The provisions of Article II, Chapter 5, and Article III, Chapter 6 (ACCESSORY OFF-STREET PARKING AND LOADING REGULATIONS), shall apply, except that the accessory parking requirements of Section 25-23 (Requirements Where Group Parking Facilities Are Provided) shall be modified to require accessory off-street parking spaces for at least 20 percent of the total number of new dwelling units. However, such modification shall not apply in R6B Districts.