Fidelity RIA Guide: Implementing the “Retroactive” Solo 401(k) for Sole Proprietors

Watch: How Fidelity RIAs can set up a retroactive Solo 401(k) for sole proprietor clients through My Solo 401k Financial

Why Fidelity RIAs Are Turning to My Solo 401k Financial

If you are a Registered Investment Advisor (RIA) who custodies client assets at Fidelity Investments, you already know that Fidelity’s own prototype Solo 401k plan has significant limitations. It does not support voluntary after-tax contributions, the Mega Backdoor Roth Solo 401k strategy, or Solo 401k participant loans.

That’s precisely why a growing number of Fidelity-based RIAs partner with My Solo 401k Financial as the plan document provider. With this arrangement, the RIA continues to custody assets at Fidelity through non-prototype (self-directed) brokerage accounts, while My Solo 401k Financial provides the custom plan document, ongoing compliance support, and contribution calculations—including for the powerful Mega Backdoor Roth strategy.

✅ Key Advantage: Thanks to the SECURE Act 2.0, it is not too late for a sole proprietor to open a Solo 401k plan in 2026 and make retroactive contributions for tax year 2025—including employee contributions, employer profit-sharing contributions, and voluntary after-tax Solo 401k contributions.

What Fidelity’s Own Solo 401k Plan Does Not Offer

Fidelity Investments offers its own prototype Solo 401k plan, but it comes with notable restrictions that limit your clients’ retirement-saving potential. Understanding these gaps is essential for any Registered Investment Advisor seeking to maximize value for self-employed clients.

Feature	Fidelity’s Solo 401k	My Solo 401k Financial
Voluntary After-Tax Contributions	❌ Not Available	✅ Available
Mega Backdoor Roth Solo 401k	❌ Not Available	✅ Available
In-Plan Roth Conversions	❌ Not Available	✅ Available
Solo 401k Participant Loans	❌ Not Available	✅ Available (investable in equities, private equity, etc.)
SECURE Act Auto Contribution Credit	❌ Not Available	✅ Built In ($1,500 over 3 years)
Custody at Fidelity	✅ Yes (prototype)	✅ Yes (non-prototype brokerage accounts)
RIA Institutional Access	✅ Yes	✅ Yes

SECURE Act 2.0: Retroactive Solo 401k Setup Rules for Sole Proprietors

The SECURE Act 2.0 changed the rules to make it significantly more favorable for a self-employed sole proprietorship to open a Solo 401k plan in the current year and make contributions for the prior tax year. Here is how the deadlines work for tax year 2025 contributions:

Deadline Scenario 1: Open by April 15, 2026

If your client’s sole proprietorship opens a Solo 401k plan in time to make contributions by April 15, 2026, the client can make all contribution types for tax year 2025:

✅ All Contribution Types Eligible by April 15, 2026:

• Employee contributions (pre-tax or Roth) — up to $23,500 for 2025
• Employer profit-sharing contributions (pre-tax or Roth)
• Voluntary after-tax Solo 401k contributions (enables Mega Backdoor Roth)

Deadline Scenario 2: October 15, 2026 (With Extension)

If the individual files a personal Form 1040 tax return extension, the deadline to open the Solo 401k plan and make contributions extends to October 15, 2026. However, only certain contribution types are available:

⚠ Important Limitation — Extended Deadline (October 15, 2026):

• Employer profit-sharing contributions — ✅ Allowed
• Voluntary after-tax Solo 401k contributions — ✅ Allowed
• Employee contributions — ❌ NOT allowed after April 15, 2026 if the plan was first opened in 2026

Deadline for Solo 401k plans opened for Sole Proprietor in 2026	Employee Contributions	Employer Contributions	Voluntary After-Tax
By April 15, 2026	✅ Yes	✅ Yes	✅ Yes
April 16 – October 15, 2026 (with extension)	❌ No	✅ Yes	✅ Yes

Key Takeaways for Fidelity RIAs

📌 Summary for Registered Investment Advisors:

• Fidelity’s own Solo 401k does not support Mega Backdoor Roth, In-Plan Roth Conversions or participant loans

• My Solo 401k Financial provides the plan document, and assets are still custodied at Fidelity via non-prototype brokerage accounts

• SECURE Act 2.0 allows sole proprietors to open a Solo 401k in 2026 for retroactive 2025 contributions

• The April 15, 2026 deadline applies for employee contributions; employer and after-tax contributions extend to October 15, 2026 with a filed extension

• The auto contribution credit provides $1,500 in tax credits over three years—no contributions required to qualify

• My Solo 401k Financial handles all compliance for no additional charge for clients or their advisors who request it in timely fashion: 1099-R preparation, 5500-EZ filing, loan documents, and more

🚀 Ready to Set Up a Solo 401k for Your Client?

Next Steps:
Open a Solo 401k Account Today

Remember: This information is provided for educational purposes only. Always consult with qualified tax, legal, and investment professionals before making investment decisions with your retirement funds.