All lawful or previously nonconforming buildings, structures and uses of land or of a building, sign or other structure existing on the effective date of this chapter may be continued, altered, restored, reconstructed, changed, sold or maintained, even though such building, structure or use may not conform to the use, height, area, yard and other regulations of the district in which it is located, provided that such nonconforming conditions shall comply with the following sections of Article XXI.