Solo 401(k) Contribution Limits and Deadlines for 2026: What Self-Employed Owners Should Do Before Year-End

A self-employed consultant with a Solo 401(k) who waits until her CPA calls in March to think about 2026 contributions may find that part of her plan has already closed. The employee side of a Solo 401(k) has to be elected by December 31, and no tax extension brings it back. At a 35% federal bracket, a missed $24,500 deferral means roughly $8,575 of federal income tax she could have deferred that year.

The 2026 solo 401k contribution limits are higher than last year, but the limits are the easy part. The deadlines are where self-employed owners lose money, because the employee and employer sides of a Solo 401(k) run on different calendars, and a sole proprietor, an S-corporation owner and someone opening a brand-new plan each face a different set of dates.

Below: the 2026 numbers, the deadline calendar through the 2027 filing season, and a year-end checklist. For the formula behind the employer contribution and worked examples by entity type, see our complete Solo 401(k) guide. One boundary up front: Unified Wealth Systems (UWS) administers Solo 401(k) plans and coordinates entity formation and tax preparation, but UWS is not a law firm, CPA firm, custodian, or licensed advisor. Confirm your own numbers and dates with your tax professional.

2026 Solo 401(k) Contribution Limits at a Glance

The IRS published the 2026 figures in Notice 2025-67 and its November 13, 2025 announcement. Here is what changed from 2025.

Catch-up contributions sit on top of the $72,000 ceiling. A participant aged 50 to 59, or 64 and older, can reach $80,000 in 2026, and a participant who turns 60, 61, 62 or 63 during 2026 can reach $83,250, provided compensation supports it.

Two contribution types, two different rules

As the IRS explains on its one-participant 401(k) page, the owner wears two hats. As the employee, you make elective deferrals (the salary-reduction contribution you choose) of up to 100% of compensation, capped at $24,500. As the employer, the business makes a nonelective contribution (a profit-sharing contribution the business decides on) of up to 25% of compensation, figured on net self-employment earnings for a sole proprietor. The deferral limit is per person, not per plan, so if you also defer into a W-2 employer's 401(k), the $24,500 is shared between both plans. Our Solo 401(k) overview covers the basics, and the UWS calculator can help you size your contribution.

The 2026 Solo 401(k) Deadline Calendar

Every date below applies to the 2026 tax year of a calendar-year business.

Before December 31, 2026: the deferral election

According to IRS Publication 560, an owner-employee's deferrals must be elected by the end of the tax year, although the money itself can be deposited later, by the tax filing deadline including extensions. Put the election in writing: the dollar amount, and whether it is pre-tax or Roth. An S-corporation owner has less room still, because deferrals come out of W-2 wages, so they have to run through a 2026 payroll.

Opening a new plan after December 31

Under the SECURE Act, a business can adopt a plan as late as its tax filing deadline, including extensions, and treat it as in place for 2026, which preserves the employer contribution. The employee side is narrower. SECURE 2.0 lets a sole proprietor or single-member LLC owner make first-year deferrals in a plan adopted after year end, but only if the election is made by the return due date without extensions: April 15, 2027. An S-corporation owner cannot do this, because 2026 deferrals had to come out of 2026 wages.

Scenario: what waiting costs

Hypothetical, for illustration only. A sole proprietor with no plan nets $220,000 in 2026. If she adopts a Solo 401(k) in December and elects a $24,500 deferral by December 31, she gets the deferral plus an employer contribution she can fund as late as October 15, 2027. If she adopts the plan in March 2027 and elects by April 15, she keeps both. If she waits until May 2027 and files on extension, the employer contribution survives but the $24,500 deferral for 2026 is gone. Your actual figures depend on your net earnings and should be calculated by your CPA.

Filing season and Form 5500-EZ

Employer contributions for 2026 are due by the business's tax filing deadline, including extensions. Separately, a one-participant plan with $250,000 or more in assets at year end generally files Form 5500-EZ by the last day of the seventh month after the plan year ends, per the IRS Form 5500 corner. July 31, 2027 is a Saturday, so the 2026 filing moves to the next business day. Form 5558 requests an extension.

