What is the legal test for establishing a perpetual non-conforming use in a residential district?

The theory in zoning is that each district is an appropriate area for the location of the uses which the zone plan permits in that area, and that the existence or entrance [127 Cal.App.2d 459] of other uses will tend to impair the development and stability of the area for the appropriate uses. The public welfare must be considered from the standpoint of the objective of zoning and of all the property within any particular use district. Rehfeld v. City and County of San Francisco, 218 Cal. 83, 85, 21 P.2d 419. It was not and is not contemplated that pre-existing nonconforming uses are to be perpetual. State ex rel. Miller v. Cain, 40 Wash.2d 216, 242 P.2d 505. The presence of any nonconforming use endangers the benefits to be derived from a comprehensive zoning plan. Having the undoubted power to establish residential districts, the legislative body has the power to make such classification really effective by adopting such reasonable regulations as would be conducive to the welfare, health, and safety of those desiring to live in such district and enjoy the benefits thereof. There would be no object in creating a residential district unless there were to be secured to those dwelling therein the advantages which are ordinarily considered the benefits of such residence. It would seem to be the logical and reasonable method of approach to place a time limit upon the continuance of existing nonconforming uses, commensurate with the investment involved and based on the nature of the use; and in cases of nonconforming structures, on their character, age, and other relevant factors.