This shall be understood to include only the professional office or studio of a doctor, dentist, artist, architect, engineer, musician, accountant or attorney at law. This may also include the use of rooms for customary home occupations, including dressmaking, millinery or similar handicrafts when permitted as a special exception by the Board of Zoning Appeals in accordance with standards set forth in Article XX. The office, studio or occupational room shall be located on the basement or first floor level in the same building containing the dwelling unit in which the practitioner resides, and an exterior means of ingress and egress to same shall be required. No goods shall be publicly displayed so as to be visible from the outside of the office, studio or occupational room, and no sign or advertisement shall be shown other than one sign not larger than six inches by 12 inches, bearing only the name and profession or occupation (words only) of the practitioner. The number of units within a multiple dwelling which are used for any of the purposes set forth in this subsection shall not exceed 10% of the total number of units in the same multiple dwelling.