A first-time 401k audit can feel like a high-stakes test for employers who have never sat across from an independent auditor before. The good news is that the process follows a predictable structure, and most of the work is gathering documents you already keep. This guide walks you through who needs a 401k audit, what the auditor will request, and how the Form 5500 timeline drives every deadline on your calendar.
Quick answer: A 401(k) plan generally requires an independent audit when it has 100 or more participants with account balances at the beginning of the plan year, counted under the current Department of Labor methodology that took effect for plan years beginning on or after January 1, 2023. The audit report is attached to the plan’s Form 5500, which is due the last day of the seventh month after the plan year ends (July 31 for a calendar-year plan), with an extension available to October 15 by filing Form 5558.
Does Your 401(k) Plan Actually Need an Audit?
Not every retirement plan is required to undergo an audit, so the first step is confirming whether yours crosses the threshold. The rule turns on participant count rather than dollar value, and the counting method changed meaningfully a few plan years ago. Getting this determination right protects you from both an unnecessary audit and a missed one.
Under the revisions the Department of Labor finalized for the 2023 Form 5500, a defined contribution plan such as a 401(k) counts only participants who have an account balance at the beginning of the plan year. This replaced the older approach that counted everyone eligible to participate, even employees who never enrolled. The change means many plans that previously tripped the threshold on eligibility alone now file as small plans without an audit. You can review the specifics in the DOL’s fact sheet on the 2023 Form 5500 changes.
The practical rule of thumb: if your plan has 100 or more participants with account balances as of the first day of the plan year, you are a large plan and you attach an independent qualified public accountant’s report to Schedule H of your Form 5500. New plans use the participant count at the end of the year for their first filing. Plans with fewer than 100 participants generally file as small plans on Schedule I or the Form 5500-SF and are not audited.
There is one wrinkle worth knowing: the 80-120 participant rule. If your plan covers between 80 and 120 participants at the beginning of the plan year and a Form 5500 was filed for the prior year, you may elect to keep filing in the same category as that prior year. That election can defer your first audit by a year or more, which is why an accurate beginning-of-year count matters so much. The mechanics are spelled out in the official Form 5500 instructions.
Because the determination hinges on a single date, the beginning of the plan year, it pays to run the count well before filing season. A plan that grows quickly through new hires can cross from small to large in a single year, and the audit obligation follows. Documenting how you arrived at the count, including which participants carried account balances, gives you a defensible record if the figure is ever questioned.
The Form 5500 Timeline That Drives Everything
Your audit does not exist in isolation; it feeds the Form 5500 filing, and the filing has firm deadlines. Understanding that timeline backward from the due date tells you when fieldwork has to start. Build your project plan around these dates so the audit report is finished before the return is due.
For a calendar-year plan, the Form 5500 is due July 31, which is the last day of the seventh month after the December 31 plan year end. Most large plans cannot complete an audit that quickly, so they file Form 5558 to request an extension. A Form 5558 filed on or before the original due date automatically extends the deadline by two and a half months, pushing a calendar-year plan to October 15. The IRS describes the extension on its About Form 5558 page.
That recurring October 15 deadline is the date most plan sponsors actually work toward. Even so, treating October as your real start point is a mistake. Auditors need several weeks to perform fieldwork, resolve questions, and issue an opinion, so engaging your firm in the spring or early summer keeps you out of the autumn crunch when audit capacity is tight.
Filing itself is electronic. Form 5500 must be submitted through the DOL’s EFAST2 system, and the audit report is attached as a signed PDF to the return. A complete filing pairs the financial statements, the auditor’s opinion, and the required schedules into a single submission. If any piece is missing or the audit is late, the entire filing is considered deficient, which can trigger penalties.
A deficient or late filing carries consequences beyond the inconvenience of refiling. The penalties attach to the plan sponsor, and they accrue while the filing remains incomplete, so a missing audit report is not a problem you want to discover in October. Mapping the audit schedule against the extended deadline early in the year is the simplest way to keep the filing clean and on time.
