Billable expenses are costs a service provider pays out of pocket on behalf of a specific client’s project and then passes back to that client for reimbursement. They exist only because the engagement exists. Your office rent, your general insurance, and your everyday software subscriptions stay on regardless of who hires you, so those are overhead. A flight to a client’s headquarters, a prototype’s materials, or a software license bought solely for one project is something the client is expected to cover, on the terms the contract spells out.
Federal tax law draws a similar line. Under 26 U.S.C. ยง 162, a business can deduct “ordinary and necessary” expenses paid in carrying on a trade or business.1Office of the Law Revision Counsel. 26 U.S. Code 162 – Trade or Business Expenses Ordinary means common and accepted in your field; necessary means helpful and appropriate for the work. Most contracts borrow that same standard when defining reimbursable costs. If an expense can’t pass it, billing the client is difficult to justify.
What Kinds of Costs Are Billable
Travel
Travel is the category most providers deal with first. Airfare, train tickets, rental cars, lodging, and mileage on a personal vehicle qualify when the trip serves the client’s project. The IRS standard mileage rate for business driving in 2026 is 72.5 cents per mile, and many contracts peg reimbursement to that figure so neither side has to argue about gas prices or vehicle wear.2Internal Revenue Service. 2026 Standard Mileage Rates
Some contracts cap travel at federal per diem rates instead of requiring itemized receipts.3Federal Register. Maximum Per Diem Reimbursement Rates for the Continental United States (CONUS) Per diem simplifies recordkeeping because you’re not saving every restaurant receipt, but you still have to document the business purpose and destination of each trip.
Business Meals
Meals with clients or business associates are billable when they have a clear business purpose. One wrinkle: even if a client reimburses you for the full cost of a meal, whoever claims the tax deduction can only take 50% of it. Federal law limits the deduction for food and beverages to half the cost, provided the meal isn’t lavish and you or an employee is present.4Office of the Law Revision Counsel. 26 U.S. Code 274 – Disallowance of Certain Entertainment, Etc., Expenses Entertainment costs like game tickets and concert seats aren’t deductible at all. Food and drinks purchased separately during an entertainment event still qualify for the 50% deduction.
Materials, Subcontractors, and Technology
Project-specific materials cover a wide range: physical prototypes, specialized supplies for a construction or design job, printing for a major presentation, or lab testing fees. Subcontractors hired to handle niche tasks your firm doesn’t do in-house are billable too. So is technology purchased solely for a client project, like a dedicated software license, a cloud server instance, or heavy shipping fees for physical deliverables. Standard tools you’d own no matter who you were working for stay on your side of the ledger.
Markups on Billable Expenses
Some providers pass expenses through at cost. Others add a markup, commonly 10% to 20%, to cover the administrative burden of fronting money, managing vendors, and processing paperwork. Both approaches are common, and neither is inherently unfair. What matters is transparency. The contract should say whether expenses are billed at cost or cost-plus, and if there’s a markup, it should state the percentage. Clients who discover a hidden markup after the invoice arrives tend to dispute it, and those disputes are avoidable with one sentence in the agreement.
Documentation That Will Actually Get You Paid
Recordkeeping is what separates expenses that get reimbursed from expenses that get rejected. The IRS requires documentary evidence showing the amount, date, place, and essential character of each expense.5Internal Revenue Service. Publication 463 (2025), Travel, Gift, and Car Expenses In practice, that means keeping original receipts or digital copies that show vendor name, purchase date, total amount, and a description of what was purchased.
Mileage has its own rules. You need a log recording the date of each trip, the starting point, the destination, and the business purpose. A note like “drove to client site for project review” is fine; a bare odometer reading with no context is not.5Internal Revenue Service. Publication 463 (2025), Travel, Gift, and Car Expenses Dedicated mileage-tracking apps handle this automatically and produce reports that satisfy both client audits and tax inspections.
Keep everything in a central, searchable place. When it’s time to invoice, you want to assemble a complete packet in minutes, not hunt through email threads and bank statements for a missing hotel receipt.
