In 2026, a solo 401k has a combined limit of $72,000 (for those under 50), and a SEP IRA has a limit of roughly $71,550 (20% of net self-employment income with adjustments). If you could maintain both, someone might think they could contribute $72,000 to the solo 401k and another $71,550 to the SEP IRA for a total of $143,550. The IRS says no. Annual additions to defined contribution plans are aggregated. If you have two defined contribution plans (a solo 401k and a SEP IRA both count), your total contributions across both cannot exceed the annual additions limit, which is $72,000. You get $72,000 of contribution room, period. Splitting it between two plans doesn’t create more room.

Can You Have a Solo 401k and a SEP IRA at the Same Time

Legally, maybe. Practically, no. The real barrier is Form 5305-SEP, the standard IRS template that explicitly prohibits maintaining “any other qualified retirement plan” simultaneously. If your SEP IRA uses Form 5305, you cannot also have a solo 401k. If your provider uses a custom prototype SEP document (uncommon), you technically could maintain both, but contributions aggregate anyway—you don’t get double the limits. A $72,000 solo 401k limit plus a $71,550 SEP IRA limit doesn’t equal $143,550 in available contribution room; it equals $72,000 total across both plans. Most people who ask this question are looking for a way to contribute more than the standard limit, and the answer is: you can’t, and having two plans won’t help. The only scenario where maintaining both makes genuine sense is operating two truly separate businesses, and even then, the aggregation rules make it nearly worthless. This article cuts through the confusion with the exact legal requirements, the aggregation math that kills the strategy, and when you might actually want to keep both plans.

The Form 5305-SEP Trap: Why Most SEP IRAs Disqualify Solo 401ks

The barrier to maintaining both a SEP IRA and a solo 401k is surprisingly simple but widely misunderstood. It’s Form 5305-SEP, the IRS’s standard SEP IRA adoption form. This form includes specific language that disqualifies it from coexisting with other qualified retirement plans.

Form 5305-SEP Explicitly Prohibits Other Plans

Form 5305-SEP states that the employer must not “maintain any other qualified retirement plan (including a SEP), a SIMPLE IRA plan, or a SIMPLE 401(k) plan.” A solo 401k is a qualified retirement plan. Under Form 5305-SEP, you cannot have one. If your SEP IRA was adopted using the standard IRS Form 5305-SEP, you are prohibited from establishing a solo 401k while the SEP remains in place. This is not a gray area—the form’s language is explicit.

The practical consequence: you would need to formally close (terminate) your SEP IRA to establish a solo 401k. Terminating a SEP is simple—you notify the custodian and cease contributions—but it’s a deliberate action that reverses a prior decision. Many people don’t realize this legal constraint exists until they’ve already invested years in a SEP and then want to switch to a solo 401k for better features (employee deferrals, participant loans, Roth option).

The Workaround: Prototype SEP Documents Allow Coexistence

A workaround exists: some custodians offer prototype SEP documents (custom forms) instead of the IRS Form 5305-SEP. These custom documents omit the prohibition on maintaining other plans. If your SEP IRA was established using a prototype document rather than Form 5305-SEP, you might legally maintain both a SEP and a solo 401k. However, this workaround is rare. Most custodians use Form 5305-SEP because it’s simpler and requires no custom legal review. Asking your SEP custodian “do you use Form 5305 or a prototype document” is not a conversation most advisors have.

If you already have a SEP and suspect it might be Form 5305-based, review your adoption agreement or call your custodian. The document’s name and language will clarify. If it’s Form 5305, you would need to terminate the SEP to add a solo 401k. If it’s a prototype document, you might be able to maintain both, but aggregation rules (discussed next) make this rarely valuable.

Aggregation Rules: Why Having Both Doesn’t Double Your Limits

Even if Form 5305 isn’t a barrier and you can legally maintain both, aggregation rules ensure you don’t benefit. Contributions to both plans roll up to a single IRS limit—you don’t get the contribution room of both plans.

Total Contributions Are Capped, Not Doubled

In 2026, a solo 401k has a combined limit of $72,000 (for those under 50), and a SEP IRA has a limit of roughly $71,550 (20% of net self-employment income with adjustments). If you could maintain both, someone might think they could contribute $72,000 to the solo 401k and another $71,550 to the SEP IRA for a total of $143,550. The IRS says no. Annual additions to defined contribution plans are aggregated. If you have two defined contribution plans (a solo 401k and a SEP IRA both count), your total contributions across both cannot exceed the annual additions limit, which is $72,000. You get $72,000 of contribution room, period. Splitting it between two plans doesn’t create more room.

Why Aggregation Destroys the Strategy

The reason people ask about maintaining both plans is typically to contribute more. They think: “If I can’t max out my solo 401k at $72,000, maybe I can add a SEP for extra room.” The answer is you can’t. The room doesn’t exist. Aggregation rules mean the total limit across both plans is the same as having one plan. You don’t gain contribution capacity by splitting your money between two custodians. You gain administrative overhead, more paperwork, two sets of compliance rules, and potential for confusion. You lose nothing by having both, but you gain nothing either, which is why having both is almost universally a bad choice.

The Exception: Genuinely Separate Businesses

The only scenario where aggregation matters less is if you have two genuinely separate businesses. Imagine you operate a consulting business and own a separate rental real estate business. You might establish a solo 401k for the consulting business and a SEP for the rental business. Contributions to the consulting 401k are based on consulting income. Contributions to the rental SEP are based on rental income. But here’s the catch: the IRS still aggregates contributions across both plans for purposes of the annual additions limit. If you contributed $50,000 to the consulting 401k and $25,000 to the rental SEP, you’ve used $75,000 of your $72,000 limit and overcontributed by $3,000. The benefit of having two separate plans is organizational clarity, not contribution capacity. Many CPAs recommend against this for precisely this reason.

