Buildings, each having a ground coverage of not less than 20,000 square feet, containing administrative, business and professional offices, including banks and other financial institutions, and research facilities for development and testing of products, management systems and services, provided, however, that when any building contains the office or research facility of two or more separate firms or organizations, the access to floor area occupied by each firm or organization shall be by means of doors and corridors serving the building generally and there shall be no separate access from such floor area to the outdoors.