There shall be at least one accessible W.C. cubicle on a floor, or in that part of a floor designed for access by the persons with a disability where the total number of W.C. cubicles provided on that floor or in that part of a floor is 20 or less, or 2 where the total number exceeds 20. This paragraph shall not apply to
domestic buildings and the domestic parts of composite buildings or where there is no toilet provided on the particular floor.
Where the accessible W.C. cubicles are within a toilet with multiple cubicles, the minimum number of such accessible W.C. cubicles to be provided for each sex shall be based on the total number of W.C. cubicles for each sex on that floor or in that part of a floor designed for access by persons with a disability.
references:
http://www.ada.gov/regs2010/2010ADAStandards/2010ADAstandards.htm
http://www.schoolfurniture.uk.com/dimenstion_sitting/sitt_low_leg_data.htm
http://www.naffainc.com/x/CodeBuddy5h98/Tables/Table1115B-1.htm