48 CFR §36.207

1. (a)Generally, firm-fixed-price contracts shall be used to acquire construction. They may be priced (1) on a lump-sum basis (when a lump sum is paid for the total work or defined parts of the work), (2) on a unit-price basis (when a unit price is paid for a specified quantity of work units), or (3) using a combination of the two methods.
2. (b)Lump-sum pricing shall be used in preference to unit pricing except when—
   1. (1)Large quantities of work such as grading, paving, building outside utilities, or site preparation are involved;
   2. (2)Quantities of work, such as excavation, cannot be estimated with sufficient confidence to permit a lump-sum offer without a substantial contingency;
   3. (3)Estimated quantities of work required may change significantly during construction; or
   4. (4)Offerors would have to expend unusual effort to develop adequate estimates.
3. (c)Fixed-price contracts with economic price adjustment may be used if such a provision is customary in contracts for the type of work being acquired, or when omission of an adjustment provision would preclude a significant number of firms from submitting offers or would result in offerors including unwarranted contingencies in proposed prices.