What Tax Forms Are Needed for a Solo 401k?

Starting a Solo 401k is a big step toward building your retirement savings. The next challenge is keeping your plan compliant with IRS rules. That means knowing which tax forms apply and when they’re due. Even a small oversight could lead to avoidable penalties, so it’s worth learning the basics before filing season arrives. 

In this article, we’ll break down the key forms you might encounter and the situations that trigger them.

Annual Filing Requirements for Solo 401k Plans (Form 5500-EZ)

Even though Solo 401k plans are simpler than large employer retirement plans, the IRS still requires certain filings to track plan activity. One of the most common is Form 5500-EZ, which applies only in specific situations. Knowing when it’s required and how to file it can help you avoid unnecessary penalties.

When You Must File Form 5500-EZ

Form 5500-EZ applies to one-participant plans, typically a Solo 401k with only the business owner (and possibly a spouse) as participants. 

You must file if:

✅ Your plan’s year-end assets total $250,000 or more. This total includes the combined assets across all one-participant plans you maintain, not just one account.

✅ It is the final year of your plan, even if assets are below $250,000.

These rules ensure the IRS has a record of larger or closing plans.

Filing Deadlines and Extensions with Form 5558

If your business tax return is on extension and your plan year matches your tax year, you automatically get the same extended due date for Form 5500-EZ without filing Form 5558. Keep a copy of your business tax extension with your plan records as proof.

📝 E-file requirement: For 2025 plan years, if you file 10 or more information returns in a calendar year, you must e-file Form 5500-EZ via EFAST2. 

Why You Cannot Use Form 5500-SF for a One-Participant Plan

Form 5500-SF is not an option for Solo 401k plans. The Department of Labor and the IRS have confirmed that one-participant and certain foreign plans must use Form 5500-EZ — either on paper or through EFAST2, depending on the e-filing rules above. Filing the wrong form could result in processing delays or IRS notices.

Reporting Solo 401k Distributions, Rollovers, and Conversions

When funds leave your Solo 401k — whether through withdrawals, rollovers, or Roth conversions — the IRS requires reporting to track taxable and non-taxable events. Even if you move money between qualified accounts without triggering taxes, the transaction still needs to be documented correctly.

📌 Also read: Solo 401k Contribution Limits & Deadlines for 2024 & 2025

Using Form 1099-R for Withdrawals and Rollovers

Form 1099-R is required for any Solo 401k distribution or deemed distribution of $10 or more. This includes:

Use the correct distribution codes from the 1099-R instructions to indicate the type of transaction. For example, certain direct rollovers from a designated Roth account use Code H, while rollovers between designated Roth accounts may use Code G.

You must furnish the 1099-R to the recipient and file it with the IRS by the published deadlines listed in the General Instructions for Information Returns. Direct rollovers are not subject to the 20% mandatory withholding, but distributions paid to you directly (if not rolled over) generally are.

Reporting Tax Withholding on Form 945

If you withhold federal income tax from a Solo 401k distribution, you must report it on Form 945, which covers non-payroll income tax withholding. Deposits are made through the Electronic Federal Tax Payment System (EFTPS) according to Form 945 deposit rules. The same EIN is used for both your withholding deposits and the related 1099-R.

If your Solo 401k funds are held by a custodian who processes distributions, they usually handle Form 945 filing and deposits on your behalf. If you have direct check-writing authority over your Solo 401k, you may need to handle these filings yourself.

How to Report Solo 401k Contributions

Both employee deferrals and employer contributions must be reported correctly to claim deductions and stay within IRS limits. The reporting process depends on whether you’re paid as a W-2 employee or self-employed.

W-2 Employees Reporting Contributions and Deductions

If you receive W-2 wages from your business, such as through an S corporation, report employee elective deferrals in Box 12 of your W-2 using:

Employer nonelective or profit-sharing contributions are not shown on the W-2. Instead, they are deducted on the employer’s federal tax return, subject to the contribution limits in IRC Section 404.

📌 Also read: General Instructions for Forms W-2 and W-3

Sole Proprietors and Partners Reporting on Form 1040 Schedule 1

If you are self-employed, deduct your Solo 401k contributions on Schedule 1 of Form 1040 in the line labeled Self-employed SEP, SIMPLE, and qualified plans. Do not claim the deduction on Schedule C.

For partnerships, contributions are generally made by the partnership and shown on Schedule K-1, Box 13 with Code R. The partner then reports the deduction on Schedule 1, line 16 of their individual return.

Wrapping Up: Staying on Top of Solo 401k Tax Filings

Keeping your Solo 401k in compliance comes down to a few key checkpoints: determine if your plan assets hit the $250,000 mark at year-end — or if it’s your plan’s final year — to know whether Form 5500-EZ is required. Remember that Form 1099-R is needed whenever money leaves the plan, whether through a distribution, rollover, Roth conversion, or loan default. If federal tax is withheld, that amount must also be reported on Form 945.

For contributions, make sure payroll reporting is accurate (using W-2 Box 12 codes D and AA for elective deferrals) and that self-employed deductions are correctly entered on Schedule 1 (Form 1040). Practical next steps include setting reminders for the July 31 filing deadline (and for Form 5558 if you need an extension), clarifying who will issue your 1099-Rs, and reviewing W-2 or Schedule 1 entries before tax season.

📌 For deeper guidance on Solo 401k management, plan setup, and contribution strategies, check out these related articles below:

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