| Feature | Solo 401(k) | SEP IRA |
|---|---|---|
| Who qualifies | Self-employed, no full-time non-owner employees | Any self-employed individual, with or without employees |
| Contribution capacity | Employee deferral plus employer profit sharing | Employer profit sharing only |
| Roth option | Built into one plan | Separate Roth SEP IRA required |
| Checkbook Control | Built in, you are the trustee | Requires an LLC or Trust plus a custodian |
| UDFI exemption | Yes, for debt-financed real property | No |
| Participant loan | Yes, up to $50,000 or 50% of vested balance | Not permitted |
| Form 5500-EZ | Required if year-end assets exceed $250,000 | Not required |
Who qualifies for each plan?
The SEP IRA is open to any self-employed individual or business, with or without employees. The Solo 401(k) is narrower: it's available only to owner-only businesses, with no full-time, non-owner employees other than a participating spouse. The full test reaches past a simple headcount, since common-law employee classification and controlled-group rules can each pull an otherwise-qualifying business out of Solo 401(k) eligibility.
If your business has eligible employees, the Solo 401(k) is off the table, and the SEP becomes your primary path to high-contribution retirement savings. Each employee's SEP IRA is a separate account that can sit at a different institution, so you can self-direct your own account while employees keep theirs wherever they choose. Self-direction is your choice to make for your own account; it doesn't have to extend to the rest of your workforce.
How do the contribution limits compare?
Both plans allow the same employer profit-sharing contribution: up to 25 percent of W-2 wages, or roughly 20 percent of net self-employment income. The Solo 401(k) adds a second contribution source the SEP doesn't have, an employee salary deferral, layered on top of profit sharing rather than counted against it. The employee deferral is what actually separates these two plans at modest income levels; the profit-sharing math alone treats them almost identically. In practice, that stacking effect is most valuable to a modest-earning sole proprietor, since the SEP's straight percentage-of-income cap limits contributions more than the Solo 401(k)'s deferral option does; the advantage narrows as income climbs toward the plan maximum both structures share. Because both figures adjust annually, current dollar amounts live on Solo 401(k) - Contribution limits and in the current IRA limits reference.
How does Roth flexibility differ?
The Solo 401(k) holds tax-deferred and Roth balances as sub-accounts within a single plan, one EIN, one bank account, with both deferrals and profit-sharing contributions eligible for Roth designation. A Roth SEP IRA is available under SECURE 2.0, but it exists as a separate account from the traditional SEP IRA, with its own custodian relationship and recordkeeping. The Solo 401(k) also supports the Mega Backdoor Roth strategy, converting after-tax contributions to Roth status inside the plan, a pathway with no equivalent in an IRA-based structure.
How does Checkbook Control differ between the two?
Checkbook Control is built into the Solo 401(k): the plan is a trust, you serve as trustee, and you open the plan's bank account directly with no custodian in the chain. A SEP IRA is a custodial account by definition, so Checkbook Control requires the SEP to invest into an LLC or Trust that you manage. The custodial account remains in place for the initial investment, adding an administrative layer the Solo 401(k) doesn't carry.
Does the UDFI exemption matter for your investments?
Only if you're using debt to acquire real estate inside the plan. A Solo 401(k) is exempt from Unrelated Debt-Financed Income under IRC Section 514 on leveraged real property acquisitions, so a mortgaged rental or a debt-financed syndication stays untaxed at the plan level. A SEP IRA receives no such exemption. Important: this exemption is specific to debt-financed real property; it doesn't extend to other leveraged UBTI-generating activity.
Can I borrow from either plan?
Only the Solo 401(k) offers a participant loan, up to $50,000 or 50 percent of the vested balance, whichever is less, repaid with interest back to the plan. A SEP IRA cannot offer this feature at all. Under IRC Section 4975, a loan from an IRA to yourself is a prohibited transaction that disqualifies the account outright.
When does it make sense to keep the SEP IRA?
The Solo 401(k)'s feature set doesn't make it the automatic choice, even for owners who qualify for one. If you already have a SEP IRA in place and want to add self-direction without restructuring your plan, the simpler move is often to keep the SEP and open it with a self-directed custodian. Switching to a Solo 401(k) means confirming eligibility, establishing a new plan, and rolling over existing assets, real administrative work that isn't justified if your main goal is Checkbook Control for your own account rather than the Solo 401(k)'s deferral or loan features specifically.
The SEP is also the more durable choice for a business whose employee situation might change. Bringing on a full-time, non-owner employee doesn't disturb a SEP IRA the way it forces a Solo 401(k) into termination or an upgrade to a full ERISA plan. An owner who expects to hire, even a year or two out, avoids a disruptive transition later by starting with, or staying on, the SEP.
Which plan actually fits your situation?
If you qualify for a Solo 401(k) and expect to keep qualifying, and the deferral capacity, Roth integration, UDFI exemption, or loan feature would actually change how you save, it's the stronger plan in nearly every dimension. If you have or plan to hire eligible employees, the SEP is your path to high-contribution savings without disqualifying your plan. And if you already run a SEP and mainly want Checkbook Control for yourself, keeping the SEP and adding a self-directed custodian is usually the cleaner path; a switch to a Solo 401(k) is worth modeling with your CPA first, particularly if your employee situation could change. In practice, the eligibility question resolves this more often than the feature comparison does.
Frequently Asked Questions
Can I have both a SEP IRA and a Solo 401(k) at the same time?
You can hold both, but if they cover the same employer, combined contributions are subject to one shared annual limit rather than two separate ones. Running both doesn't multiply your contribution room.
Does the Solo 401(k)'s contribution advantage disappear at higher income?
It narrows. Both plans converge at the same overall plan maximum. The deferral advantage is most pronounced at lower income levels, where the SEP's percentage-of-income cap limits it more than the Solo 401(k)'s deferral option does.
Can my spouse participate in either plan?
Yes, in both, but differently. A Solo 401(k) can hold both spouses' contributions within one plan if the spouse earns income from the business. A SEP requires each spouse to hold a separate SEP IRA account.
If I already have a SEP IRA, do I have to close it to open a Solo 401(k)?
No, but ongoing contributions to both for the same self-employment income share one combined limit, so most investors transition to a single primary vehicle rather than running parallel plans.
Does self-directing a SEP IRA require anything different from a standard SEP?
No. Self-direction happens at the custodian level. You establish your SEP with a self-directed custodian instead of a conventional brokerage; the plan mechanics themselves are unchanged.
What happens to my Solo 401(k) if I hire a full-time employee?
The plan must be terminated or upgraded to a full ERISA-covered plan once you have non-owner employees who don't qualify for an exclusion. Balances roll to an IRA or another qualified plan.
Next Steps
Not sure which plan fits your situation? Plan Finder gives you a recommendation based on your employment status, funding sources, and goals.