Restoration of Damaged Nonconforming Structures. A nonconforming structure which is destroyed or damaged by fire, other casualty, act of God or by a public enemy to the extent of 50%, or more, of its cast of restoration to the condition in which it was before the occurrence, shall thereafter conform with the regulations of the district in which it is located. In the event such damage or destruction is less than 50% of its cost of restoration, no repairs or restoration shall be made unless a building permit is secured and reconstruction is started within 180 days from the date of the partial destruction and is diligently prosecuted to completion. Determination of the reproduction value shall be made by the director of code administration.