Timing is everything when it comes to maximizing your retirement contributions—and one of the most common questions we hear is:
“If I file a business tax return extension, can I wait until September or October 2026 to make my 2025 Solo 401(k) contributions?”
The short answer is:
Yes—but only for certain contribution types, and only if you follow specific IRS rules.
Watch: Complete breakdown of how you can still open a solo 401k in 2026 for making year 2025 contributions
The extended deadlines at a glance
When you file a business tax return extension, your Solo 401(k) contribution window extends along with it. Here are the key extended deadlines for tax year 2025 contributions made in 2026:
Critical distinction
These extended deadlines apply to employer profit-sharing and voluntary after-tax contributions. Employee salary deferrals have stricter deadlines tied to when your plan was established.
Let’s break it down so you can plan strategically and avoid costly mistakes.
Key Rule: Contribution Deadlines Follow Your Tax Filing Deadline
For self-employed individuals, Solo 401(k) contribution deadlines are generally tied to your business tax return due date—including extensions. The rules changed significantly with the passage of SECURE Act 1.0 and 2.0. Your entity structure now determines both your contribution deadline and which contribution types you can still make for 2025.
That means:
- Without extension: Contributions are due by your normal filing deadline (March or April 2026 for 2025 taxes)
- With extension: You may have until:
- September 15, 2026 (S-Corps & Partnerships)
- October 15, 2026 (Sole Proprietors and C-Corps)
This extended window can be a powerful planning tool—but not all contributions are treated equally.
Contribution Types Matter (A Lot)
Your ability to delay contributions depends on what type of contribution you’re making:
1. Employer Profit-Sharing Contributions
- Can be made by your extended tax deadline
- Applies even if you open the plan in 2026
- Available across all entity types
This is the most flexible contribution type.
2. Voluntary After-Tax Contributions (Mega Backdoor Roth)
- Also allowed by the extended deadline
- Plan must allow:
- After-tax contributions
- Roth conversions or in-service distributions
This is key for executing the Mega Backdoor Roth strategy.
3. Employee Salary Deferrals
This is where most people get tripped up.
- Must generally be elected by December 31, 2025
- If you did NOT have a plan in place by year-end, you likely:
- ❌ Cannot make employee contributions for 2025
- (Exception: limited flexibility for sole proprietors before April deadline)
Bottom line: You can’t retroactively create employee deferrals after year-end.
What If You Didn’t Open a Solo 401(k) by 12/31/2025?
Good news—you still have options.
If you did not have a Solo 401(k) plan in place by December 31, 2025, you can still open one in 2026 and make employer profit-sharing and voluntary after-tax contributions for tax year 2025 — as long as you open and fund the plan by your extended business tax return deadline.
Employee salary deferrals are the one exception: for S-Corps and partnerships, those contributions required a plan to be in place by December 31, 2025. For sole proprietorships, you technically have until April 15, 2026 — but with that date now days away, it’s effectively out of reach for most.
If you open a Solo 401(k) in 2026:
- You CAN still make:
- Employer contributions
- Voluntary after-tax contributions
- You CANNOT make:
- Employee deferrals (in most cases)
This is a major opportunity many self-employed individuals overlook.
Important: income must be from 2025
Even if you’re making contributions in 2026, they must be based on your 2025 earned self-employment income — not 2026 income. Contributions to a Solo 401(k) for tax year 2025 cannot exceed what your 2025 business income supports.
Deadlines by Business Type
Sole Proprietors / Single-Member LLCs
- Without extension: April 15, 2026
- With extension: October 15, 2026
✔ Can fund employer + after-tax contributions until October
✔ Employee contributions only if plan was opened on time
S-Corporations & Partnerships
- Without extension: March 15, 2026
- With extension: September 15, 2026
✔ Can open and fund plan by September for:
- Employer contributions
- After-tax contributions
Employee contributions require prior-year setup
Already Have a Solo 401(k)? You May Have Even More Flexibility
If your plan was established by December 31, 2025, then:
- You can make ALL contribution types (including employee deferrals)
- Up until your extended deadline in 2026
Additionally, you may be able to:
- Restate your plan (e.g., from Fidelity Investments or Charles Schwab)
- Add Mega Backdoor Roth features
- Make after-tax contributions retroactively
There’s also an option for those who already have a basic Solo 401(k) at Fidelity, Schwab, or another institution but want to add the Mega Backdoor Roth feature. Restating that existing plan to a MySolo401k Financial plan — as long as the original plan was open by December 31, 2025 — allows you to make voluntary after-tax contributions for 2025 by your extended deadline.
Important Requirements to Keep in Mind
To take advantage of extended deadlines:
- You must file a timely tax extension
- You must have earned self-employment income in 2025
- Contributions must be based on 2025 income (not 2026)
Common Mistakes to Avoid
- Assuming all contributions can be made late
- Missing the December 31 employee deferral deadline
- Opening a plan that doesn’t support Mega Backdoor Roth
- Confusing IRA rules with Solo 401(k) rules
2025 & 2026 Contribution Limits Reminder
For planning purposes:
- 2025 total contribution limit: $70,000
- Catch-up (age 50+): $7,500
- Super catch-up (age 60–63): $11,250
- 2026 total contribution limit: $72,000
Note that you cannot claim both the standard catch-up and the super catch-up — only one applies. And all contributions must be supported by your earned self-employment income for the tax year in question.
Reader Q&A
Final Takeaway
Filing a business tax return extension is a genuinely powerful retirement planning tool — it can push your Solo 401(k) contribution window out to September or October 2026 for tax year 2025 contributions. But the benefit is limited to employer profit-sharing and voluntary after-tax contributions, and only if you’ve filed the extension and have the self-employment income to support the contribution.
If you haven’t yet opened a Solo 401(k), it’s still not too late to do so and capture meaningful 2025 deductions. The key is acting before your extended deadline and working with both MySolo401k Financial (for the plan and calculations) and your CPA (for your final 2025 income figures) so everything is coordinated before you fund the account.
Yes, you can delay certain 2025 Solo 401(k) contributions until fall 2026
But primarily for:
- Employer contributions
- After-tax (Mega Backdoor Roth) contributions
Employee contributions require earlier action
