All ordinary deadlines for making 2024 Solo 401(k) contributions have passed. The dates depended on your business’s tax classification, whether you were making employee salary deferrals or employer contributions, whether the plan already existed, and whether you filed a valid tax-return extension. These are historical 2024 dates, not current-year deadlines.
Why there was no single 2024 contribution deadline
A Solo 401(k) can involve different acts with different deadlines: adopting the plan, electing employee deferrals, depositing contributions, and filing an annual plan return. A tax-return extension can affect the deadline for certain employer contributions, but it does not automatically extend every plan or deferral deadline.
The IRS’s Publication 560 (2024) describes owner-employee deferrals as requiring an election by the end of the tax year, with deposits handled separately. The IRS also describes a special post-year-end plan-adoption rule for qualifying sole proprietors in its Issue Snapshot on 401(k) employer-contribution deductibility. Those rules should not be collapsed into a claim that every owner could elect a 2024 deferral by an extended tax-return deadline.
2024 dates by business type and contribution
| Business / situation | Employee deferral or plan adoption | Employer contribution deadline |
|---|---|---|
| Sole proprietor or single-member LLC taxed as a sole proprietor | For an owner-employee, the general election deadline was year-end. A qualifying sole proprietor with no employees could use the special rule to adopt a new plan for 2024 by the 2024 individual return filing deadline, without regard to extensions: ordinarily April 15, 2025. | Generally the individual return deadline, including a valid extension: ordinarily April 15, 2025, or October 15, 2025 with a valid extension. |
| S corporation | Salary deferrals are tied to W-2 compensation; the IRS material does not establish the sole-proprietor post-year-end rule for corporate owners. | Generally the corporate return deadline: March 17, 2025 for a calendar-year 2024 Form 1120-S, or September 15, 2025 with a valid extension. |
| C corporation | The sole-proprietor post-year-end adoption rule should not be assumed to apply to a corporate owner. | Generally the corporate return deadline: April 15, 2025 for a calendar-year 2024 Form 1120, or October 15, 2025 with a valid extension. |
| Partnership | Rules depend on the plan and participant circumstances; do not assume the sole-proprietor special rule applies. | Generally the partnership return deadline: March 17, 2025 for a calendar-year 2024 return, or September 15, 2025 with a valid extension. |
The dates for calendar-year returns above are practical applications of federal return schedules; check the business’s actual filing year, return, and extension status. An extension generally extends the time to file a return, not the time to pay tax, and it extends a contribution deadline only where the applicable contribution rule permits that.
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What the sole-proprietor special rule did—and did not—allow
For 2024, an individual calendar-year return was ordinarily due April 15, 2025. A valid extension generally allowed filing through October 15, 2025. But the IRS’s special rule for a sole proprietor with no employees allowed a new plan for a prior year to be adopted by the return filing deadline without regard to extensions. For this route, the ordinary cutoff was April 15, 2025; extending the return did not move that adoption cutoff.
This is specifically a rule about adopting a new plan and the related prior-year self-employment deferral opportunity described by the IRS. It is not a blanket extension for every existing-plan deferral, every business form, or every contribution. The IRS’s Publication 560 separately describes the general year-end election rule and deposit timing. Review the plan document and circumstances before applying either rule.
2024 contribution limits
| Limit type | 2024 amount |
|---|---|
| Employee elective deferrals | $23,000 |
| Age-50 catch-up elective deferrals, if eligible | $7,500 |
| Annual additions limit, generally excluding eligible catch-up contributions | $69,000 |
These are IRS 2024 limits from Publication 560, not promises that every participant could contribute those amounts. The allowable amount can be lower depending on compensation, self-employment earnings, age, plan terms, the calculation rules, and participation in other employer plans. For an S corporation shareholder-employee, salary deferrals are based on W-2 compensation, not shareholder distributions; see the IRS S corporation contribution FAQ.
S corporation owners: W-2 wages matter
An S corporation shareholder-employee’s 401(k) salary deferrals are based on compensation reported as W-2 wages. Distributions from the corporation do not provide the compensation base for those deferrals. The IRS explains this distinction in its retirement plan FAQ for S corporations. Do not use the sole-proprietor special post-year-end rule as a substitute for the corporate plan and payroll rules.
Separate deadline: Form 5500-EZ
Filing Form 5500-EZ is a reporting obligation, not a contribution deadline. If a calendar-year 2024 one-participant plan was required to file, its regular due date was July 31, 2025—the last day of the seventh month after the plan year ended. The IRS generally provides an exception when year-end plan assets are $250,000 or less, unless it is the plan’s final year. See the IRS About Form 5500-EZ page for filing requirements.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to check what applied to your 2024 plan
- Identify the business tax classification. Determine whether the business was a sole proprietorship, partnership, S corporation, or C corporation for the relevant year.
- Separate the action in question. Was the issue plan adoption, an employee deferral election, depositing employee deferrals, an employer contribution, or Form 5500-EZ reporting?
- Check whether a plan already existed. The special post-year-end adoption route was for qualifying sole proprietors with no employees, not a general rule for all existing plans or entities.
- Confirm the return and extension. Use the actual federal return due date and confirm that any extension was timely and valid. An extension does not automatically change every retirement-plan deadline.
- Review plan terms and contribution calculations. Confirm compensation, other-plan participation, and the plan document before deciding whether a contribution was permitted and how much.
The IRS overview of one-participant 401(k) plans provides general context, but the applicable date for a specific plan depends on its entity, contribution type, plan history, and filing status.
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