Section 719.42 - What categories of costs are unallowable?(a) Specific categories of unallowable costs are contained in the cost principles at 48 CFR (FAR) part 31 and 48 CFR (DEAR) part 931 and 48 CFR 970.31 . See also 41 U.S.C. 4304 .(b) Costs that are customarily or already included in billed hourly rates are not separately reimbursable.(c) Interest charges that a contractor incurs on any outstanding (unpaid) bills from retained legal counsel are not reimbursable.