Structures legally existing as of the effective date of these regulations but that do not conform to the provisions of this chapter shall be considered legal non-conforming structures. Such structures may be modified and enlarged, provided that modifications or enlargement increase consistency with the site development standards for the zone in which the structure is located. At such time as a structure is substantially destroyed such that restoration costs exceed 75 percent of the assessed value of record when the damage occurred, reconstruction shall be consistent with the requirements of this chapter.