Most owner-operated businesses choose a retirement plan exactly once. It was just you on payroll, someone reasonably told you a SEP IRA or a Solo 401(k) was the simple answer, and they were right. Then you hired a second person, and a third. The plan is still doing exactly what it was designed to do for a business that no longer exists. The rules did not change. Your facts did.
What Actually Happens to a Solo 401(k) When You Hire Someone
A Solo 401(k), which the IRS calls a one-participant 401(k), is not a special type of plan. It is an ordinary 401(k) that happens to cover an owner and possibly a spouse. What makes it feel special is the absence of nondiscrimination testing: with no common-law employees, nobody could have been treated worse than you. The IRS is blunt about what comes next. That advantage vanishes once you hire employees who meet the plan’s eligibility requirements. They must be included, and their deferrals become subject to testing unless the plan is a safe harbor plan.
Notice the conditional. Hiring someone on March 1 does not vaporize your plan on March 2, because your plan document controls eligibility and a 401(k) may generally require up to a year of service. But eligibility cannot be pushed past two consecutive 12-month periods in which an employee works at least 500 hours, so long-term part-timers get in eventually. The question is whether you will have made a decision by then or absorbed one.
The SEP Eligibility Trap Most Owners Do Not See Coming
For 2026, an employee is generally eligible for your SEP once they have reached age 21, worked for you in at least three of the last five years, and received at least $800 in compensation from you during the year.
Read those last two conditions together. A “year” of service is not a full-time year, and $800 is a very low bar. The person who helps you a few weeks each summer can be building toward eligibility while nobody tracks it. IRS guidance states plainly that all eligible employees must participate, including part-time and seasonal workers.
Eligibility then turns into economics. SEP contributions must follow a written allocation formula that does not favor highly compensated employees, and you must contribute for every eligible participant who performed services that year. In practice, that means the same percentage for everyone, including you.
What that looks like in dollars
A Southlake owner has historically put 20% of her compensation into her SEP. Three employees have now passed the three-of-five-year mark.
| Participant | Compensation | Required at 20% |
| Owner | $200,000 | $40,000 |
| Employee A | $55,000 | $11,000 |
| Employee B | $48,000 | $9,600 |
| Employee C | $34,000 | $6,800 |
| Employee subtotal | $137,000 | $27,400 |
Hypothetical, for illustration only.
Keeping her own $40,000 now costs an extra $27,400 she never budgeted for. She can cut her own contribution instead, but that is the trade. Her retirement funding and her payroll cost move on a single dial.
Where a Traditional or Safe Harbor 401(k) Starts to Fit
A 401(k) is not automatically better, and it costs more to operate. What changes is who funds the accounts. A SEP is entirely employer-funded. A 401(k) lets employees fund themselves through salary deferrals, up to $24,500 in 2026 plus an $8,000 catch-up at age 50 or older, so your employer contribution becomes a design decision rather than the only mechanism.
The catch in a traditional 401(k) is testing. If your employees defer at low rates while you defer at the maximum, the plan can fail, and the usual correction is refunding part of your own contribution back to you as taxable income. That is a real problem for an owner counting on the deduction.
Safe harbor design addresses it. In exchange for a prescribed employer contribution that vests immediately, the plan is relieved of certain annual testing. Two common formulas: a basic match of 100% of the first 3% deferred plus 50% of the next 2%, or a nonelective contribution of at least 3% of compensation for every eligible non-highly compensated employee whether they defer or not. Safe harbor is relief, not amnesty. Notices, documents, and filings still apply.
Why Waiting Until December Narrows Your Options
October 1 is not a universal safe harbor deadline, and you should be skeptical of anyone who presents it as one. Current law offers real flexibility on the nonelective side: a plan may generally be amended to adopt safe harbor nonelective status before the 30th day before the close of the plan year, and later still, into the following plan year, if the nonelective contribution is at least 4% rather than 3%.
The practical point is not the deadline. It is everything in front of it: a clean census, a determination of who is actually eligible, a cost model at realistic participation rates, a provider and administrator, documents, and payroll setup. Starting in November means a rushed decision or a higher percentage for deciding late.
Comparing the Real Cost, Not the Sticker Price
“SEP is cheap, 401(k) is expensive” is the wrong comparison once you have payroll.
| SEP IRA | 401(k) with safe harbor design | |
| Who funds it | Employer only | Employee deferrals plus employer contribution |
| Employer cost driver | Uniform percentage for every eligible participant, including you | Set by plan design, often well below a uniform SEP percentage |
| Annual testing | None | Relief from certain testing under safe harbor |
| Administration | Minimal custodial cost | Recordkeeping, third-party administration, advisory, filings |
| Owner flexibility | Your rate is everyone’s rate | Deferral and employer pieces move independently |
Then subtract the credits, which can absorb a real share of the cost of standing up a new plan.
| Credit | Amount | Duration |
| Startup costs | 100% of qualified startup costs (1 to 50 employees), 50% (51 to 100). Capped at the greater of $500 or the lesser of $250 per eligible non-highly compensated employee or $5,000 | First credit year plus 2 years |
| Employer contributions | Up to $1,000 per employee. 100% in years one and two, 75% year three, 50% year four, 25% year five, reduced above 50 employees | 5 years |
| Automatic enrollment | $500 per year for adding an eligible automatic contribution arrangement | 3 years |
Not every business qualifies. Employers that maintained a plan for substantially the same employees in the prior three years are excluded, and the contribution credit disregards contributions for higher-paid employees.
How to Tell It Is Time to Re-Evaluate
You are probably past the point where the original arrangement fits if any of these are true:
- Employees are approaching the three-of-five-year mark in your SEP.
- You use seasonal or part-time help and have never tracked service history.
- You want to contribute meaningfully for yourself but not at the same percentage for everyone.
- You have hired, or expect to hire, within the next 12 months.
- Your plan document has not been reviewed since you were the only participant.
None of these mean you must abandon what you have. A SEP can still be right for a business with one or two employees and an owner comfortable funding a uniform percentage. Texas owners have an added reason to get it right: with no state income tax to deduct against, the entire benefit of your plan is federal, so an inefficient structure has nothing else working in its favor.
Frequently Asked Questions
Can you have a Solo 401(k) with employees? Not indefinitely. Once employees satisfy your plan’s eligibility terms they must be included, and the plan follows the ordinary 401(k) rules, including testing unless it is a safe harbor plan.
When does an employee become eligible for a SEP IRA? Under the most restrictive terms the IRS permits, at age 21 with service in three of the last five years and at least $800 of compensation in 2026. Your plan may be less restrictive, not more.
Can I exclude part-time or seasonal workers from my SEP? Generally no, if they meet the eligibility conditions. IRS guidance specifically names part-time and seasonal employees among those who must participate.
Does calling someone a contractor solve the problem? Classification is a facts-and-circumstances determination, not a labeling choice, and a misclassified common-law employee creates separate exposure.
Anything else new plans should budget for? 401(k) plans established on or after December 29, 2022 generally must include automatic enrollment for plan years beginning after 2024, subject to limited exceptions covering certain new and very small businesses.
The Bottom Line
The SEP that felt effortless becomes expensive the moment your team grows, and the Solo 401(k) stops being solo on a timeline set by your plan document rather than your calendar. Before your next contribution, model what the current arrangement will cost at next year’s headcount and what a different design would cost for the same result. If you would like that run against your own census and payroll, we are glad to build it with you.
This article is educational and is not individualized tax, legal, or investment advice. Outcomes depend on your plan document, worker classification, ownership and controlled-group structure, and other facts specific to your business.