Most self-employed investors know they can open a SEP IRA or a Solo 401(k). Far fewer know they can hold both simultaneously, and that in specific income and timing situations, running both accounts at once produces larger tax deductions and greater retirement savings than either account could deliver alone. For a foundational comparison of both account types before diving into the combined strategy, see IRA Financial’s guide to Why Choose a Solo 401(k) Plan vs. a SEP IRA.

Can You Contribute to Both a SEP IRA and a Solo 401(k) in the Same Year?

Yes, you can contribute to both a SEP IRA and a Solo 401(k) in the same year, but your total contributions across both accounts cannot exceed the IRS annual additions limit of $72,000 for 2026, and the accounts must be established for different business entities or under specific circumstances.

The most important clarification upfront: the IRS does not prohibit holding both account types simultaneously. What it prohibits is exceeding the annual additions limit across all defined contribution plans in which you participate. The $72,000 limit, plus an $8,000 catch-up contribution for those age 50 and older bringing the total to $80,000, or $83,250 for those ages 60 to 63, is a combined ceiling, not a per-account ceiling. Whether contributions go into a SEP IRA, a Solo 401(k), or both, they count toward the same limit.

IRA Financial works with self-employed clients across a range of income levels and business structures to determine when running both accounts simultaneously maximizes tax savings within the IRS limits.

Real more: Best Small Business Retirement Plans for 2026

2026 Contribution Rules: What You Need to Know Before Running Both

Both accounts have distinct contribution structures that directly affect how the combined strategy works. For the full 2026 contribution limits for both a SEP IRA and a Solo 401(k), including the employee deferral rules, profit-sharing calculations, and catch-up provisions, see IRA Financial’s SEP IRA Contribution Calculation Guide and Solo 401(k) contribution limits overview.

The key distinction to understand before looking at the combined strategy is that a Solo 401(k) allows both employee and employer contributions while a SEP IRA allows only employer contributions. That difference is what makes the combined strategy work at income levels where the SEP IRA alone cannot reach the annual limit.

When Does It Make Sense to Run Both a SEP IRA and a Solo 401(k) Simultaneously?

Running both a SEP IRA and a Solo 401(k) simultaneously makes sense when you have self-employment income from two separate businesses, one eligible for a Solo 401(k) and one where a SEP IRA is the more practical option, allowing combined contributions that approach or reach the $72,000 limit more efficiently.

The most common scenario involves a self-employed individual with two distinct income streams from separate business entities. Consider a physician who operates a private practice through an S-corporation where a Solo 401(k) is established, and also earns consulting income as a sole proprietor where a SEP IRA is established for that entity. The IRS treats each business as a separate employer for retirement plan purposes, subject to controlled group rules, allowing contributions to both plans up to the combined $72,000 ceiling.

A second scenario involves timing. An investor who already has a SEP IRA from a prior year and establishes a Solo 401(k) mid-year will find that SEP IRA contributions already made reduce the available room for Solo 401(k) employer contributions dollar-for-dollar, but the Solo 401(k)’s employee elective deferral remains fully available, potentially allowing additional contributions the SEP IRA alone could not have captured. IRA Financial’s tax team performs this calculation for each client before recommending a combined strategy. For the controlled group rules that determine whether two businesses are treated as one employer, see Solo 401(k) Plan Controlled Group Rules.

Read more: Solo 401(k) and SEP IRA: Can You Have Both at the Same Time?

How Do the IRS Limits Interact When Contributing to Both Accounts?

When contributing to both a SEP IRA and a Solo 401(k), the employer profit-sharing contributions from both accounts are aggregated and cannot exceed 25% of total compensation, but the Solo 401(k)’s employee elective deferral is separate and does not reduce the SEP IRA’s employer contribution limit.

This is where the math gets specific and where most general-purpose coverage gets it wrong. The IRS applies two distinct limits simultaneously:

Limit 1, the $72,000 annual additions limit: All contributions to defined contribution plans, SEP IRA employer contributions plus Solo 401(k) employer contributions plus Solo 401(k) employee deferrals, cannot exceed $72,000 in total from a single employer.

