The final plan, or parts of the final plan as finally approved, shall be filed in the Building Official's office and building permits may be issued only for structures conforming to the OPD-H plan. In the event commercial uses are approved as a part of a planned development, no building permit for a commercial use shall be issued until Certificates of Occupancy for a minimum of 25 percent of the housing units planned for the area (or approved portion) have been issued or unless 25 percent of the housing units planned for the area (or approved portion) will be constructed simultaneously. (Separate building permits shall be obtained by the developer for the construction of housing and commercial uses where separate buildings are to be used.)