Solo 401(k) plans offer one of the most flexible and generous retirement-saving opportunities for self-employed entrepreneurs. But for business owners operating as C-corporations or LLCs taxed as C-corporations, one critical rule catches many by surprise:
Your Solo 401(k) must be adopted by December 31, 2025, if you want to make employee (pretax or Roth) contributions for the 2025 tax year—even though those contributions can be funded in 2026.
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Below is a clear breakdown of what you must know to stay compliant and maximize your contribution opportunities.
Solo 401(k) Contributions (all types) Are Always Due by the Business Tax Return Date (Including Extensions)
For all business types—sole proprietors, S-corps, C-corps, and partnerships—the funding deadline for Solo 401(k) contributions is the business tax return due date, including extensions.
That means:
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April 15, 2026, for C-corps that do not extend
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October 15, 2026, for C-corps that file a timely extension
This deadline applies to employee, employer, and voluntary after-tax contributions.
However—and this is where C-corps differ—your ability to make certain types of contributions depends on when the plan is adopted.
The Critical 12/31/2025 Deadline for New C-Corp Solo 401(k)s
If your business is taxed as a C-corporation (or LLC taxed as a C-corp), you must adopt/open the Solo 401(k) plan by December 31, 2025, if you want to preserve the right to make:
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Employee pretax deferrals, and
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Employee Roth Solo 401(k) contributions
for the 2025 tax year—even though you’ll fund them in 2026.
“Adopting” simply means signing the Solo 401(k) plan documents by December 31. No account funding is required by year-end.
Missing the deadline does not eliminate all contribution options—but it does meaningfully limit them.
What If You Open the Plan After 12/31/2025?
If you wait until January 1, 2026 or later—but still adopt the plan by your 2025 business tax return due date (including extension)—you may still make:
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Employer profit-sharing contributions, and
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Voluntary after-tax contributions (for Mega Backdoor Roth strategies)
But you will not be allowed to make employee pretax or Roth contributions for 2025 because the plan documents were not signed by 12/31/2025.
This is one of the biggest pitfalls for C-corp owners who assume they can wait until tax season to open their plan.
Why This Matters So Much for C-Corp Owners
Employee contributions—pretax or Roth—are often the largest portion of the Solo 401(k) annual limit and give business owners greater flexibility in tax planning.
Failing to adopt the plan by 12/31/2025 may result in losing:
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Up to $23,500 in employee deferrals for 2025 (or more with catch-ups, depending on final IRS inflation updates).
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The opportunity to shift income into Roth in a high-earning year.
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The ability to combine employee + employer + after-tax contributions for full Mega Backdoor Roth potential.
With contribution limits rising again in 2025, the value of meeting the deadline is even greater.
Key Takeaway for 2025
To maximize your Solo 401(k) opportunities as a C-corporation or LLC taxed as a C-corp:
✅ Sign your Solo 401(k) plan documents by December 31, 2025
This preserves your ability to make 2025 employee contributions in 2026.
