401(k) Plan Administration: The Payroll Professional's Handbook

Payroll does not administer a 401(k) plan, but payroll determines whether the plan is compliant. Every deferral amount, every compensation figure fed to the recordkeeper, every deposit date, and every loan repayment originates in payroll — and a plan failure traced to a payroll error is corrected under the plan's rules, at the employer's expense, with the fiduciary exposure that comes with it.

This handbook covers what payroll actually owns.

The 2026 Limits

Two points on the catch-up rules that create payroll work:

The age 60–63 "super catch-up" is available in place of the standard $8,000 amount, if the plan permits it. Whether it applies depends on the participant's age during the year and on plan design, so payroll needs the plan's election to configure the deduction limit correctly.

The Roth catch-up requirement. Participants whose prior-year wages exceeded the applicable threshold must make catch-up contributions on a Roth basis. This is not optional for them, and it requires the plan document, the recordkeeper's expectation, and the payroll deduction code to agree. A mismatch — payroll taking a pre-tax catch-up for someone required to use Roth — is an operational failure requiring correction.

Our 401(k) Training & Certification Program and retirement plan administration page cover the plan-side rules.

The FICA Rule

The single most important taxation fact:

A traditional 401(k) elective deferral reduces federal income tax wages but NOT Social Security, Medicare, FUTA, or state unemployment wages.

A participant deferring 6% of $100,000 has $94,000 in Box 1 of their Form W-2 and $100,000 in Boxes 3 and 5. That is correct, not an error.

The failure mode is a system configured with one "pre-tax" flag applied to all wage bases. Consequences compound quietly: employees under-withheld for FICA all year, the employer under-paying its matching share, wrong Forms 941 for four quarters, and a Form W-2 that will not reconcile. Correction after year end means amended 941s, W-2c forms, and recovering employee FICA that was never withheld — which the employer frequently ends up absorbing.

Roth deferrals reduce nothing. They are after-tax for all purposes, and both Box 1 and Boxes 3 and 5 reflect the full compensation.

Employer contributions — match and non-elective — are not wages to the employee at all and appear in no W-2 wage box.

See our pre-tax vs. post-tax deductions guide for the full matrix.

Deposit Timing Is a Fiduciary Matter

This is where payroll creates the most serious exposure, and it is genuinely different from tax deposit rules.

Elective deferrals become plan assets as soon as they can reasonably be segregated from the employer's general assets. Holding them longer is not a late payment — it is a prohibited transaction under ERISA, treated as the employer using plan assets.

Practical implications:

• The standard is "as soon as administratively feasible," judged against your own demonstrated capability. If you have ever transmitted in two days, the argument that seven days is your feasible limit is weak.
• There is a safe harbor for small plans — generally deposit within 7 business days of withholding — but it does not extend to large plans.
• Late deposits require correction: depositing the deferrals plus lost earnings, filing the appropriate reporting, and potentially paying an excise tax.
• Late deposits must be reported on Form 5500, which flags them for the Department of Labor. This is a self-identifying failure.
• The same rule applies to loan repayments withheld from pay.

Payroll's control: transmit deferrals on the same cadence as the payroll itself, with a documented standard timeline and a monitored exception log. Do not batch deferral transmissions for administrative convenience.

Plan Compensation Definitions

The plan document defines "compensation" for deferral, match, and testing purposes — and that definition frequently differs from gross wages, from Box 1, and from what payroll's default compensation field contains.

Questions the plan document answers, and payroll must implement:

• Are bonuses included?
• Are commissions included?
• Is overtime included?
• Are taxable fringe benefits and imputed income included?
• Is severance included? Generally it is not eligible compensation, and deferring from severance is a common error.
• Is compensation measured before or after pre-tax deductions?
• What compensation counts for a partial year of participation?

Using the wrong definition produces incorrect deferrals and an incorrect match for every affected participant, which is an operational failure requiring correction — often a corrective employer contribution plus earnings. Because the error is systematic, it typically affects the whole population.

The practical control is to obtain the plan's compensation definition in writing and map it explicitly to specific payroll earnings codes, then re-verify whenever a new earnings code is created. New codes default to whatever the person creating them copied, which is how definitions drift.

Eligibility and Enrollment

Payroll typically executes eligibility, and the plan document sets the rules — age, service, hours, entry dates, and the treatment of rehires and part-time employees.

Two areas of current attention:

Long-term part-time employees. Rules now require eligibility for certain part-time employees meeting consecutive-year service thresholds. This requires tracking hours for employees you may not have tracked for plan purposes before, which is a payroll data problem before it is a plan problem.

Automatic enrollment. Many plans use automatic enrollment with automatic escalation, and some plans are now required to. Payroll must implement default deferral percentages, escalate on schedule, and honor opt-outs and permissible withdrawal requests within the required windows. Missing an automatic enrollment is a failure to implement a plan provision, corrected by a contribution on the employee's behalf.

Loans

Plan loans run through payroll and carry their own failure modes:

• Repayments must begin promptly and follow the amortization schedule
• Missed repayments can cause a deemed distribution — a taxable event for the participant — if not cured within the plan's grace period
• Leaves of absence have specific rules for suspending and resuming repayments, including for military leave
• Termination typically triggers an offset or acceleration governed by the plan
• Repayments are plan assets and subject to the same deposit timing standard as deferrals

The recurring problem is an unpaid leave during which repayments simply stop and nobody restarts them. Build a return-from-leave checklist item.

See our Plan Loans Training & Certification Program.

Testing Support and Year-End

Payroll supplies the data for compliance testing — ADP and ACP tests, coverage testing, top-heavy testing — and the quality of that data determines whether results are meaningful.

Data payroll must provide accurately: compensation per the plan definition, deferral amounts by type, employer contributions, hours of service, employment dates, ownership and family relationship information for determining highly compensated and key employees, and terminations with dates.

