2013 California Standards for Accessible Design Pocket Guide

11B-306.3.4 Clearance reduction.

Between 9 inches (229 mm) and 27 inches (686 mm) above the finish floor or ground, the knee clearance shall be permitted to reduce at a rate of 1 inch (25 mm) in depth for each 6 inches (152 mm) in height.

Exception:  The knee clearance shall not be reduced at built-in dining and work surfaces required to be accessible by Section 11B-226.1.