Guide
Your workers' comp premium is an estimate until the year-end audit reconciles it against what actually happened. Here's what auditors actually check, where the bill blows up, and how to walk in prepared.
If your policy runs on a calendar year, the audit notice usually shows up in the first few months after renewal. If it runs on another cycle, it lands a few weeks past your policy anniversary. Either way, most owners get the notice, treat it like paperwork, and hand over whatever the bookkeeper has on file — then get hit with a bill weeks later that's hundreds or thousands more than expected.
The premium you pay when you buy or renew a policy is an estimate based on projected payroll. The audit is the carrier checking that estimate against what you actually paid out over the year — in wages, in overtime, and in money to subcontractors. The gap between estimate and reality is where surprise bills come from, and it's entirely avoidable with a bit of prep.
An auditor (sometimes a person, increasingly a remote or self-reported audit) asks for a defined set of records covering your policy period: payroll registers, quarterly tax filings (Form 941), W-2s, 1099s for anyone you paid as a contractor, and documentation for any subcontractors you used. They classify your payroll into class codes — the same class codes your policy was rated on — and compare the actual dollar amounts to your original estimate.
If actual payroll came in higher than estimated, you owe the difference. If it came in lower, you get a credit. Straightforward in theory. In practice, three things routinely blow the number up: subcontractors without their own coverage, payroll that wasn't split correctly between office and field work, and overtime that wasn't documented in a way the carrier can exclude the premium portion of.
This is the one that catches contractors off guard most often. If you paid a subcontractor and didn't collect a valid certificate of insurance (COI) showing they carried their own workers' comp coverage for the exact dates they worked on your jobs, the carrier treats that money as if it were your own employee payroll — usually charged at your highest applicable class code rate, not the sub's lower rate.
The trap isn't always "no certificate at all." It's often a certificate collected at the start of a job that expired two or three months before the work wrapped. A six-month job with a COI that lapsed in month two means four months of that sub's pay gets reclassified as yours. On a mid-size sub relationship, that alone can add several thousand dollars to your audit bill.
Rule of thumb: collect a COI before the sub sets foot on a job, confirm the effective dates cover the entire time they'll be working for you, and calendar the expiration date so you can request a renewed certificate before it lapses — not after.
Office vs. field payroll. Office staff — scheduling, dispatch, bookkeeping — are rated at a much lower class code than field labor. If your payroll records lump everyone together, or if a field employee occasionally covers the phones, the auditor may default the whole thing to the higher field rate unless your records clearly separate the hours.
Overtime premium. In many states, the extra half-time (or double-time) premium portion of overtime pay can be excluded from the payroll the audit rates you on — you only get charged comp premium on the straight-time equivalent. But this only works if your payroll records break out regular hours from overtime hours clearly. Lump-sum payroll totals don't get you the exclusion.
Keep this documentation for five to seven years — check your state and your policy for the exact figure, and check with your insurance agent or accountant on anything specific to your situation, since audit rules vary by state and carrier.
An audit bill is really a delayed labor cost — it just shows up months after the work was billed. If your workers' comp premium creeps up year over year (more field hours, a rate class change, a bad claim), that cost needs to land in your hourly rate and your job pricing, not just get absorbed. Run your numbers through the hourly rate calculator after your next audit settles to see whether your rate still covers true labor cost, and check a recent job through the job profitability calculator to see how much a comp rate increase actually eats into margin on a typical job.
If chasing down current certificates from a rotating list of subs, plus organizing payroll records before renewal, is the kind of admin task that keeps sliding to "next week" — that's exactly the repetitive, easy-to-drop work a done-for-you admin service is built to take off your plate before it turns into a surprise bill. A simple subcontractor-COI tracking sheet, the kind included in the Operations Kit ($19), also works fine if you just need a system to stop relying on memory.
Most carriers audit at the end of your policy term, once a year, based on actual payroll and subcontractor payments over the past 12 months versus the estimate you gave when you bought the policy. Some carriers also run a mid-term or voluntary audit if your payroll changes significantly. If your policy runs on a calendar year, expect the notice early the following year; on another cycle, it typically lands a few weeks after your renewal date.
Paying a subcontractor who didn't carry their own workers' comp coverage, or whose certificate expired partway through the job. The carrier then treats everything you paid that sub as your own payroll, usually at your highest class code rate, and bills you retroactively. This is consistently the largest single source of unexpected audit bills for contractors who use subs.
Typically five to seven years, covering payroll registers, 1099s, subcontractor agreements, and every certificate of insurance with its effective dates. Confirm the exact requirement with your state and your policy documents, since retention rules vary.