The level of a public plaza, inclusive of major and minor portions, shall not at any point be less than the average elevation of curb level of the nearest adjoining street nor more than two feet above the average curb level of the nearest adjoining street in front of the major and minor portions of the public plaza. However, a public plaza with an area of 10,000 square feet or more may additionally have a maximum of 20 percent of its area at an elevation more than two feet above, but not more than four feet above curb level of the nearest adjoining street in front of the major and minor portions of the public plaza, provided that such higher portion may not be located within 25 feet of any street line. Public plazas that front on streets with slopes greater than 2.5 percent along the frontage of the public plaza may not at any point be more than one foot below the curb level of the adjoining street.