No changes may be made in the location, dimension, character or duration of the encroachment or use as granted by the permit except upon written authorization of the director of public works. No permits shall be required for the continuing use or maintenance of encroachments installed by public utilities, or for changes, additions or extensions therein or thereto where such use, maintenance, changes, additions or extensions require no excavation of the right-of-way or watercourse. (Ord. No. 1201, § 2 (part).)