41 CFR 301-11.22: -11.22 Circumstances for prescribing a reduced per diem rate.

An agency may prescribe a reduced per diem rate lower than the prescribed per diem rate under the following circumstances:

(a) When the agency can determine in advance that lodging and/or meal costs will be lower than the per diem rate, such as when two employees share a room or kitchen facilities are available, reducing the need for buying prepared meals; and

(b) The lowest authorized rate must be stated in the travel authorization before travel or the traveler must be given sufficient notice once travel has begun to adjust spending (i.e., finding and occupying alternative lodging).