A Solo 401(k) checklist tracks three things: key dates, required documents, and contribution decisions. The Internal Revenue Service defines a Solo 401(k) as a traditional 401(k) covering a business owner with no employees, or the owner plus spouse. You use the list to stay eligible, fund by the right deadline, and keep proof the plan exists. It also helps you choose employee and employer amounts within 2026 caps.
Table of Contents
- Do you still qualify?
- How much can you contribute in 2026?
- When must you adopt and fund?
- What papers prove the plan exists?
- When does Form 5500-EZ apply?
Do you still qualify?
The Internal Revenue Service treats a one-participant plan under the same rules as any other 401(k), as described in the One-Participant 401(k) Plans page. It covers a business owner with no employees, or the owner plus spouse.
Hiring other employees ends one-participant eligibility. Check status before each plan year and after any hire. A spouse on payroll fits the rule, but a non-spouse employee does not.
- Review new hires, part-time help, and job offers for employee status.
- Pause Solo contributions if a new hire ends eligibility.
- Change plan type before the hire begins work if needed.
How much can you contribute in 2026?
The IRS newsroom announcement puts the 2026 employee elective-deferral limit at $24,500, up from $23,500 in 2025. That is the pay you set aside as the employee. That limit sets the employee leg of a Solo 401(k). The Internal Revenue Service caps 2026 total annual additions at $72,000 or 100% of compensation, as stated on the 401(k) contribution-limits page. Annual additions mean employee plus employer amounts for the year.
The cap rises to $80,000 with the $8,000 age-50-plus catch-up. It rises to $83,250 for ages 60-63 with the $11,250 super catch-up when the plan allows it. For example, a $24,500 deferral plus employer funds must fit inside the total-additions cap. SW Accounting & Consulting reports that catch-up amounts must be Roth in 2026 for participants whose prior-year FICA wages from that employer topped $150,000. Owners who expect a catch-up should confirm the plan offers Roth.
When must you adopt and fund?
The Internal Revenue Service explains in the post-year-end contributions snapshot that SECURE Act section 401(b)(2) permits retroactive setup for tax years after 2019. An employer may establish a 401(k) by the tax-return due date including extensions and elect treatment as adopted on the prior year's last day. The IRS Fix-It Guide requires employer matching and nonelective funds by the employer's income-tax filing deadline including extensions.
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Nonelective means employer funds paid even if the worker defers nothing. Taking an extension extends the funding and deduction window. Put adoption and funding deadlines on one tax calendar. Sign before the extended due date when you want prior-year treatment.
What papers prove the plan exists?
The Internal Revenue Service requires every 401(k) to start with a formal written plan document, as described on the Establishing a 401(k) Plan page. That document controls how the plan operates. Keep the signed adoption agreement and each amendment with it.
The same IRS guidance says owners must amend for tax-law changes and keep records by plan type. Store copies where you can find them at tax time. For example, file the 2026 amendment with the original adoption papers, not in email. Missing papers slow rollovers, distributions, and audits.
- Signed plan document and adoption agreement.
- Dated amendments and restatements.
- Contribution records and account statements.
When does Form 5500-EZ apply?
The IRS operating guidance requires a one-participant plan to file Form 5500-EZ when combined year-end assets of all its one-participant plans exceed $250,000. The form is due the last day of the seventh month after plan year-end, July 31 for calendar-year plans. Most small one-participant plans at or below $250,000 are otherwise exempt from annual filing.
The same guidance still requires a final Form 5500-EZ on termination regardless of asset value. For example, two Solo plans with $150,000 each total $300,000 and trigger the filing rule. Mark July 31 on a calendar-year checklist and add a termination filing task.
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