One such office shall be permitted on any one property. Such office shall be conducted wholly within the principal building, by appointment only, and shall be clearly incidental and secondary to the principal use of the premises for permitted purposes, pursuant to Subsections 20-13.1 and 20-14.1. To this end, no such office shall occupy more than 10% of the total floor area of the principal building or 2,500 square feet, whichever is less.