Side yards. Each lot shall have two side yards with a total width of not less than 20 feet, neither one of which may be less than eight feet in width at the front and rear building line. When a lot less than 70 feet in width exists in separate ownership on the effective date of this chapter, the total width of the two side yards may be reduced by one foot for each foot of lot width less than 70 feet, but not to a total width of less than 12 feet. When the total side yard becomes less than 16 feet, the two side yards shall be of equal width.