The Solo 401(k) is the most generous retirement plan available to self-employed people, but it’s also the one where deadlines sneak up fastest. Some decisions have to be made by December 31, others can wait until you file your tax return, and one filing comes due the following summer. This Solo 401(k) year-end checklist walks through what to do before the year closes, what you can still do after, and what’s new for 2026.

2026 Solo 401(k) Contribution Limits

Before you plan any year-end contributions, know your ceiling. As a Solo 401(k) owner you wear two hats: as the employee you make elective deferrals, and as the employer you make profit-sharing contributions. The IRS limits for 2026 are:

2026 limit	Under 50	Age 50 to 59 and 64+	Age 60 to 63
Employee deferral	$24,500	$32,500	$35,750
Total contributions (employee + employer)	$72,000	$80,000	$83,250

The employee deferral can be up to 100% of your compensation, up to the dollar limit. The employer contribution is generally up to 25% of compensation for an S or C corporation owner. For a sole proprietor or single-member LLC, it works out to about 20% of net self-employment earnings, because IRS Publication 560 requires the contribution itself to be taken into account when calculating net earnings. If you have a 401(k) at another job too, remember that the $24,500 deferral limit is per person across all plans, not per plan.

The practical step for year-end: estimate your 2026 net business income now, so you know how much room you have before December 31.

Solo 401(k) Year-End Deadlines

The biggest mistake I see is assuming every contribution can wait until April. It can’t. Here’s how the deadlines line up for a calendar-year plan and a calendar-year business:

What	Deadline for the 2026 plan year
Elect your 2026 employee deferrals	December 31, 2026
W-2 owners (S or C corp): run deferrals through payroll	Final 2026 payroll
Employer profit-sharing contributions	Your business tax return due date, including extensions (March 15, 2027 for most S corporations; April 15, 2027 for sole proprietors)
Adopt a new Solo 401(k) for 2026	Your business tax return due date, including extensions
File Form 5500-EZ, if required	July 31, 2027, or October 15, 2027 with a Form 5558 extension

Elective Deferrals Have to Be Elected by December 31

IRS Publication 560 is clear that employee deferrals must be elected by the end of the tax year. For a sole proprietor, the money itself can be deposited up to your tax filing deadline, but the written election has to exist by December 31. Put it in writing and keep it with your plan records. If you’re an S corporation owner, deferrals come out of your W-2 wages, so they need to run through payroll before the year closes.

The SECURE Act lets you set up a brand-new plan as late as your tax filing deadline and still make employer contributions for the prior year, and SECURE 2.0 added a narrow first-year exception for sole proprietors’ deferrals. Both are useful safety nets, but setting the plan up before December 31 keeps every option open.

Form 5500-EZ

A Solo 401(k) has no annual filing until total assets across your one-participant plans exceed $250,000 at year-end, and a final return is always required in the year the plan distributes everything. According to the Form 5500-EZ instructions, the return is due by the last day of the seventh month after the plan year ends, which is July 31 for calendar-year plans. Don’t let this slide. The IRS late-filing penalty is $250 per day, up to $150,000 per return, though a relief program lets late filers catch up for $500 per return.

New for 2026: The Roth Catch-Up Rule

This is the first year the SECURE 2.0 Roth catch-up requirement is in effect, and it catches some Solo 401(k) owners off guard. If you’re 50 or older and your prior-year FICA wages from the business sponsoring the plan exceeded $150,000, every dollar of your 2026 catch-up contribution has to be made as a Roth contribution. Your regular $24,500 deferral can still be pre-tax.

Who does this hit? The rule is built around W-2 wages, so it mainly affects owners of S corporations and C corporations who pay themselves a salary. Self-employment income reported on Schedule C or a partnership K-1 isn’t FICA wages, and neither is investment income or wages from a different employer, so sole proprietors generally aren’t caught by the wage test. I still tell clients to confirm their own situation with their tax advisor before year-end.

The year-end action item is simple. If you’re affected, make sure your plan document allows Roth deferrals before you make a catch-up contribution, because a plan without a Roth option can’t accept catch-up dollars from an affected participant at all. I explain the details in The New 2026 Solo 401(k) Roth Catch-Up Rule.

Year-End Housekeeping for a Self-Directed Solo 401(k)

Value Your Alternative Assets

If your Solo 401(k) holds real estate, a private company stake, a promissory note, or precious metals, you need a reasonable December 31 value for each one. That number decides whether you’ve crossed the $250,000 Form 5500-EZ threshold, and it’s what you report if you do file. For a rental property, a broker’s opinion or recent comparable sales usually works. For a private investment, ask the sponsor for a year-end statement.

Check Your Participant Loan

If you borrowed from your plan, confirm that every payment due this year was made. Under the IRS plan loan rules, payments have to be made at least quarterly, and a loan that falls out of compliance can be treated as a taxable distribution. Year-end is a good time to catch a missed payment before it becomes a tax problem.

Take Any Required Minimum Distribution

If you’ve reached age 73, your Solo 401(k) is subject to required minimum distributions. Business owners can’t use the “still working” exception that lets some employees delay 401(k) RMDs, so plan for it. Your RMD generally has to come out by December 31, except your first one, which can wait until April 1 of the following year. Roth Solo 401(k) accounts are no longer subject to lifetime RMDs under SECURE 2.0.

Review Beneficiaries and Records

Confirm your primary and contingent beneficiaries are current, especially after a marriage, divorce, or birth. A 401(k) generally requires your spouse’s written consent if you name someone else. Then tidy your records: keep the plan’s bank and brokerage statements separate from your business accounts, note which contributions were employee deferrals and which were employer contributions, and make sure income from plan investments went back into the plan.

Your Solo 401(k) Year-End Checklist

Before December 31, 2026

• Estimate 2026 net business income or W-2 wages to size your contributions
• Sign a written election for your 2026 employee deferrals
• S or C corporation owners: run deferrals through your final payroll
• If you’re 50+ and had prior-year FICA wages over $150,000, confirm your plan allows Roth catch-up contributions
• Take your RMD if you’re 73 or older (first-year RMDs can wait until April 1)
• Confirm all participant loan payments for the year were made
• Set up a new Solo 401(k) now if you want the most flexibility for 2026

At year-end

• Record a December 31 value for every alternative asset in the plan
• Total all one-participant plan assets to see if you’re over $250,000
• Review primary and contingent beneficiaries

By your 2026 tax filing deadline

• Deposit remaining employee deferrals (sole proprietors)
• Make your employer profit-sharing contribution
• Work with your CPA to report contributions correctly on your return

By July 31, 2027

• File Form 5500-EZ if plan assets exceeded $250,000 at year-end, or file Form 5558 to extend

Final Thoughts

Most Solo 401(k) problems I see at tax time trace back to something that should have happened in December: a deferral election that was never signed, a loan payment that got skipped, or a catch-up contribution made into a plan that couldn’t accept it. None of these take long to fix in advance. They’re much harder to fix after the fact.

This is also why I built IRA Financial’s Solo 401(k) with Form 5500-EZ preparation and plan document updates included in the $399 annual fee, so the paperwork side doesn’t fall through the cracks. If you have a plan loan, I cover the repayment rules in more depth in The Definitive Guide to the Solo 401(k) Participant Loan. Block an hour on your calendar this month, work through the list above, and you’ll start 2027 with a clean plan and every contribution you’re entitled to.

This content is for educational purposes only and does not constitute legal, tax, or investment advice. Consult a qualified professional before making any investment decisions.