Within the districts established by this title or amendments that may later be adopted, there exist structures and uses which were lawful before this title was adopted or amended, but which would be prohibited under the terms of this title or future amendment to this title. It is the intent of this title to permit these nonconformities to continue until they are terminated but not to encourage their survival nor to permit their enlargement, expansion, or extension. To avoid undue hardship, nothing in this title shall be deemed to require a change in the plans, construction, or designated use of any building, where a building permit has been issued prior to the effective date hereof, and pursuant to such permit, construction is diligently carried to completion. (Ord., 3-8-1971)