Incidental uses customarily incident to any use, authorized by this section, may be carried on in not more than one room or compartment, when located within the same building or upon the same lot in which or on which the use authorized by this section is actually made of the premises, if the following conditions are strictly fulfilled: Such incidental use must be definitely incidental and not a principal use, and must be indispensable necessary to the enjoyment of the premises of any one of the uses permitted by this section. The incidental use must be customary according to the then prevailing custom in the area and adjacent areas which are exclusively residential in character. Nothing herein shall authorize or be construed to permit the use of any part of the premises within a single-family dwelling district for the purpose of conducting or carrying on the business, trade or professional practice, or carrying on the business of practice of rendering personal trade or professional service for which a charge is made or for which any kind of remuneration is required or received by occupants of the premises, except that professional services may be rendered in unusual and occasional emergency cases when such service is essential to the preservation of life or to the rendering of first aid.