The odd thing was that the aircraft ahead of us was cleared for a CAT II approach while we did a CAT I. But he was far enough ahead that I did not hear if they had specifically requested that approach.
If Low Visibility Procedures are in force at the airport (and they had to be, if the preceding a/c was cleared for cat II), you can do whatever category you are authorized for at that particular airport. You don't need specific clearance from the ATC for CAT I, II, or III