The Planning Commission, by written finding, may determine that a particular public utility facility or installation, nonconforming to the requirements of this chapter, is necessary to serve the areas in which it is located. Said public utility facility may then be extended or altered, provided said facility does not extend beyond the boundaries of the existing site and provided, that said addition, extension or alteration complies with all other performance standards provisions of this chapter.
('64 Code, § 51.20)