When testing fails, corrections have payroll consequences: corrective distributions to highly compensated employees, which are taxable and must be reported, or qualified non-elective contributions funded by the employer. Both have deadlines, and missing them increases the cost of correction.

The timing point that matters: request testing results early. A failure identified in February is corrected within the deadline; one identified in November may not be.

A Payroll Checklist for 401(k) Administration

• [ ] Confirm traditional deferrals reduce FIT wages only, not FICA or unemployment wages
• [ ] Confirm Roth deferrals reduce nothing
• [ ] Load the current-year limits, including the age 60–63 catch-up if the plan permits it
• [ ] Confirm the Roth catch-up requirement is applied to participants above the wage threshold, consistently with the plan and recordkeeper
• [ ] Obtain the plan's compensation definition in writing and map it to specific earnings codes
• [ ] Re-verify the mapping whenever a new earnings code is created
• [ ] Confirm severance is excluded from deferrals if the plan excludes it
• [ ] Transmit deferrals and loan repayments on the payroll's own cadence, with a documented timeline
• [ ] Monitor and log any transmission exception, and treat delays as correctable failures
• [ ] Track hours for long-term part-time eligibility
• [ ] Implement automatic enrollment defaults, escalation, and opt-out windows
• [ ] Restart loan repayments after any leave of absence
• [ ] Supply testing data on the plan's schedule and request results early
• [ ] Reconcile total deferrals per payroll to the recordkeeper's records quarterly

That final reconciliation is the highest-value control on the list. A quarterly tie-out between payroll and the recordkeeper catches definition errors, missed enrollments, and transmission gaps while they are still cheap to correct.

Where Payroll Sits in the Fiduciary Picture

Payroll professionals are sometimes told that fiduciary responsibility belongs to the plan committee and does not reach them. That is broadly true as to formal fiduciary status, and misleading as to practical consequence.

The reason is that fiduciary duty attaches to functions, not only to titles. A person exercising discretionary authority over plan administration or plan assets can be a functional fiduciary regardless of their job description. Payroll typically does not exercise that discretion — it executes plan provisions — which keeps it outside formal fiduciary status in most arrangements.

But the failures payroll can cause are fiduciary failures for the employer:

• Late deferral deposits are a prohibited transaction, and the employer is the party using plan assets
• Missed automatic enrollments are failures to implement plan terms
• Wrong compensation definitions produce incorrect contributions across the population
• Unremitted loan repayments are the same prohibited transaction as late deferrals

Each of those is corrected under the plan's correction procedures at the employer's expense, and each requires disclosure that draws regulatory attention. So the practical position is that payroll does not carry the liability but reliably creates or prevents it.

Two implications worth acting on. First, document your standard timelines and your exceptions — a documented same-day transmission practice with a logged exception is a far better record than an undocumented practice with no exceptions noted. Second, escalate rather than improvise: when a plan question arises that payroll cannot answer from a written instruction, route it to the plan administrator. Improvising a reasonable-sounding answer is how operational failures begin.

Frequently Asked Questions

Does a 401(k) contribution reduce Social Security and Medicare taxes?

No. A traditional 401(k) elective deferral reduces federal income tax wages only. Social Security, Medicare, FUTA, and state unemployment wages are unaffected, which is why Box 1 of Form W-2 is legitimately lower than Boxes 3 and 5 for participants. Roth deferrals reduce no tax base at all, and employer contributions are not wages to the employee.

What is the 401(k) contribution limit for 2026?

The elective deferral limit is $24,500. The catch-up contribution for participants age 50 and older is $8,000, and participants aged 60 to 63 may contribute $11,250 instead if the plan permits. The IRA limit is $7,500. Participants whose prior-year wages exceeded the applicable threshold must make catch-up contributions on a Roth basis.

How quickly must 401(k) deferrals be deposited?

As soon as they can reasonably be segregated from the employer's general assets. This is a fiduciary standard, not a tax deposit deadline — holding deferrals longer is a prohibited transaction under ERISA rather than a late payment. Small plans have a safe harbor of 7 business days; large plans do not. The standard is judged against your own demonstrated capability, so a history of faster transmissions undermines a claim that a slower pace is feasible.

What happens if 401(k) deferrals are deposited late?

Correction is required: depositing the deferrals plus lost earnings, filing the appropriate reporting, and potentially paying an excise tax. Late deposits must also be disclosed on Form 5500, which flags them for the Department of Labor — making this a self-identifying failure. The same rule and consequences apply to plan loan repayments withheld from pay.

Why does plan compensation differ from gross wages?

Because the plan document defines compensation for deferral, match, and testing purposes, and that definition frequently excludes or includes items differently than gross wages do — bonuses, commissions, overtime, imputed income, and severance are all common variables. Using the wrong definition produces incorrect deferrals and match for every affected participant, which is an operational failure typically requiring a corrective employer contribution plus earnings.

What is the payroll role in 401(k) compliance testing?

Supplying the data: compensation per the plan definition, deferral amounts by type, employer contributions, hours of service, employment dates, ownership and family relationship information for identifying highly compensated and key employees, and termination dates. When testing fails, payroll also processes the corrections — taxable corrective distributions to highly compensated employees or employer-funded qualified non-elective contributions — both of which have deadlines worth requesting results early to meet.

Going Deeper

• 401(k) Training & Certification Program — plan rules and administration
• Retirement plan administration — the broader plan landscape
• Plan Loans Training & Certification Program — loan mechanics
• 401(k) Procedures Manual & E-Alerts — documented procedures

Contribution limits change annually and retirement plan rules have been amended substantially in recent legislation. Confirm current limits and the plan's own provisions before configuring payroll, and coordinate with the plan administrator on any correction.