SEP IRA vs. Solo 401(k): Choosing a Retirement Plan for Your Business

If you run your own business, whether that means freelancing, consulting, or owning a small company with no employees, one of the more consequential retirement decisions you’ll make is which type of plan to use. Two of the most common options for owner-only businesses are the SEP IRA and the Solo 401(k). Both allow tax-advantaged retirement savings, but they work differently, and the right choice depends on your income, how much administrative complexity you’re willing to take on, and whether you expect to hire employees down the road.

At a high level, a SEP IRA is funded only by employer contributions, while a Solo 401(k) allows both an employee salary deferral and an employer contribution. That single structural difference drives most of the practical decisions business owners face when choosing between the two. This article is general education only and is not individualized tax or legal advice.

Key takeaways

A SEP IRA is employer-funded only and tends to be simpler to set up and maintain.
A Solo 401(k) allows an employee deferral in addition to an employer contribution, which can meaningfully increase how much a lower- to moderate-income owner can contribute in a given year.
Solo 401(k)s can offer features SEP IRAs do not, including a Roth option and participant loans (if the plan document allows them).
Once a Solo 401(k)’s assets reach a certain threshold, annual filing (Form 5500-EZ) is generally required; SEP IRAs generally do not have this requirement.
If your business later hires employees, both plan types have rules about extending coverage that are worth reviewing with a professional before that happens.

What is a SEP IRA?

A Simplified Employee Pension (SEP) IRA is a retirement account funded entirely through employer contributions, calculated as a percentage of compensation, up to the annual limits set by the IRS. There is no employee deferral component. For a self-employed individual, this means the business itself makes the contribution on your behalf.

SEP IRAs are generally attractive for their simplicity. They typically involve less paperwork to establish and maintain than a Solo 401(k), and current guidance indicates a SEP can generally be established as late as your tax filing deadline, including extensions, for the year you want the contribution to count. That flexibility can be useful if you’re deciding on a retirement strategy after your business year has already ended.

What is a Solo 401(k)?

A Solo 401(k) (also called a one-participant 401(k)) is designed for a business owner with no employees other than, in some cases, a spouse. It has two contribution components:

Employee deferral — a portion of your compensation you elect to defer, similar to how a traditional workplace 401(k) deferral works.
Employer profit-sharing contribution — an additional contribution the business makes on your behalf, calculated similarly to a SEP contribution.

Because both components are available, a Solo 401(k) can allow a lower- or moderate-income business owner to contribute more in total than a SEP IRA would allow at the same income level, since the employee deferral isn’t tied to a percentage-of-compensation formula the way the employer portion is.

Many Solo 401(k) plans also offer a Roth option for the employee deferral portion, letting you contribute after-tax dollars in exchange for tax-free qualified withdrawals later. Some plans also permit participant loans, which SEP IRAs do not allow. Unlike a SEP IRA, current guidance suggests a Solo 401(k) generally needs to be adopted before year-end if you want to make employee deferrals for that same year, so the timing decision matters more.

Key differences to compare

Contribution structure. SEP IRA: employer only. Solo 401(k): employee deferral plus employer contribution.
Roth option. SEP IRA: not available. Solo 401(k): often available.
Participant loans. SEP IRA: not available. Solo 401(k): may be available, depending on the plan document.
Administrative complexity. SEP IRA: generally simpler. Solo 401(k): more plan documentation, plus annual filing (Form 5500-EZ) once assets reach a certain threshold.
Setup deadline. SEP IRA: generally by the tax filing deadline, including extensions. Solo 401(k): generally must be adopted before year-end for that year’s employee deferrals.
Best suited for. SEP IRA: owners who want simplicity and flexibility on timing. Solo 401(k): owners who want to maximize contributions or want Roth/loan features.

Who tends to prefer each option

Business owners who value simplicity, who are deciding late in the year, or who anticipate hiring employees relatively soon often lean toward a SEP IRA. Owners who want to contribute as much as possible at a given income level, who want access to a Roth savings option, or who might want the ability to borrow against their retirement plan tend to favor a Solo 401(k). Neither is universally “better” — the right fit depends on your income pattern, cash flow, and long-term business plans.

Common mistakes to avoid

Assuming both plans allow the same total contribution at every income level. They often don’t, because only the Solo 401(k) includes an employee deferral component.
Waiting until after year-end to consider a Solo 401(k) if you wanted to make an employee deferral for that year. Timing rules differ from SEP IRAs.
Overlooking the Form 5500-EZ filing requirement once Solo 401(k) assets grow past the applicable threshold.
Not revisiting the plan choice once the business hires its first employee, since eligibility and coverage rules can change.

How MRA can help

Choosing between a SEP IRA and a Solo 401(k) is rarely just a retirement plan question. It connects to your business’s cash flow, your personal tax situation, and your broader financial plan, including how much you’re already saving elsewhere and what your retirement income needs might look like. At MRA, we believe every financial decision is connected, and a retirement plan decision for your business is a good example: what makes sense this year may change as your income, staffing, or goals evolve. If you’d like to talk through which option may fit your situation, meet an MRA advisor to get started.

Frequently asked questions

Can I have both a SEP IRA and a Solo 401(k)?

Generally, IRS rules limit how contributions across multiple plan types interact, and combining plans adds complexity. This is a detailed, individual determination that should be reviewed with a qualified tax professional before you set anything up.

What happens if I hire employees?

Both plan types have eligibility and coverage requirements that can change once you have employees. A SEP IRA generally must be extended to eligible employees under its terms, and a Solo 401(k) is specifically designed for owner-only (or owner-and-spouse) businesses, so hiring staff is a trigger to revisit your plan structure with a professional.

Are contributions tax-deductible?

Contributions to both SEP IRAs and traditional Solo 401(k) accounts are generally made on a pre-tax basis for the business or individual, within IRS limits. Specific deductibility depends on your business structure and individual tax situation, so this should be confirmed with your tax professional.

Do contribution limits change every year?

Yes. Both plan types are subject to annual IRS limits that are adjusted periodically. Always confirm the current-year figures directly with the IRS or your tax professional rather than relying on a prior year’s numbers.

This article is for general educational purposes only and is not individualized investment, tax, or legal advice. Contribution limits, filing thresholds, and deadlines are set by the IRS and can change from year to year; please confirm current details with the IRS, a qualified tax professional, or your retirement plan administrator before making a decision. Investing involves risk, including the possible loss of principal.