Many self-employed individuals believe that if they didn’t open a Solo 401(k) by December 31, 2025, they permanently lost the opportunity to make 2025 contributions. For sole proprietors, that belief is incorrect.
Thanks to changes under SECURE Act 2.0, sole proprietors can still open a Solo 401(k) in 2026 and make certain — and in some cases all — contribution types for tax year 2025, depending on timing.
This article explains how the rules work and what deadlines matter most.
Who This Applies To
The rules discussed here apply specifically to sole proprietors, including:
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Independent contractors receiving 1099-NEC income
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Single-member LLCs taxed as sole proprietors
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Schedule C filers reporting net self-employment income
To be eligible for a Solo 401(k), you must:
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Perform material services in the business (earned income)
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Have no common-law W-2 employees working 1,000+ hours (excluding spouse)
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Report self-employment income on Schedule C, Line 31
2025 Solo 401(k) Contribution Limits (Recap)
For tax year 2025, the overall Solo 401(k) contribution limit is:
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$70,000 if under age 50
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$77,500 if age 50+ (includes $7,500 catch-up)
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$81,250 if age 60–63 (includes $11,250 super catch-up)
These limits include all contribution types combined.
Key Deadlines for Sole Proprietors Opening in 2026
April 15, 2026 — The Critical Date
If a sole proprietor adopts a Solo 401(k) plan by April 15, 2026, they may still make all three contribution types for tax year 2025:
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Employee contributions (pretax or Roth)
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Employer profit-sharing contributions
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Voluntary after-tax contributions (Mega Backdoor Roth)
However, the employee contribution must also be funded by April 15, 2026 to count for 2025.
This flexibility is unique to sole proprietors and was expanded under SECURE Act 2.0.
After April 15, 2026 (With Extension)
If the plan is adopted after April 15, 2026, but the sole proprietor files a timely tax return extension, the following contributions for 2025 are still allowed:
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Employer profit-sharing contributions
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Voluntary after-tax contributions (Mega Backdoor Roth)
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Employee contributions are no longer permitted for 2025
In this case, employer and after-tax contributions may be made up to October 15, 2026.
Understanding the Contribution Types
Employee Contributions (Pretax or Roth)
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Based on net self-employment income
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Aggregated across all 401(k) plans
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Must be:
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Plan adopted by April 15, 2026
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Contribution made by April 15, 2026
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Employer Profit-Sharing Contributions
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Calculated as 20% of net self-employment income
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Based on Schedule C, Line 31, after deducting ½ self-employment tax
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May be made:
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By April 15, 2026, or
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By October 15, 2026 with extension
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Voluntary After-Tax Contributions (Mega Backdoor Roth)
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Can be converted to Roth
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Same deadline as employer contributions
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Still available even if the plan is opened in 2026
This makes the Mega Backdoor Roth one of the most flexible planning tools available to sole proprietors who missed the year-end setup deadline.
Why December 31 Still Matters (But Isn’t the End)
While December 31, 2025, remains the best practice for preserving maximum flexibility, missing it does not eliminate all opportunities for sole proprietors.
Opening earlier:
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Reduces deadline pressure
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Ensures employee deferrals are preserved
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Simplifies compliance and documentation
But opening in 2026 can still unlock significant tax-advantaged savings if done correctly.
How MySolo401k Financial Supports This Strategy
With MySolo401k Financial:
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You choose your own bank or brokerage
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We draft custom Solo 401(k) plan documents
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We support:
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Roth contributions
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Voluntary after-tax contributions
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Mega Backdoor Roth conversions
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Participant loans
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We prepare required IRS filings, including:
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Form 1099-R, upon your timely request
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Form 5500-EZ, upon your timely request
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We support the $1,500 SECURE Act auto-contribution tax credit
Your funds are always held in accounts you control.
Bottom Line
✔ Sole proprietors can still open a Solo 401(k) in 2026
✔ Employee contributions for 2025 are possible if the plan is adopted by April 15, 2026
✔ Employer and voluntary after-tax contributions can be made as late as October 15, 2026 with extension
✔ Mega Backdoor Roth strategies remain fully available
✔ Proper timing is critical to avoid losing contribution buckets
With the right plan and guidance, it is not too late to capture powerful 2025 Solo 401(k) tax benefits.
