment purposes, then such destruction shall be deemed complete destruction and the nonconformity may not be rebuilt, restored or repaired except upon the demonstration of a hardship leading to the issuance of a variance in accordance with § 150-301 of this chapter. Nothing in this chapter shall prevent the strengthening or restoring to a safe condition any wall, floor or roof which has been declared unsafe.