Nothing in this chapter shall be deemed to require any change in the plans, construction or designated use of any building for which a building permit was duly issued and on which actual construction was lawfully begun and diligently carried on prior to the adoption of this chapter, provided that the building shall be completed within 12 months after the passage of this chapter. "Actual construction" is hereby defined to be the actual placing of construction materials in their permanent position, fastened in a permanent manner, except that where a basement or cellar is being excavated, such excavating shall be deemed to be actual construction, or where demolition or removal of an existing structure has been begun preparatory to rebuilding, such demolition or removal shall be deemed to be actual construction. Similarly, whenever a district shall be changed hereafter, the provisions of this chapter with regard to building permits issued prior to the passage of this chapter shall apply to building permits issued for construction in that changed district before the time the amendment effecting the change becomes effective.