A cost plus contract reimburses a contractor for its allowable costs and adds an agreed amount on top. Owners use this structure when the work is complex and the final cost is hard to estimate, which is common in capital project management programs. The model shares cost risk between the parties, but it places a heavy burden on the owner to confirm that each reimbursable cost is real, allowable, and documented. Those checks are easiest before payment, when the backup is complete and the people who incurred the costs can still answer questions.
How cost plus contracts work
In a cost plus contract, also called a cost reimbursable contract, the owner reimburses the contractor's allowable costs and pays an agreed additional amount on top. The contract defines which costs qualify, how they must be documented, and what the additional amount is based on. Because the owner carries more of the cost exposure, the reimbursement rules matter as much as the scope of work.
Cost plus structures suit work where scope is still forming and the final cost cannot be known in advance. A new process unit, a major equipment replacement, or a multi-year capital program may all fit. The structure protects the contractor from absorbing costs it cannot predict, which is why it appears on large projects. It also makes the owner's review a central control rather than an afterthought.
Cost plus is related to, but different from, time and materials billing. Time and materials bills hours at set rates plus materials. Cost plus reimburses actual costs and then adds an agreed amount. The review questions overlap, but the source of each line is different, so reviewers should know which structure governs an invoice before they start.
What counts as reimbursable
Reimbursable costs are the expenses the contract names as allowable, such as direct labor, materials, equipment, and subcontract amounts. A cost outside that list, or one without proper support, should not be paid simply because it was incurred. Clear definitions in the contract are the first line of review, and they should be read before the first invoice arrives.
- Direct labor. Wages and burden rates for people working on the project, limited to the categories and rates the contract defines.
- Materials and equipment. Items charged at cost or at the rental and rate basis in the contract, supported by receipts or equipment logs.
- Subcontracts. Amounts paid to subcontractors for approved scope, supported by their invoices and any lien waivers the contract requires.
- Excluded costs. Many contracts exclude the cost of correcting the contractor's own errors, entertainment, and certain penalties. Confirm each exclusion in the agreement rather than relying on general practice.
Overhead needs careful reading. Some contracts reimburse overhead only through the agreed additional amount, while others list specific indirect costs. A reviewer who treats overhead as a reimbursable line by default may pay the same cost twice. Markups on labor and materials follow their own rules, which are covered in how owners review markups on invoices.
Where cost and the added amount drift
Drift in cost plus billing usually comes from three sources: costs that fall outside the contract's definition, burdens applied twice, and the additional amount calculated on a base larger than the contract allows. Each source is visible in the backup if the reviewer knows where to look.
- Double burdens. Labor rates that already include burden are sometimes burdened again on the invoice.
- Idle equipment. Equipment may be billed during standby or breakdown time at rates the contract does not support.
- Cross-job charges. Costs for one project may be coded to another, which shifts them outside the contract's scope and often outside its period.
- Wrong base. The additional amount may be calculated on total costs that include items the contract excludes.
- Duplicate subcontract lines. The same subcontract invoice may appear in two periods or under two job codes.
Cost coding is often the cause rather than the intent. Crews work across jobs, shared equipment moves between sites, and codes get selected in a hurry. The reviewer's job is to find the pattern, then ask the contractor to correct the coding and document the change.
How owners review backup before paying
Backup documentation is the evidence behind each reimbursable cost. A review checks that every line has support, that the support matches the contract's definition, and that the arithmetic holds. Missing backup is a request for information, not a conclusion, and it should be handled that way in writing.
The review usually runs in a consistent order:
- Confirm every cost line maps to a category the contract allows.
- Match labor lines to signed timesheets and payroll summaries.
- Match materials and equipment to receipts, logs, and the agreed rate basis.
- Check subcontract invoices against the approved scope and the period billed.
- Recalculate the additional amount on the correct base.
- Confirm each cost falls within the contract period and the project scope.
Keep the trail. Each request for backup should name the invoice line, the contract clause, and the document needed. Record the answer, the date, and any credit issued. A file built this way lets anyone who picks up the project later see what was reviewed and why.
Cost codes that make review faster
Coding structure has a large effect on review speed. When job and cost codes mirror the contract's categories, a reviewer can trace each reimbursable line without rebuilding the ledger. Owners can ask for that alignment at the start, when it is easy to agree.
Teams that want help setting up that structure can look at Capital Projects Reviews for contractor cost control.
Keeping the working relationship intact
Cost plus work runs for months, and reviewers and contractors talk often. A review framed as a documentation check tends to produce faster answers than one framed as suspicion. Owners do best when they ask for specific support, cite the contract basis for each question, and record the response.
Ask for the backup the contract already requires, and name the clause. Most questions resolve quickly when both sides see the same records. An organized review protects the relationship the owner depends on for the rest of the program.
Next step
If your team pays reimbursable costs on capital or turnaround work, a pre-payment review can show where backup is strong and where it needs to be requested. We start with the contract definitions, the coding structure, and a recent set of invoices, so the first conversation stays specific to your terms.
Revenew brings 27+ years of audit results and experience to this work.
Request a No-Risk Review to start that review. There is no upfront fee.