The joint or collective parking provision of required off-street parking areas shall comply with the following standards and requirements:
(a) All required parking spaces shall be located on the same lot with the building or use served, except that where an increase in the number of spaces is required by a change or enlargement of use, or where such spaces are provided collectively or jointly by two or more buildings or establishments, the required spaces may be located not farther than 500 feet from the building served.
(b) The total of such off-street parking spaces supplied collectively shall be not less than the sum of the requirements for the various uses computed separately.
(c) In any case where the required parking spaces are not located on the same lot with the building or use served, or where such spaces are collectively or jointly provided and used, a written agreement thereby assuring their retention for such purposes shall be properly drawn and executed by the parties concerned, approved as to form by legal counsel of the City and filed with the application for a zoning permit.