The audit letter arrives after the policy year ends, which is rude timing but normal procedure.
You feel like you already paid for that year. Then the carrier asks for payroll records, class code detail, subcontractor certificates, and a little emotional maturity. The extra bill, if there is one, feels like a penalty.
Most of the time, it is not. It is a reconciliation. The cheap quote can look brilliant for twelve months because it was built on hopeful payroll and friendly class codes. Then the audit asks for records. Payroll is higher than estimated. A worker who was supposedly lower-risk spent half the year in the field. A subcontractor certificate is missing.
The spreadsheet stops being polite. Workers comp was priced on estimated payroll and class codes. The audit compares the estimate with what actually happened. If you want the compact version first, start with Class Codes and Audits: The Back Premium Trap.
What is really going on
Class codes are the pricing language of workers comp. They tell the policy what kind of work the payroll represents. A roofer, clerical employee, carpenter, and project manager do not carry the same injury risk. So they do not get priced the same way.
The quote starts with an estimate. The audit checks reality. If payroll was understated, the premium was too low. If payroll was in the wrong class code, the premium was too low or too high. If a subcontractor lacked proper coverage, that exposure may get added to your audit.
That is how back premium happens. The carrier is not usually inventing a new bill. It is correcting an old estimate.
Still not fun. But different.
Tradeoffs and gotchas
Mixed duty is the audit classic. If someone splits time between lower-risk and higher-risk work, the split needs real records. Without them, the higher code can win.
Subcontractor paperwork can sting too. Missing certificates can bring subcontractor exposure into your audit. That is a fast way for “we hired help” to become “we bought more premium.” Office exceptions need clean facts.
Clerical or supervisory treatment usually needs clean separation. If the person also does field work, the audit will want proof of the split. Payroll system changes can make a normal audit feel twice as hard. If you switch systems mid-year and lose the clean trail, the audit gets harder. Auditors are not mind readers. They are spreadsheet readers, which is somehow less forgiving.
If payroll itself is swinging, read Payroll Volatility: How It Moves Workers Comp More Than You Expect.
Price levers or decision factors
These are the levers that keep the audit calmer:
- Accurate class codes. Match codes to actual duties.
- Clean payroll splits. Track hours if a worker does more than one kind of work.
- Subcontractor certificates. Keep them current, not just somewhere in an inbox.
- Job descriptions. Make the work easy to understand before the auditor asks.
- Mid-year review. Catch drift before it becomes a bill.
- Owner and officer treatment. Elections and exclusions can change the payroll base.
An audit binder is not glamorous.
Payroll summaries, class code notes, certificates, job descriptions, and time records. Nothing glamorous. It works because it gives the audit fewer places to guess.
Simple decision rule
If you cannot defend the class code with real records, assume the auditor may pick the higher code. Documentation is not decoration. It is the steering wheel.
Next step
Before the audit, pull one month of payroll and match each role to the class code.
If anything feels fuzzy, fix it while there is still time to explain it clearly. The audit does not reward late clarity. Minnesota note: seasonal crews and subcontractor use make certificates and payroll splits worth checking before year-end. An audit surprise is like the plow ridge at the end of the driveway: technically predictable, still rude when you meet it.