Limit 2, the 25% compensation limit: Employer contributions specifically, meaning SEP IRA plus Solo 401(k) profit-sharing, cannot exceed 25% of W-2 compensation or approximately 20% of net self-employment income in aggregate.

The Solo 401(k) employee elective deferral does not count toward the 25% employer contribution limit. It counts only toward the $72,000 annual additions ceiling. This distinction is what makes the Solo 401(k) a more powerful accumulation vehicle than the SEP IRA at lower income levels: the $24,500 elective deferral is available regardless of how much profit-sharing room exists.

What Does the Math Look Like for a Combined SEP IRA and Solo 401(k) Strategy?

The math shows that a Solo 401(k) alone reaches the $72,000 limit at lower income than a SEP IRA alone, but a combined strategy using two separate business entities can allow contributions from both accounts toward a single investor’s total retirement savings in the same year.

Scenario 1: One business, Solo 401(k) only (income: $150,000 net self-employment)

• Employee elective deferral: $24,500
• Employer profit-sharing (20% of net after SE tax deduction, approx. $141,732): $28,346
• Total Solo 401(k) contribution: $52,846
• Gap to $72,000 limit: $19,154

Scenario 2: One business, SEP IRA only (income: $150,000 net self-employment)

• Employer contribution (20% of approx. $141,732): $28,346
• Total SEP IRA contribution: $28,346
• Gap to $72,000 limit: $43,654

Scenario 3: Two businesses, Solo 401(k) for Business A ($150,000) + SEP IRA for Business B ($80,000)

• Solo 401(k) employee deferral (Business A): $24,500
• Solo 401(k) profit-sharing (Business A, 20% of net): $28,346
• SEP IRA contribution (Business B, 20% of net approx. $75,424): $15,085
• Total combined contributions: $67,931
• Remaining room to $72,000 limit: $4,069

The combined two-business scenario produces $39,585 more in tax-deductible retirement contributions than the SEP IRA alone, on the same total income. At a 37% marginal rate, that differential represents $14,646 in additional tax savings in a single year. IRA Financial calculates this analysis for clients with multiple income streams to identify the optimal contribution structure before year-end. For a guide to how high earners approach Solo 401(k) planning specifically, see A High Earner’s Guide to the Solo 401(k).

What Are the Deadlines for Contributing to Both a SEP IRA and a Solo 401(k)?

The Solo 401(k) must be established by December 31 of the tax year to make employee elective deferrals. The SEP IRA can be established and funded as late as the tax filing deadline including extensions, giving it a significant setup flexibility advantage.

This deadline difference is one of the most practically important distinctions between the two accounts. The Solo 401(k) plan document must be signed and the plan established before December 31 for elective deferrals to be made for that tax year. Employer profit-sharing contributions to the Solo 401(k) can be made up to the tax filing deadline including extensions, the same deadline that applies to SEP IRA contributions.

For investors who missed the December 31 Solo 401(k) establishment deadline, the SEP IRA remains available as a backstop. It can be opened and funded in full by April 15, or October 15 with extension, for the prior tax year. This is one of the scenarios where running both accounts makes practical sense: the Solo 401(k) captures the elective deferral opportunity for investors who plan ahead, while the SEP IRA remains available for late-year tax planning on the employer contribution side. For a complete guide to Solo 401(k) plan documents and establishment requirements, see Solo 401(k) Plan Documents.

Can You Make Roth Contributions to Both a SEP IRA and a Solo 401(k)?

A Solo 401(k) allows Roth elective deferrals. The SEP IRA traditionally does not offer a Roth option, though SECURE Act 2.0 created a Roth SEP IRA contribution option beginning in 2023 that relatively few custodians currently support.

The Roth contribution landscape for self-employed investors has changed significantly since SECURE Act 2.0. The traditional SEP IRA was exclusively pre-tax. SECURE Act 2.0 authorized Roth SEP IRA contributions beginning in 2023, allowing self-employed investors to designate SEP contributions as after-tax Roth. However, adoption among custodians has been uneven and not all SEP IRA providers currently offer the Roth SEP option.

