For partners and LLC members taxed as a partnership, the Solo 401k remains one of the most powerful tools for maximizing tax-advantaged retirement savings. With the ability to contribute in two roles—as both employee and employer—partners can take advantage of high contribution limits and flexible funding deadlines. However, these benefits only apply if the Solo 401k is established on time.
If you plan to set up a Solo 401k for tax year 2025, it’s essential to understand the deadlines that determine whether you can make both employee and employer contributions—or only employer contributions—when funding the plan in 2025 or 2026.
This guide breaks down the key rules, contribution limits, and setup deadlines for partnerships and LLCs taxed as partnerships.
Why Solo 401k Deadlines Matter for Partnerships
To make Solo 401k contributions for the 2025 tax year, you must establish (adopt) the plan by signing the Solo 401k plan documents by the applicable deadline. The IRS treats each partner as wearing two hats:
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Employee – eligible to make employee salary-deferral contributions
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Employer – eligible to make profit-sharing contributions based on K-1 income
Because each contribution type follows different rules, the date you establish the plan determines which contributions you can still make.
2025 Solo 401k Contribution Limits for Partnerships
For partners and LLC members taxed as partnerships, the 2025 contribution limits are extremely favorable.
Employee Contributions (the “Employee Hat”)
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Up to $23,500 for 2025
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Plus $7,500 catch-up if age 50 or older, or as super catch-up amount of $11,250 if age 60, 61, 62, or 63 in 2025
Employer Contributions (the “Employer Hat”)
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Up to 20% of adjusted net earnings, calculated using:
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Schedule K-1, Line 14, Code A
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Total Solo 401k Contribution Limit for 2025
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Up to $70,000
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Or $77,500 if age 50+
Compared to SEP IRAs or SIMPLE IRAs, the Solo 401k offers significantly higher contribution potential, primarily because of the employee deferral component.
Critical Setup Deadline: December 31, 2025
To preserve your full range of 2025 contribution options—including employee deferrals—your Solo 401k must be established by December 31, 2025.
This means:
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You must sign your Solo 401k plan documents by 12/31/2025
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You can then fund the plan in:
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2025, or
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2026, up to your business tax return due date including extensions
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This gives partnerships and LLCs plenty of flexibility to finalize tax planning after year-end while still obtaining the full benefit of employee and employer contributions.
Voluntary After-Tax Contributions: The Mega Backdoor Roth for 2025
If your Solo 401k supports voluntary after-tax contributions, you gain access to the Mega Backdoor Roth strategy, which can significantly increase your Roth retirement wealth.
Good news:
You can still make 2025 voluntary after-tax contributions if:
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You file a personal tax extension, and
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You adopt the Solo 401k by September 15, 2026
The deadline to fund these after-tax contributions is also September 15, 2026.
This gives partnership businesses exceptional flexibility to use the Mega Backdoor Roth even after the original tax year ends.
What Happens If You Wait Until 2026 to Adopt the Solo 401k?
Thanks to the SECURE Act, partnerships and LLCs taxed as partnerships can still open a Solo 401k after the close of the tax year—but with limitations.
If you adopt the plan in 2026, but on or before your business tax return deadline (including extension):
✔ You CAN make:
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Employer profit-sharing contributions for 2025
✘ You CANNOT make:
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Employee contributions (pretax or Roth) for 2025
To make employee contributions, the solo 4o1k plan must must be adopted by December 31, 2025, so waiting until 2026 eliminates the ability to make those deferrals.
This means you lose access to:
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The $23,500 employee deferral
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The $7,500 catch-up contribution or the $11,250 super catch-up (if eligible)
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The Roth employee deferral option
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The ability to maximize the full $70,000 / $77,500 limit/$81,250 limit
For most partners, this is a significant reduction in available savings and tax planning flexibility.
Summary of 2025 Deadlines for Partnerships & LLCs Taxed as Partnerships
| Action | Deadline | Contribution Types Allowed |
|---|---|---|
| Adopt Solo 401k by December 31, 2025 | 12/31/2025 | ✔ Employee contributions (pretax & Roth) ✔ Employer contributions (pretax & Roth)✔ Voluntary after-tax contributions |
| Adopt Solo 401k in 2026 by business tax return + extension | Typically 3/15/2026 or 9/15/2026 | ✔ Employer contributions only
✔ Voluntary after-tax contributions |
Final Takeaway
To unlock the full power of the Solo 401k—including employee deferrals, catch-up contributions, voluntary after-tax, and tax-efficient profit sharing—partnerships and LLCs taxed as partnerships should adopt the Solo 401k by December 31, 2025.
Waiting until 2026 still allows employer contributions and voluntary after-tax contributions, but at the cost of losing the employee deferral portion—often the most valuable part of the plan.
If you’re ready to set up your Solo 401k for tax year 2025, now is the time to begin your plan adoption process to stay ahead of the deadline and maximize your retirement and tax benefits.
