As used in this chapter, the following terms shall have the meanings indicated:
Any building or structure, including any appurtenances thereto, which has been unoccupied for a period of 60 days or more or which has become unoccupied due to fire, flood or other natural calamity or which has been ordered vacated because the building has been declared unfit for habitation or as a hazard to life, limb or the general welfare of the occupants or the general public; except that this shall not apply to any building which is unoccupied because of a current alteration being performed under application and plan approved by the construction official having jurisdiction.