I don't know about cochins, but my sussexes, who are of decently large size, are rather on the thump-y and ungainly side when getting down off their ~4' high roost; I would not personally build one taller for them, unless there was a very easy-access multiple-chickens-using-at-same-time way for them to get down gradually.
JMO, good luck, have fun,
Pat