Updated October 16, 2026. Quick answer: A solo 401(k) has two clocks. Publication 560 says owner/employee deferrals must be elected by the end of the tax year and can then be made by the tax return filing deadline, including extensions, and deductible contributions can be made up to the due date of the return plus extensions. A plan generally must be set up by the last day of the year to deduct contributions for it, with a narrow exception for a sole proprietor with no employees.
Two clocks: electing the deferral and funding it
Publication 560 separates the date a deferral must be elected from the date it can be funded. For an owner, the definition of an elective deferral says: “The employee deferrals must be elected by the end of the tax year and can then be made by the tax return filing deadline, including extensions.”
For a tax year that ends December 31, 2026, the election side lands on December 31, 2026. The funding side lands on the tax return filing deadline, which is April 15, 2027 without an extension (April 15, 2027 is a Thursday, computed here, not published by the IRS). With an extension, the publication’s wording moves the funding side to the extended deadline.
| Item | What Publication 560 prints |
|---|---|
| Elective deferral by an owner/employee | “The employee deferrals must be elected by the end of the tax year and can then be made by the tax return filing deadline, including extensions.” |
| Deductible contributions for the year | “You can make deductible contributions for a tax year up to the due date of your return (plus extensions) for that year.” |
| Setting up (adopting) the plan | “your plan must be set up (adopted) by the last day of that year.” |
The employer contribution deadline
Employer money follows the return, extensions included: “You can make deductible contributions for a tax year up to the due date of your return (plus extensions) for that year.”
A self-employed owner makes the employer contribution on their own behalf only while the business has earnings: “You can make contributions on behalf of yourself only if you have net earnings (compensation) from self-employment in the trade or business for which the plan was set up.” “If you have a net loss from self-employment, you can’t make contributions for yourself for the year”. For how the employee and employer pieces add up, see the solo 401(k) contribution calculator.
Setting up the plan: the last day of the year
The setup rule is stricter than the funding rule. The publication says: “To take a deduction for contributions for a tax year, your plan must be set up (adopted) by the last day of that year.” A written document is part of that step: “You must adopt a written plan.”
For plans adopted after year end, the publication adds a narrow exception: “For 2023 and later years, a sole proprietor with no employees can adopt a section 401(k) plan after the end of the tax year, provided the plan is adopted by the tax filing deadline (without regard to extensions).”
It then ties the exception to funding: “your elective deferrals must be paid to the plan before the time for filing your return for that tax year (determined without regard to any extensions)” in order for them to count as made by the end of the first plan year. Notice that the extension is switched off for this exception. A related comparison of simpler plans that can be opened later is on the solo 401(k) versus SEP IRA page.
The 2026 limits
Notice 2025-67 sets the 2026 amounts. The elective deferral limit reads: “The limitation under section 402(g)(1) on the exclusion for elective deferrals described in section 402(g)(3), which includes elective deferrals made to the Thrift Savings Plan, is increased from $23,500 to $24,500.”
| 2026 item | As printed in Notice 2025-67 |
|---|---|
| Elective deferral limit, other than catch-up | “is increased from $23,500 to $24,500.” |
| Catch-up, age 50 or over | “is increased from $7,500 to $8,000.” |
| Catch-up, age 60 to 63 | “age 60, 61, 62, or 63 in 2026 remains $11,250.” |
| Defined contribution plan limit, other than catch-up | “is increased in 2026 from $70,000 to $72,000.” |
Publication 560 repeats the main figures: “The limit on elective deferrals, other than catch-up contributions, is $23,500 for 2025 and $24,500 for 2026.” “The limit on contributions, other than catch-up contributions, for a participant in a defined contribution plan is $70,000 for 2025 and increases to $72,000 for 2026.”
Because both limits are stated as other than catch-up contributions, they stack. With the age 50 catch-up, $24,500 plus $8,000 is $32,500 of employee money, and $72,000 plus $8,000 is $80,000 overall (illustration, not an IRS example). The notice also prints a catch-up that applies at ages 60 to 63, shown in the table. The related page on catch-up contributions outside the 415(c) limit explains the stacking, and the 2026 solo 401(k) limit page walks the arithmetic for a self-employed owner.
The annual return: Form 5500-EZ
A plan that holds money may owe a yearly report. Publication 560 says: “You may have to file an annual return/report by the last day of the seventh month after the plan year ends.” “Form 5500-EZ. You may be able to use Form 5500-EZ if the plan is a one-participant plan, as defined below.”
The dates, the extension and the one-participant tests are covered on the Form 5500 due date and extension page and the Form 5500-EZ page.
Plan around the dates, then the investments
The calendar above decides when money can go in. What the money buys once it is inside is a separate decision, and an owner picking between a solo plan and a SEP IRA is comparing setup dates, limits and the cost of running the plan. This page covers dates only.
The editions read for this page are IRS Publication 560 (2025), for use with 2025 returns, and Notice 2025-67 for the 2026 amounts, which are the revisions irs.gov served when they were read on 2026-10-16. Check the revision of the publication you are using, because a later edition can change the dates and the dollar limits.
Common mistakes
- Skipping the election by the end of the tax year. The publication says owner/employee deferrals must be elected by the end of the tax year and can then be made by the filing deadline, so the choice is not left open until April.
- Opening the plan after year end and counting on the extension. The exception for a sole proprietor with no employees uses the tax filing deadline without regard to extensions, and deferrals must be paid before the time for filing the return, determined without regard to any extensions.
- Forgetting that the report can come due. The publication says a plan may have to file an annual return/report by the last day of the seventh month after the plan year ends.
Related: 2026 solo 401(k) contribution limit, solo 401(k) contribution calculator, solo 401(k) versus SEP IRA, Form 5500 due date and extension, and Form 5500-EZ one-participant plan.
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Answer againSources
- IRS Publication 560, Retirement Plans for Small Business, for 2025 returns: https://www.irs.gov/pub/irs-pdf/p560.pdf
- IRS Notice 2025-67, 2026 Amounts Relating to Retirement Plans and IRAs: https://www.irs.gov/pub/irs-drop/n-25-67.pdf
Read from the IRS documents linked above on October 16, 2026. General information, not tax advice. Your own facts decide the outcome, and a preparer, the account provider or the IRS is the right place to confirm anything consequential.