Residential professional offices shall provide at least one off-street parking space for each 100 square feet of floor space devoted to such office use, but in no case fewer than three such spaces, said parking space to be to the rear of the building line. Residential professional offices shall be limited to those of the practice of medicine, dentistry, law, architecture and engineering. The conduct of such profession shall permit the employment of not more than one employee and shall be limited to 1/4 of the gross floor area of the building. The balance of the area shall be occupied by the professional practitioner as his principal residence.