A sole proprietor can sometimes adopt a new one-participant 401(k) after year-end for the prior tax year. For a qualifying first plan year, the employee deferral election generally must be made by the unextended tax-return due date, while employer contributions can generally be funded by the return due date including extensions. Provider processing deadlines can be earlier.
An April 2026 forum question captures conflicting online claims about December 31, the filing deadline, and extensions. IRS Publication 560 provides the official special rule for a sole proprietor's new one-participant plan.
Start here: a practical action plan
- Open the official source and confirm the rule, deadline, or plan term for the exact facts.
- Collect the notices, statements, forms, invoices, and confirmations named in the guide.
- Replace the worked-example figures with actual amounts and check every calculation.
- Record the next deadline and save written proof after each action is completed.
- Relying on a forum answer instead of the governing form, plan document, policy, or agency source.
- Spending disputed money or a reserve before the final amount is confirmed in writing.
- Losing dated records needed to prove timing, calculation inputs, or a correction request.
Separate adoption, election, and deposit dates
Ask the provider for the signed plan adoption date, employee deferral election deadline, funding deadline, and whether its platform can accept a prior-year election. An extension can extend an employer contribution deadline without extending every employee election deadline.
The special after-year-end rule applies narrowly to a sole proprietor with a new section 401(k) plan and only-participant facts. An S corporation employee, partnership, existing plan, or business with eligible employees needs different analysis.
Worked example: profit does not equal contribution room
Example: a sole proprietor has $27,000 of Schedule C net profit before the self-employment tax deduction. Assume one-half of self-employment tax is $1,907 for this illustration. Adjusted net earnings are $25,093.
A 20 percent employer contribution planning estimate is $5,018.60. An employee deferral may add room, but total limits, other workplace deferrals, compensation, timing, and current-year law apply. Do not promise a $27,000 contribution from $27,000 of gross contractor receipts.
| Item | Example | Why it matters |
|---|---|---|
| Schedule C profit | $27,000 | Example after expenses |
| Half SE tax | $1,907 | Assumed calculation |
| Adjusted earnings | $25,093 | Planning compensation |
| 20% employer amount | $5,018.60 | Simplified estimate |
Make the election in writing before the applicable deadline
A contribution deposited later is not a substitute for a timely deferral election. Save the signed adoption agreement, election, extension, deposit confirmation, and provider correspondence. Use the correct contribution year in the custodian portal.
If the provider refuses retroactive processing, tax law alone does not make the operational step happen. Compare providers early and leave time for an EIN, account review, funding transfer, and correction of rejected paperwork.
Coordinate other jobs and future employees
Employee elective deferral limits generally apply across the person's plans, so a workplace 401(k) at another job can reduce solo-plan deferral room. Employer contribution calculations are separate and depend on business structure.
A one-participant plan can require Form 5500-EZ after plan assets cross the filing threshold or in the final plan year. Hiring an eligible employee can end the simple owner-only design. Review the plan annually rather than treating setup as the final task.
Frequently asked questions
Can a sole proprietor open a solo 401(k) after December 31?
A qualifying new one-participant plan can use a special after-year-end adoption rule, but election and funding deadlines still apply.
Does a tax extension extend the employee deferral election?
For the special first-year sole-proprietor rule, the election deadline is generally the unextended return due date.
Can employer contributions be funded by the extended due date?
They generally can when the return is extended and the other deduction requirements are satisfied.
Is the employer contribution 25 percent of Schedule C profit?
For a sole proprietor, the effective rate is generally lower because the contribution calculation is self-referential; 20 percent is a common simplified rate before other adjustments.
Do deferrals to another employer plan matter?
Yes. The individual employee elective deferral limit generally applies across plans.
What documents prove the contribution year?
Keep the plan adoption, signed election, extension, provider acceptance, bank transfer, and year-end plan statement.
Sources and review notes
The calculations and scenarios in this guide are original BreadWinnr worked examples. We recalculated each example and checked that its parts match the stated total. Official sources support the financial concepts. Forum links document the reader problem that prompted the guide and are not treated as financial authority.
- IRS Publication 560, retirement plans for small business
- IRS retirement plans for self-employed people
- Reddit: Schedule C and solo 401(k) options
Read our research, testing, corrections, and source policy.
This guide provides general financial education. It does not account for your complete financial, tax, legal, or contractual situation.
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