There shall be provided at the time of erection or enlargement of any principal building or structure minimum off-street parking space with adequate provision for ingress and egress as follows:
a.
Two parking spaces for each dwelling unit.
b.
Professional offices of chiropractors, dentists, medical and osteopathic physicians and optometrists, in addition to those required by Subsection a above, three parking spaces plus one for each employee.
c.
Professional offices other than those listed in Subsection b above, in addition to those required by Subsection a above, one parking space for each employee.