Zoning regulations are generally prospective, that is, they apply to uses of land proposed to be established after the regulations go into effect. This chapter provides rules for a variety of situations in which uses were established, lots were created, buildings were constructed, etc., prior to the enactment of zoning. After the enactment, questions arise as to continuance, change in use, expansion, etc. Within the zoning districts established by this chapter, there may exist uses, buildings, structures, and lots that do not conform to the provisions of this chapter. The purpose of this article is to specify those circumstances and conditions under which these nonconforming uses, buildings, structures and lots shall be permitted to continue.