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SEP IRA and Solo 401(k): Which Retirement Account Fits Your Side Income?

When your side income isn't just a hobby—when you're treating it as a real business—you can no longer ignore retirement savings. Many freelancers and solo entrepreneurs default to a regular individual retirement account (IRA), which caps contributions at $7,000 per year (for 2024). But two accounts stand out as game-changers: the SEP IRA and the Solo 401(k). Both allow you to shelter far more of your self-employment income from taxes and accelerate wealth-building. The catch? They have very different rules, and picking the wrong one costs you thousands in opportunity.

Quick definition: A SEP IRA and a Solo 401(k) are retirement accounts designed for self-employed individuals and small-business owners that allow much higher annual contributions—often 25-30% of net self-employment income—compared to traditional IRAs, which cap at $7,000 yearly.

Key Takeaways​

  • Both SEP IRAs and Solo 401(k)s let self-employed earners contribute far more than a regular IRA (often $69,000+ per year).
  • SEP IRAs are simpler to set up and maintain but offer less flexibility in borrowing and early access.
  • Solo 401(k)s allow you to take loans against your balance and offer more control over investments.
  • Your choice depends on whether you prioritize simplicity, contribution room, or flexibility.
  • Set up either account before December 31 to contribute for that tax year (Solo 401(k)) or by the tax-filing deadline (SEP IRA).

What Makes These Accounts Different from a Regular IRA​

A traditional or Roth IRA lets you set aside $7,000 annually (2024). That's a solid start, but if you're earning $50,000 from side income, you're leaving massive tax deferral on the table. A SEP IRA or Solo 401(k) changes the equation entirely.

With a SEP IRA, you can contribute roughly 20% of your net self-employment income. A Solo 401(k) lets you contribute both as an employee (up to $23,500 in 2024) and as an employer (another 20% or so of net income). The result: a freelancer earning $75,000 from side work might sock away $15,000–$25,000 annually in either account, versus just $7,000 in an IRA.

This matters compounded over decades. An extra $15,000 per year at 7% annual returns for 30 years grows to approximately $1.7 million more than the IRA-only path. That's not hypothetical—that's a direct result of the tax shelter these accounts provide.

SEP IRA: The Simple Choice​

SEP stands for Simplified Employee Pension. The name says it: this account is simple. If you want the least administrative burden, the SEP IRA usually wins.

How SEP IRA Contributions Work​

With a SEP IRA, you contribute up to 20% of your net self-employment income, capped at $69,000 per year (2024). The calculation is straightforward: take your business profit, subtract half of your self-employment tax, and contribute up to 20% of what's left.

Example: You earned $80,000 from freelance writing. Your net self-employment income after expenses is $60,000. Half your self-employment tax is roughly $4,240. Your SEP contribution limit is 20% of ($60,000 − $4,240) = 20% × $55,760 = $11,152. You can contribute anything from $0 to $11,152 and deduct it from your income.

The beauty is flexibility: some years you might contribute $15,000; another year when business slows, you might contribute $5,000 or nothing at all. There's no mandatory minimum. This matters for freelancers whose income fluctuates.

Mechanics: Setup and Maintenance​

Opening a SEP IRA takes 15 minutes. You fill out a simple form (Form 5305-SEP) with your brokerage, and you're done. Annual paperwork is minimal—just ensure your contributions are made by your tax-filing deadline (including extensions, usually April 15 the following year). If you have employees, you must make an equal percentage contribution for them too—a key limitation if you plan to hire.

Withdrawal and Loan Rules​

SEP IRA money is invested in mutual funds, stocks, or bonds, just like a regular IRA. Unlike a Solo 401(k), you cannot borrow from a SEP IRA. If you need cash before age 59½, you withdraw and pay income tax plus a 10% penalty (with a few exceptions: disability, medical bills, first-home purchase). Once you reach 72, you face required minimum distributions (RMDs)—the government forces you to start withdrawing and paying tax.

Solo 401(k): Maximum Flexibility and Contribution Room​

A Solo 401(k) (also called an individual 401(k) or self-employed 401(k)) is a full-featured retirement plan that mimics a company 401(k) but for one person or a married couple. It's more complex to set up and administer but offers unique perks.

How Solo 401(k) Contributions Work​

You contribute as both an employee and an employer. The employee deferral maxes at $23,500 (2024). The employer contribution maxes out at roughly 20% of net self-employment income. Combined, you can contribute up to $69,000 per year (2024), matching the SEP IRA cap.

Example using the same freelancer: $80,000 earned, $60,000 net after expenses, $4,240 self-employment tax. Employee deferral: up to $23,500. Employer contribution: 20% × ($60,000 − $4,240) = $11,152. Total: $23,500 + $11,152 = $34,652—more than double the SEP IRA in this scenario.