Catch-Ups and the Roth Catch-Up Rule in 2026

Catch-up eligibility depends on the age you reach during 2026, so a participant who turns 50 on December 30 qualifies for the full $8,000. The bigger change is the Roth catch-up rule from SECURE 2.0. If your 2025 FICA wages (Social Security wages) from the business sponsoring the plan exceeded $150,000, your 2026 catch-up contributions must be made as Roth. The IRS's final regulations apply from 2027, and plans must follow a reasonable, good-faith reading of the statute for 2026.

This matters most for S-corporation owners paid on a W-2. If your plan document has no Roth option and the rule applies to you, you cannot make catch-up contributions at all, so check the document before year end. Sole proprietors and partners receive no FICA wages from their own business, and the prevailing reading is that they fall outside the rule; confirm your treatment with your plan provider and CPA. Our guide to tax-free investing and our mega backdoor Roth guide cover the wider Roth picture.

Your 2026 Solo 401(k) Year-End Checklist

1. Estimate your 2026 net earnings with your CPA using real fourth-quarter numbers.

2. Put your deferral election in writing by December 31: amount, and pre-tax or Roth.

3. S-corporation owners: run deferrals through the final 2026 payroll and confirm your W-2 wages support the employer contribution you plan to make.

4. Check catch-up eligibility and whether the Roth catch-up rule applies; add a Roth feature if needed.

5. Count deferrals at other jobs. The $24,500 is shared with any W-2 employer's 401(k).

6. No plan yet? Decide whether to adopt now or use the 2027 window, knowing the April 15 cutoff for deferrals.

7. Calendar your deposit deadline and, if your balance is $250,000 or more, the Form 5500-EZ.

8. Hiring in 2027? Employees who meet the plan's eligibility rules must be covered, which ends the one-participant setup.

Where year-end planning goes wrong

• Using last year's limits. Funding to $70,000 leaves $2,000 of 2026 room unused.

• Assuming an extension saves the deferral. It extends deposits, not the election.

• Over-deferring across two plans. Excess deferrals must be corrected, as the IRS fix-it guide explains.

• Funding an employer contribution your compensation does not support. The 25% limit is figured on compensation, not on what is in the bank.

Frequently Asked Questions

What are the 2026 Solo 401(k) contribution limits?

For 2026, you can defer up to $24,500 as the employee, and total contributions per participant can reach $72,000 before catch-ups. Participants aged 50 and over can add $8,000, for $80,000, and those who turn 60 to 63 during 2026 can add $11,250, for $83,250, if their compensation supports it.

What is the Solo 401(k) contribution deadline for 2026?

It depends on the contribution type. Sole proprietors must elect 2026 deferrals by December 31, 2026, then can deposit them and the employer contribution by the tax filing deadline: April 15, 2027, or October 15, 2027 with an extension. S-corporation owners run deferrals through 2026 payroll and have until March 15, 2027, or September 15, 2027 on extension, for employer contributions.

Can I still open a Solo 401(k) for 2026 after December 31?

Yes, for the employer contribution. A plan adopted by your tax filing deadline, including extensions, can count for 2026. A sole proprietor can also make first-year employee deferrals in that new plan, but only if the election is made by April 15, 2027. S-corporation owners cannot make retroactive deferrals because they had to come out of 2026 wages.

Does a tax extension give me more time for employee deferrals?

Only for depositing deferrals you already elected. The election is due by December 31 for an existing plan, or April 15, 2027 for a new plan adopted after year end, and an extension moves neither date.

Do my 2026 catch-up contributions have to be Roth?

Only if your 2025 FICA wages from the business sponsoring the plan exceeded $150,000, which mainly affects S-corporation and C-corporation owners paid on a W-2. The prevailing reading is that sole proprietors and partners, who receive no FICA wages from their own business, are outside the rule. Confirm your situation with your plan provider and CPA.

When do I need to file Form 5500-EZ for my Solo 401(k)?

You generally file it when plan assets were $250,000 or more at the end of the year. The deadline is the last day of the seventh month after the plan year ends, and because July 31, 2027 is a Saturday, the 2026 filing moves to the next business day. Form 5558 requests an extension.

Ready to take control of your retirement?

The 2026 limits are set; the dates that decide how much of them you can use are coming up fast. A short conversation can confirm your election, your deposit deadline and whether your plan document fits the new Roth rules before December 31.

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Related Reading:Solo 401(k): The Complete Guide | Solo 401(k) vs. SEP-IRA | Mega Backdoor Roth Guide