SEP IRA vs Solo 401(k) for Self-Employed

A SEP IRA vs solo 401(k) comparison matters to any self-employed person or sole proprietor deciding where to save for retirement. Both plans offer high contribution room, tax deductions, and no employees to cover—but they differ sharply in how much paperwork they demand, what investment options you get, whether you can borrow from the plan, and whether a Roth version is available.

Contribution Limits: The Headline Difference

The SEP IRA and solo 401(k) reach the same annual ceiling—$69,000 for 2024—but the way you get there differs.

A SEP IRA lets you contribute up to 25% of your net self-employment income after the deduction for half your self-employment tax. This is the employer contribution formula: you are both employee and employer, so you write one check, and it goes into your SEP IRA as a deductible business expense.

A solo 401(k) splits contributions into two buckets. You defer salary (like a W-2 employee would), and you contribute an employer match on top. The total still caps at roughly 25% of net self-employment income, but the mechanics matter if you want to max out: you can contribute more as employee deferrals (up to $23,500 in 2024), and top it off with employer contributions. For most self-employed people, the total reaches the same $69,000 ceiling. However, the solo 401(k) architecture makes it simpler to visualize and control how much is employee versus employer, which can be relevant for business structuring and loan rules.

Administrative Burden and Paperwork

This is where real-world complexity emerges.

SEP IRA wins on simplicity. You open a SEP IRA account, sign a one-page SEP-IRA adoption agreement, and you’re done. No annual filing. No IRS forms to track employee deferrals. If you hire an employee later, you must contribute the same percentage of their salary as you contribute for yourself—but no employee has one until that moment, so most solo operators face zero annual bureaucracy.

Solo 401(k) demands more care. You adopt a written plan document (often a prototype plan from your custodian or accountant), contribute money, and track deferrals versus employer contributions. If your plan’s assets exceed $250,000 at year-end, you must file Form 5500-N with the Department of Labor and IRS. This filing requirement can trigger accountant fees; a solo 401(k) with $300,000 in assets will likely cost $400–$800 annually just to file the form. Below $250,000, you skip the filing requirement, which is why many solopreneurs treat solo 401(k)s as the “no filing” option until they hit that threshold.

For a freelancer with one account and $50,000 in assets, a SEP IRA is clearly easier. For someone with $350,000 saved and plans to borrow, the solo 401(k) cost is justified.

Roth Contributions and Tax Planning

SEP IRA offers no Roth option. You get only traditional, pre-tax contributions. This is firm and unchanging. If you want Roth retirement savings as a self-employed person, you must use a separate Roth IRA or Roth solo 401(k).

Solo 401(k) allows both traditional and Roth deferrals within the same plan. You can put $10,000 as Roth and $13,500 as traditional, for example, splitting your $23,500 employee deferral allowance. The employer contribution portion is always pre-tax, but the ability to do Roth deferrals offers tax diversification: some of your retirement income later will be tax-free (Roth), and some will be taxable (traditional). This flexibility becomes valuable if you expect your tax bracket to rise, or if you simply want to reduce your future required minimum distributions.

Loans Against Your Balance

One of the most overlooked differences is borrowing.

SEP IRA does not permit loans. Your money is in an IRA, and IRA rules forbid loans from any IRA, whether Roth or traditional. If you need cash, you must withdraw and pay income tax plus a 10% early-withdrawal penalty if you’re under 59½.

Solo 401(k) allows loans. You can borrow up to 50% of your vested balance or $50,000, whichever is less, and repay it over up to five years (or longer if the loan is for a primary residence). The interest you pay goes back into your plan, not to a bank. For a self-employed person facing a cash crunch or wanting to avoid a taxable withdrawal to fund a business opportunity, this is a material advantage.

Investment Options

Both plans impose no restrictions on investment choices. You can hold stocks, bonds, mutual funds, ETFs, and (with certain custodians) real estate or private equity. Neither the SEP IRA nor the solo 401(k) specifies what you must buy; the choice is yours. If this matters to you—for example, you want to own alternative assets—both work equally well. The difference is not in what you can invest in, but in the friction to set it up and the reporting burden once you do.

Hiring Employees: When Plans Break

Both plans protect you only as long as you are truly solo.

SEP IRA: If you hire a W-2 employee, you must contribute the same percentage of salary to their SEP IRA as you do for yourself. If you contribute 15% for yourself and hire one part-time employee, you now must contribute 15% of their salary too. This can grow expensive fast, which is why some business owners cap SEP contributions to avoid this liability. Contractors and 1099 vendors do not trigger this requirement; only W-2 employees do.

Solo 401(k): The same rule applies: once you have a W-2 employee, you must offer them the chance to participate in the plan under 401(k) rules. You cannot leave them out. This ends the “solo” nature of the plan, and you must adopt a full 401(k) plan. At that point, the administrative cost (and Form 5500 filing) becomes mandatory for all asset levels.

For either plan, if you are truly planning to stay solo, this is not a worry. If you think you might hire someone soon, the economic math shifts.

Which Plan Makes Sense When

Choose a SEP IRA if:

• You want the absolute minimum paperwork.
• You do not need to borrow from your retirement account.
• You have no plans to hire W-2 employees.
• You are comfortable with traditional (pre-tax) contributions only.
• Your business is very young and you are still building savings.

Choose a solo 401(k) if:

• You want the option to take loans from your retirement account.
• You want to split contributions between traditional and Roth.
• Your plan balance is likely to exceed $250,000 and you accept the Form 5500 filing fee as a cost of managing larger assets.
• You want to maximize flexibility in how you structure employee versus employer contributions.

Common Mistakes

Confusing the deadline is a perennial error. You can contribute to either plan through the tax-filing deadline (April 15, or October 15 if you file an extension), not the extension deadline itself. If you get an extension to October 15, you must contribute by April 15 for that tax year.

Another trap: naming an employee beneficiary incorrectly or updating a spouse after a divorce. Both plans pass to beneficiaries, and a stale beneficiary form will override your will.

Finally, underestimating the solo 401(k) Form 5500 cost. Many solopreneurs discover they owe their accountant $500 for the filing and regret the complexity. Know the threshold ($250,000) before you pick the plan.

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