The higher contribution room comes from the two-bucket structure. For some freelancers, this extra $10,000–$15,000 per year compounds into a meaningful difference.

Mechanics: Setup and Maintenance​

This is the tradeoff. Opening a Solo 401(k) requires more paperwork: you complete a plan document (often provided by your brokerage or a custodian), designate yourself as plan administrator, and set up payroll withholding. Some custodians charge setup fees ($200–$500).

Annual maintenance is more complex. You must file Form 5500-SF (or Form 5500, depending on assets) every year—a federal filing that SEP IRA owners don't face. If you make a mistake, there are penalties. Many freelancers hire an accountant to manage this ($300–$800 annually).

The Loan Feature: A Solo 401(k) Superpower​

This is where Solo 401(k) shines. You can borrow against your Solo 401(k) balance—up to 50% of your account value or $50,000, whichever is less. You repay with interest (set by your plan), and the interest goes back into your account. The loan is not taxable, and there's no 10% penalty for early withdrawal (unlike a traditional IRA).

Example: Your Solo 401(k) has grown to $120,000. You need $30,000 for a new laptop, software, or to cover a business gap. You borrow $30,000 at 5% interest, repaying over five years. No tax bill, no penalty, no lender approval hassle. A SEP IRA owner can't do this—they'd have to withdraw, pay income tax, and face a penalty.

For freelancers managing cash flow volatility, this feature alone justifies the extra complexity.

Roth and After-Tax Options​

Solo 401(k)s also allow Roth deferrals and mega backdoor Roths (in some setups). If you expect your tax bracket to be higher in retirement, you can contribute after-tax dollars that grow tax-free. This is an advanced move, but it's not available with a SEP IRA.

Decision Framework: SEP IRA or Solo 401(k)?​

Choose a SEP IRA if:

  • You want the absolute minimum administrative burden.
  • Your side income fluctuates unpredictably, and you want flexibility in contribution amounts.
  • You don't have employees and don't plan to hire.
  • You're confident you won't need to access your retirement savings early (even as a loan).

Choose a Solo 401(k) if:

  • You anticipate wanting to borrow against your retirement savings for business or personal needs.
  • You want maximum contribution room and expect your side income to grow.
  • You're comfortable (or willing to hire help) with annual form filing.
  • You want Roth options and more investment control.

Contribution Limits and the Tax Benefit Calculation​

Let's talk numbers. Understanding the tax shield is why you're doing this.

Suppose you're a W-2 employee earning $120,000 from your day job and $60,000 from freelance consulting. Your combined taxable income is $180,000, putting you in the 24% federal tax bracket (and possibly 6% state tax in many states). That's a 30% marginal rate.

If you contribute $15,000 to a SEP IRA, you reduce your taxable income to $165,000. You save $15,000 × 30% = $4,500 in taxes that year. Over 20 years, if that $15,000 grows at 7% annually, it becomes $58,000. Compare that to keeping it as taxable income: the $15,000 after taxes is only $10,500, which grows to $40,600. The difference: $17,400 in extra wealth—a direct result of the tax shelter and compound growth.

The contribution limits reset every January, so plan to fund these accounts before December 31 to take the deduction on that tax year's return (for Solo 401(k), the deadline is Dec 31 of the year; for SEP IRA, it's the tax-filing deadline, typically April 15 the next year).

Real-World Examples​

Example 1: The Graphic Designer

Maya does freelance graphic design. She earned $95,000 net from side work in 2024, plus a $70,000 W-2 salary. She opened a SEP IRA at age 28.

Her SEP contribution: 20% × ($95,000 − $6,732 SE tax) = $17,653. She deducts this, reducing her taxable income. If she's in the 24% + 5% state bracket, she saves $4,876 in taxes immediately.

At age 65 (37 years later), assuming 7% annual growth, that single year's contribution of $17,653 will have become approximately $342,000—purely from the tax-deferred growth. Over her career, if she contributes $15,000–$20,000 per year for 30 years, her side-income retirement savings could easily exceed $1.5 million.

Example 2: The Software Consultant

James earns $150,000 from consulting and wants maximum retirement savings. He opens a Solo 401(k).

Employee deferral: $23,500. Employer contribution: 20% × ($150,000 − $9,235 SE tax) = $28,153. Total: $51,653 per year. That's $34,000 more than a regular IRA would allow.

Two years in, his Solo 401(k) holds $110,000. He needs $25,000 for business equipment, so he borrows from his plan at 5% interest, repaying over five years. No tax hit, no penalty. By contrast, a SEP IRA owner in his position would have to withdraw $25,000, pay income tax on it, plus a 10% penalty—costing him roughly $9,000–$10,000 in taxes and penalties.