How Long to Keep the Records
The IRS generally requires you to retain records supporting income, deductions, and credits until the period of limitations expires for the return in question. For most returns, that period is three years from the filing date. If you underreport gross income by more than 25%, the window extends to six years. If you have employees, employment tax records must be kept for at least four years.6Internal Revenue Service. Topic No. 305, Recordkeeping A safe default is to hold expense documentation for at least three years after filing the return that includes the deduction, longer if your client contract requires it.
How Reimbursements Are Taxed
Employees Under an Accountable Plan
For employees, reimbursements paid through an accountable plan are excluded from wages entirely and don’t show up on a W-2. An accountable plan requires three things: the expense must be business-related, the employee must substantiate it to the employer within a reasonable time, and any excess reimbursement must be returned. “Reasonable time” generally means substantiating expenses within 60 days of incurring them and returning any excess within 120 days.7Internal Revenue Service. Publication 15 (2026), (Circular E), Employer’s Tax Guide
Independent Contractors and the 1099-NEC Trap
Independent contractors don’t fall under the accountable plan rules of Section 62. When a contractor adequately substantiates expenses to the client, the reimbursement can still be excluded from income under a separate provision covering working condition fringe benefits.8Internal Revenue Service. Nonresident Aliens and the Accountable Plan Rules The standard is the same: document every element of the expense and present that documentation to the payer.
Here’s where it bites contractors. When a contractor does not account for reimbursed expenses to the client, those reimbursements get lumped into Box 1 of Form 1099-NEC as nonemployee compensation, meaning you’ll owe income tax and self-employment tax on money that was only ever meant to cover costs.9Internal Revenue Service. Instructions for Forms 1099-MISC and 1099-NEC Submit itemized expense reports with receipts to every client, and make sure the contract requires separate reporting of fees and reimbursed expenses.
Invoicing and Payment Terms
List reimbursable expenses on the invoice as separate line items, distinct from your service fees. Each entry should identify the date, the vendor or description, and the amount. Attach receipts so the client can verify each charge without having to ask follow-up questions. The less work you create for the person approving the invoice, the faster you get paid.
Most service contracts specify payment terms in “Net” language. Net 30 means the client has 30 calendar days from the invoice date to pay; Net 60 gives them 60. Some contracts offer an early-payment discount, such as 2% off if paid within 10 days. Assume the client will use the full window. If the contract is silent on payment terms, you lose leverage on any late-payment dispute, so nail this down before work begins.
Missing or incomplete documentation is the single most common reason reimbursements get delayed or denied. A rejected line item doesn’t always mean the expense was illegitimate. Often the receipt was missing or the description too vague. When a client pushes back on a charge, respond with the supporting receipt and a brief written explanation tying the cost to the project. If the expense was pre-approved, reference the approval. Formal dispute resolution rarely becomes necessary when the paperwork is solid.
Caps and Pre-Approval Clauses
Well-drafted contracts don’t leave expense reimbursement open-ended. Two safeguards show up routinely.
- Dollar-amount caps set a ceiling on total reimbursable expenses, sometimes across the engagement and sometimes per category. Once you hit the cap, additional costs come out of your margin unless the client agrees in writing to raise it.10Justia. Expenses Contract Clause Examples
- Pre-approval thresholds require written authorization before you incur any individual expense above a set amount. Skipping the approval step is risky. Even if the expense was legitimate and necessary, a client who didn’t authorize it may refuse to pay.10Justia. Expenses Contract Clause Examples
If your contract doesn’t include either mechanism, negotiate one in. Providers benefit too, because caps and pre-approvals create a paper trail proving the client agreed to the cost before it was incurred. That trail is your best defense if the invoice is later challenged.
When a Client Pays Late
If a client misses the deadline, your contract’s late-fee clause controls what happens next. Late-payment interest on commercial invoices varies by jurisdiction, with state statutory caps ranging from around 1% to 1.5% per month in states that set limits, while many states impose no statutory ceiling at all. Regardless of the legal maximum, a late fee is generally enforceable only if the contract discloses it before work begins. If your agreement is silent on late fees, collecting interest on overdue reimbursements becomes much harder. A clear late-payment provision, even a modest one, gives the client a reason to pay on time and gives you a contractual basis to collect if they don’t.