2013 California Standards for Accessible Design Pocket Guide

11B-411.1.1 Floor designations.

In facilities served by destination-oriented elevator systems, floor designations shall be numeric characters only. Floor designations shall be "one" (1) or "zero" (0) at the main entry level and shall increase by one for each successive higher story or level. The initial floor below the main entry level shall be designated “minus one” (-1) and the designation for each successive lower story or level shall decrease by one. Stories or levels shall not be designated by alphabetic characters.

Exceptions:

                           1.      In existing facilities where new elevators are installed or existing elevators are altered into a destination-oriented elevator system, levels within stories, such as mezzanines located above or below the main entry level shall be permitted to be designated with an alpha-numeric character such as "M2", indicating "mezzanine" and the "story number", respectively, in which it is located, provided there is no duplication with alphanumeric designations of elevator cars in the facility.

                           2.      Non-successive floor numbering shall be permitted where a specific floor number is omitted  or where a floor is frequented only by service personnel for maintenance, repair or occasional monitoring of equipment.