What the Auditor Will Request and Review
Once you confirm an audit is required, the next question is what the engagement actually involves. A retirement plan audit examines the plan’s financial statements and tests whether contributions, distributions, and participant data were handled correctly. Knowing the request list in advance lets you assemble everything before fieldwork begins.
Expect the auditor to ask for the signed plan document and all amendments, the most recent IRS determination or opinion letter, and the summary plan description. These establish the rules the auditor will test your operations against. Discrepancies between the written plan and actual practice are among the most common findings, so reconcile them early.
On the financial side, you will provide the trust statements, the recordkeeper’s certified or uncertified report, payroll records, and a census of eligible employees. The auditor reconciles contributions withheld from payroll to amounts deposited in the trust and checks the timing of those deposits. Late deposits of employee deferrals are a frequent issue and may require correction and reporting.
Whether the recordkeeper’s report is certified matters to the scope of the work. A certified report from a qualifying institution lets the auditor limit testing of the certified investment information, while an uncertified report generally means broader testing. Confirming which type your provider issues, and requesting certification where available, can streamline the engagement before it begins.
The auditor also tests a sample of participant transactions: new enrollments, distributions, loans, rollovers, and forfeitures. For each sample item they verify eligibility, accurate compensation, correct deferral percentages, and proper authorization. Clean, well-organized records make this phase faster and reduce follow-up requests that drag out the engagement. If your internal recordkeeping needs strengthening before the audit, ongoing client accounting services can keep payroll, contribution, and reconciliation data audit-ready year-round.
How to Prepare So Your First Audit Goes Smoothly
Preparation separates a calm first audit from a stressful one. The plan sponsor who hands over a complete, reconciled file in week one almost always gets a cleaner, faster engagement. Use the months before fieldwork to close gaps rather than scrambling during it.
Start by selecting a qualified, independent auditor with employee benefit plan experience. The DOL stresses that an auditor’s training and experience with benefit plans directly affects audit quality, and it offers guidance for sponsors in its publication on selecting an auditor for your employee benefit plan. The auditor must be licensed or certified as a public accountant by a state regulatory authority and independent of the plan and its sponsor.
Next, reconcile your records before the auditor arrives. Match payroll deferrals to trust deposits for every pay period, confirm employer match and profit-sharing calculations, and verify that deposit timing met the DOL’s requirements throughout the year. Pull together the plan document, amendments, determination letter, and the prior-year Form 5500 if one exists.
Pay particular attention to compensation, since deferral and match calculations depend on the plan’s definition of eligible pay. If bonuses, commissions, or other pay types are treated inconsistently with the plan document, the auditor will likely flag it, and the correction can reach back across the year. Catching those mismatches during your own review is far less costly than addressing them as audit findings.
Finally, line up the engagement timeline against your October 15 extended deadline and work backward. Engage the firm early, deliver the document request list promptly, and assign one internal point of contact to coordinate responses. Pease Bell’s audit and assurance services team handles employee benefit plan audits and can scope your first engagement so the report is ready well before the Form 5500 is due.
Frequently Asked Questions
When does a 401(k) plan require an audit?
A 401(k) plan generally requires an independent audit when it has 100 or more participants with account balances at the beginning of the plan year, using the counting method the DOL adopted for plan years beginning on or after January 1, 2023. Plans with 80 to 120 participants may elect to keep their prior-year filing status under the 80-120 rule, which can delay a first audit.
What is the deadline for filing the Form 5500 with the audit?
For a calendar-year plan, Form 5500 is due July 31, the last day of the seventh month after the plan year ends. Filing Form 5558 on or before that date extends the deadline by two and a half months to October 15.
Who can perform a 401(k) plan audit?
The audit must be performed by an independent public accountant who is licensed or certified by a state regulatory authority and not affiliated with the plan or its sponsor. The DOL recommends choosing a firm with specific employee benefit plan audit experience, since that experience strongly affects audit quality.
What documents should I gather before the audit starts?
Assemble the signed plan document and amendments, the IRS determination or opinion letter, the summary plan description, trust and recordkeeper statements, payroll records, an employee census, and the prior-year Form 5500. Reconciling payroll deferrals to trust deposits in advance prevents the most common findings and keeps the engagement on schedule.