Example 3: The Inconsistent Freelancer

Ali does contract work but his income swings wildly—$40,000 one year, $120,000 the next. He opened a SEP IRA because of its flexibility.

Year 1 (low income): He contributes only $4,000 to his SEP. No problem. Year 2 (strong income): He contributes $20,000. The SEP accommodates both.

A Solo 401(k) would work too, but it requires an annual Form 5500 filing regardless of contribution amount. For someone with uneven income, the SEP's flexibility and minimal paperwork appeal more.

Common Mistakes with Self-Employed Retirement Accounts​

Mistake 1: Missing the Deadline

For Solo 401(k), you must establish the plan by December 31 of the tax year. For SEP IRA, you can open it until your tax-filing deadline (April 15 the next year). Many freelancers realize in March that they should have saved more and find themselves locked out of the Solo 401(k) option. Set a calendar reminder for November.

Mistake 2: Confusing SEP IRA with a Traditional IRA

A SEP IRA is not the same as a regular IRA. You cannot make both a SEP IRA contribution and a regular IRA contribution in the same year (there are phase-out limits). If you open a SEP IRA, your $7,000 regular IRA contribution room disappears. Read the rules carefully when your brokerage asks about existing IRA accounts.

Mistake 3: Forgetting the Self-Employment Tax Adjustment

The contribution percentage for SEP IRAs is roughly 20%, not 25%, because you have to subtract a portion of your self-employment tax first. Many freelancers oversimplify and try to contribute 25% of gross income, then discover their numbers are wrong when filing taxes. Use an online calculator or ask your accountant to get it right.

Mistake 4: Ignoring the Loan Repayment

If you borrow from a Solo 401(k) and default on repayment, the IRS treats it as a distribution subject to income tax and a 10% penalty. It's not a consequence-free line of credit. Borrow only if you're confident you'll repay consistently.

Mistake 5: Not Increasing Contributions as Income Grows

A freelancer making $50,000 from side work might open a SEP IRA and contribute $6,000 annually. Three years later, the income has grown to $120,000, but they're still contributing $6,000. The contribution limit has grown too—they're leaving $15,000+ of tax-sheltering room unused each year. Re-evaluate your contribution annually.

FAQ​

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

No. In any given tax year, you can use one or the other, not both. However, you can switch: use a SEP IRA one year, convert to a Solo 401(k) the next. Just be careful about overlapping tax years.

What happens to my SEP IRA or Solo 401(k) if I get a full-time job and stop freelancing?​

You keep the account. Contributions stop (you can only contribute if you have self-employment income), but the balance continues to grow tax-deferred. You can still withdraw at 59½. This is actually a benefit: your side-income savings are segregated from your W-2 retirement accounts.

Can my spouse contribute to a Solo 401(k) with me?​

Yes. If you're married and both have self-employment income from the same business, you can have a spousal Solo 401(k). Each of you gets your own deferrals and contributions. This is powerful for couples: combined, they might shelter $50,000+ annually.

What if my side income is small—$5,000 per year—should I bother?​

Maybe not immediately. The setup and maintenance cost (especially for Solo 401(k)) might exceed the tax savings. But if you expect the income to grow, opening a SEP IRA early gives you years of compound growth ahead. A SEP IRA with minimal paperwork is worth opening even for $5,000 of income.

Do I need to report my SEP IRA or Solo 401(k) on my taxes every year?​

SEP IRAs: You report the contribution on Schedule C (self-employment income), but no separate IRS filing is required unless assets exceed $250,000.

Solo 401(k)s: You file Form 5500-SF (or Form 5500) every year, even if assets are small. This is a bigger paperwork burden.

Can I invest in anything in a SEP IRA or Solo 401(k)?​

SEP IRAs: You're limited to assets your custodian offers (stocks, bonds, mutual funds, CDs, and some alternative investments like real estate via SDIRA custodians).

Solo 401(k)s: Generally the same menu, though some plans allow checkbook control for alternative investments. Talk to your custodian about options.

Summary​

A SEP IRA and Solo 401(k) are your two best tools for sheltering self-employment income in a tax-deferred account. A SEP IRA prioritizes simplicity: open it, contribute up to 20% of net income, and move on. A Solo 401(k) offers more contribution room (often $10,000–$15,000 more per year), the ability to borrow from your balance without penalty, and Roth options—but at the cost of annual form filing and more complexity. For most freelancers just starting out, a SEP IRA is the easier path. As your side income grows and you want maximum savings and flexibility, consider converting to a Solo 401(k). Whichever you choose, set it up before the deadline (Solo 401(k): December 31; SEP IRA: April 15 the next year) and contribute consistently. Over decades, the tax savings compound into hundreds of thousands of dollars in extra retirement wealth.

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