The lawful nonconforming use of a structure or land, including but not limited to fences, parking and zoning setbacks, existing at the time of the adoption or amendment of this chapter may be continued although the use does not conform to the provisions of this chapter. However, only that portion of the land in actual use may be so continued, and the structure may not be extended, enlarged, reconstructed, substituted, moved or structurally altered except when required to do so by law or order or so as to comply with the provisions of this chapter.