Ordinances enacted by local authorities pursuant to this chapter may incorporate appropriate provisions of this title, of Article 9 (§ 16.1-278 et seq.) of Chapter 11 of Title 16.1, and of Article 2 (§ 18.2-266 et seq.) of Chapter 7 of Title 18.2 into such ordinances by reference. Nothing contained in this title shall require the readoption of ordinances heretofore validly adopted. Local authorities may adopt ordinances incorporating by reference the appropriate provisions of state law before the effective date of such state law; provided that such local ordinances do not become effective before the effective date of the state law. The provisions of this section are declaratory of existing law.