No building, structure or other improvement shall hereafter be erected or placed within the official plan lines of streets and highways as adopted by this chapter, except that this provision shall not apply to garden and agricultural crop planting and such ordinary front yard fences and such more or less nonpermanent sprinkling and irrigation structures as, in the opinion of the Planning Commission, will not defeat the purpose of this chapter. (Ord. 567, § 8, 6/21/1966)