The regulations prescribed in Section 13.12.040 and 13.12.050 of this chapter shall not be construed to require the removal, lowering or other change or alteration of any structure or tree not conforming to the regulations as of the effective date hereof, or otherwise interfere with the continuance of any nonconforming use except as herein provided. Nothing herein contained shall require any change in the construction, alteration, or intended use of any structure, the construction or alteration of which was begun prior to the effective date of this chapter, and is diligently pursued and completed within a reasonable time thereof. (Ord. 938, 2/3/64; 3433, 8/23/83)