The Solo 401(k) has offered Roth elective deferrals for significantly longer and most established providers, including IRA Financial, support the Roth Solo 401(k) option. For investors who want Roth treatment on the maximum possible contribution, the Solo 401(k)’s $24,500 Roth deferral option captures far more after-tax contribution room than a Roth SEP IRA allows on the same income. For a complete overview of the Roth SEP IRA rules introduced under SECURE Act 2.0, see SECURE Act 2.0: New SEP Roth IRA Contributions. For the Roth Solo 401(k) catch-up rules, see 2026 Solo 401(k) Roth Catch-Up Rule.

What Are the Administrative Differences Between Running Both Accounts?

A SEP IRA requires almost no ongoing administration. A Solo 401(k) requires an annual IRS filing once plan assets exceed $250,000, making the combined strategy slightly more complex but well within reach of most self-employed investors with professional support.

In practice, the administrative comparison looks like this:

SEP IRA administration: No plan document required. No annual IRS filing until assets exceed $250,000, at which point Form 5500-EZ is required. Contribution is reported on the tax return. Minimal ongoing complexity.

Solo 401(k) administration: Requires a written plan document provided by IRA Financial at setup. Annual Form 5500-EZ required once plan assets exceed $250,000. Roth deferral elections must be made before year-end. Loan provisions, if used, require documentation.

For investors running both accounts, the combined administrative load is additive but manageable, particularly with IRA Financial’s in-house tax filing and IRS reporting services handling the Solo 401(k) compliance requirements. The additional administrative step of the Form 5500-EZ filing is a minor annual task that is vastly outweighed by the additional contribution room and tax savings the combined strategy produces.

Read more: IRS Form 5500-EZ: Solo 401(k) Filing & Reporting Requirements

Frequently Asked Questions

Can a W-2 employee with a side business contribute to both a workplace 401(k) and a Solo 401(k) for their side business?

Yes, but the $24,500 employee elective deferral limit is shared across all 401(k) plans. A W-2 employee who contributes $10,000 to their employer’s 401(k) can contribute only $14,500 in elective deferrals to their Solo 401(k). The employer profit-sharing contribution to the Solo 401(k) is separate and fully available. For more on this scenario, see Can I Open a Solo 401(k) Plan If I Have a Job?.

Does contributing to a SEP IRA affect my ability to contribute to a Roth IRA?

No. SEP IRA contributions do not affect Roth IRA eligibility. Roth IRA contributions are limited only by income and the annual contribution limit. For 2026, the Roth IRA phase-out begins at $153,000 for single filers and $242,000 for married filing jointly. The annual contribution limit is $7,500, or $8,600 if age 50 or older. For the full Roth IRA contribution rules, see Can I Contribute to a Traditional IRA and Roth IRA in the Same Year?.

Can I convert my existing SEP IRA to a Solo 401(k)?

You cannot convert a SEP IRA directly into a Solo 401(k), but you can roll the SEP IRA balance into a Solo 401(k) once the Solo 401(k) is established. This is a common approach for investors who want to consolidate to a single account with greater investment flexibility and loan provisions. For the mechanics of this rollover, see Convert a SEP IRA to a Solo 401(k).

What happens if I contribute too much across both accounts?

Excess contributions to a Solo 401(k) are subject to a 10% excise tax under IRC Section 4979 and must be corrected by the tax filing deadline to avoid additional penalties. Excess SEP IRA contributions are subject to a 6% excise tax per year until corrected. IRA Financial’s tax team monitors contribution calculations for clients running both accounts to prevent over-contribution before year-end.

Can a self-directed Solo 401(k) and self-directed SEP IRA invest in the same alternative assets?

Yes. Both accounts can be self-directed to invest in real estate, private equity, precious metals, cryptocurrency, and private lending, subject to the same prohibited transaction rules that govern all self-directed retirement accounts. Running both as self-directed accounts doubles the capital available for alternative investment strategies while keeping each account’s compliance requirements separate. For a guide to self-directed retirement account investing, see Essential Self-Directed IRA Rules.