The provisions of sections 54 and 55 of this title shall not be construed to require the removal, lowering or other change or alteration of any structure or tree not otherwise conforming as of the effective date hereof, or otherwise interfere with the continuance of any nonconforming use. 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 July 1, 1957, and is diligently prosecuted and completed within two years thereof.