When Maintaining Both Plans Actually Made Sense (And Doesn’t Anymore)

There was a narrow historical window where maintaining both plans was strategically valuable, but that window has largely closed.

Legacy Scenario: Avoiding Solo 401k Paperwork

In prior decades, solo 401ks required more compliance paperwork than they do now. A SEP IRA was significantly simpler—just three pages, no annual Form 5500 filing, minimal documentation. Some small business owners chose a SEP to avoid 401k complexity, even though solo 401ks offered better features. This was a legitimate trade-off: simplicity versus features. Modern solo 401ks are now nearly as simple as SEP IRAs, especially with online providers. The complexity argument for maintaining a legacy SEP while avoiding a solo 401k is largely gone.

Why Solo 401k Is Superior for New Plans

Solo 401ks now dominate the market for self-employed individuals precisely because they offer employee deferrals (a SEP IRA doesn’t), catch-up contributions at age 50 (a SEP IRA offers limited catch-up for high earners only), participant loans (a SEP IRA doesn’t), and Roth options (a SEP IRA doesn’t). A SEP IRA’s only advantage is simplicity, but that gap has narrowed. For anyone starting fresh, a solo 401k is almost always superior. The only exception is if you have employees (non-spouse, non-owner), in which case a SEP becomes the better choice because it’s easier to administer contributions for multiple employees. A self-employed sole proprietor has no reason to choose a SEP over a solo 401k.

The Practical Scenario: You Already Have a SEP and Want a Solo 401k

If you’re in this position, the options are clear but require action.

Verify Your SEP’s Document Type First

Before taking any action, confirm whether your SEP uses Form 5305-SEP or a prototype document. Contact your SEP custodian (your bank, brokerage, or IRA provider) and ask for a copy of your SEP adoption agreement or plan document. Look for the form title or language. If it explicitly states “Form 5305-SEP” or includes language prohibiting “any other qualified retirement plan,” you have a Form 5305 SEP. If it’s a custom prototype document, you might have flexibility. Most custodians will confirm by phone if you ask directly.

Terminating Your SEP IRA Properly

If you want to switch to a solo 401k and your SEP is Form 5305-based, you need to terminate the SEP. Terminating is simple: notify your custodian in writing (an email usually suffices) that you’re closing the plan and ceasing contributions. The money doesn’t go anywhere—you don’t withdraw it. It sits in your IRA/SEP custodial account. You can then establish a solo 401k and, if desired, perform a trustee-to-trustee transfer of the SEP balance to the solo 401k (assuming the solo 401k plan document allows it). Most do, but confirm with your new provider before closing the SEP. The transfer avoids taxes and consolidates your retirement savings in one place.

The Mechanics of Rolling a SEP to a Solo 401k

Once you’ve terminated your SEP, contact your new solo 401k provider and ask if they accept SEP IRA rollover transfers. Most major brokers (Fidelity, Schwab, Vanguard) do. Provide them with your SEP custodian’s contact information. The new solo 401k provider then files paperwork with your old SEP custodian requesting a trustee-to-trustee transfer. The SEP balance moves directly to the new solo 401k account with no tax consequences. You don’t take a distribution; the custodians handle the transfer. This typically takes 5-10 business days. Once the transfer completes, your SEP is empty and closed, and your retirement savings are consolidated in the solo 401k. You’re now positioned to use the solo 401k’s superior features (employee deferrals, catch-up, loans, Roth).

FAQ

If I have both a SEP IRA and a solo 401k for different businesses, do I file two tax forms?

You report contributions to both plans on your personal tax return (Form 1040, Schedule 1). If either plan balance exceeds $250,000 at year-end, you file Form 5500-EZ for that plan (solo 401ks file 5500-EZ if they exceed the threshold; SEP IRAs don’t file 5500 at all). So you might file no forms, a 5500-EZ for the solo 401k, or both. The aggregation rule applies regardless—your combined contributions across both plans cannot exceed the annual limit. Most CPAs recommend avoiding this complexity and choosing one plan per business.

Can I have a solo 401k in one year and switch to a SEP IRA the next year?

You can close a solo 401k and open a SEP IRA for a subsequent year, or vice versa. The key is not maintaining both simultaneously for the same business. If you had a solo 401k in 2025 and close it before establishing a SEP IRA in 2026, that’s fine. You would perform a trustee-to-trustee transfer of the solo 401k balance to a SEP IRA if you want to consolidate. Most people prefer to stay with whatever plan they open initially because switching creates administrative friction. Solo 401k is almost always the better long-term choice.

What if I have a SEP IRA and I hire an employee?

A SEP IRA allows you to cover an employee with a SEP contribution (employer-only). You cannot have a solo 401k with employees. If you hire an employee and want to maintain a solo 401k, you must close it and switch to a different plan (like a SIMPLE 401k or regular 401k that covers the employee). If you hire an employee and have a SEP, the SEP remains valid and you can cover that employee as well. This is one situation where a SEP becomes superior—it scales easily to multiple employees, whereas solo 401ks do not. But for a sole proprietor, solo 401k is almost always better.

Is there any reason to keep a SEP IRA if I also have a solo 401k?

No. If you’ve legally managed to maintain both (using a prototype SEP document), there’s no strategic advantage. You get no additional contribution room. You’re paying two custodians, filing potentially two tax forms (if balances are high), and dealing with aggregation complexity. The only reason to have both would be if you have genuinely separate businesses and want to keep finances completely separate for accounting clarity. Even then, most advisors would recommend closing one plan and consolidating. Having both is legal confusion without legal benefit.