A solo 401(k) is a retirement plan designed for self-employed business owners who have no employees other than themselves and, optionally, a spouse. It follows the same IRS rules as any other 401(k), but because you’re the only participant, you skip the complex nondiscrimination testing that larger plans require. The result is a streamlined way to save aggressively for retirement while reducing your taxable income.
Who Can Open a Solo 401(k)
You qualify if you run a business with no common-law employees. That includes freelancers, independent contractors, sole proprietors, single-member LLC owners, and partners in a partnership. The business can be a side hustle; you don’t need to be self-employed full time. If your spouse works in the business, they can participate in the plan too, which effectively doubles the household contribution capacity.
The key restriction is hiring. If you bring on employees who meet the plan’s eligibility requirements, you must include them in the plan. At that point, the plan loses its simplified status, and their contributions become subject to nondiscrimination testing (unless you convert to a safe harbor plan). For most people, the moment you have a W-2 employee who isn’t your spouse, a solo 401(k) stops being practical.
How Contributions Work
The solo 401(k) lets you contribute in two separate roles: as the employee of your business and as the employer. This dual contribution structure is what makes the plan so powerful compared to simpler alternatives.
As the “employee,” you can make elective deferrals from your compensation. For 2024, the limit on those deferrals is $23,000. If you’re 50 or older, you can add a catch-up contribution of $7,500, bringing the employee side to $30,500.
As the “employer,” you can contribute up to 25% of your net self-employment earnings on top of the employee deferral. If you operate as a corporation, the employer piece is based on your W-2 salary from the business instead. The combined total of both sides cannot exceed $69,000 for 2024 (or $76,500 with the catch-up contribution for those 50 and older). These limits adjust annually for inflation, so check the IRS figures for the current year.
To put that in practical terms: if your net self-employment income is $100,000, you could defer $23,000 as the employee, then add roughly $18,587 as the employer contribution (25% of net earnings after the self-employment tax deduction), for a total around $41,587. Someone earning $250,000 or more can potentially hit the full combined cap.
Traditional and Roth Options
Your employee deferrals can go in as traditional pre-tax contributions, Roth (after-tax) contributions, or a mix of both, as long as your plan document allows Roth contributions. Traditional contributions reduce your taxable income now, and you pay taxes when you withdraw in retirement. Roth contributions don’t give you a tax break today, but qualified withdrawals in retirement are completely tax-free.
If you choose the Roth option, your plan provider will set up a separate Roth account within the solo 401(k). The employer profit-sharing portion, however, always goes in as pre-tax money regardless of your Roth election.
Loans and Hardship Withdrawals
Unlike a SEP IRA, a solo 401(k) can be set up to allow loans from your own account balance. The general 401(k) loan rules apply: you can borrow up to 50% of your vested balance or $50,000, whichever is less. You repay the loan with interest back into your own account, typically over five years. Not every plan provider offers the loan feature, so if this matters to you, confirm it before opening the account.
Some plans also permit hardship distributions for immediate and heavy financial needs, though these come with income taxes and, if you’re under 59½, a 10% early withdrawal penalty.
Setting Up the Plan
Opening a solo 401(k) involves adopting a written plan document that spells out the rules, such as contribution formulas, loan provisions, and Roth eligibility. Many brokerage firms offer pre-approved plan documents at no cost when you open an account with them, which makes setup straightforward for most people.
To make contributions for a given tax year, the plan must be established by December 31 of that year. You then have until your tax filing deadline, including extensions, to actually deposit the employer profit-sharing contribution. Employee elective deferrals generally need to be made by the end of the calendar year, though rules can vary slightly depending on your business structure.
Filing Requirements
Solo 401(k) plans with total assets of $250,000 or more at the end of the plan year must file Form 5500-EZ with the IRS annually. If your plan balance is below that threshold, no annual filing is required. This is a simple, one-page form, but missing the deadline can trigger penalties, so mark your calendar once your balance crosses that line.
Solo 401(k) Compared to a SEP IRA
The SEP IRA is the other popular retirement plan for self-employed people, and the two are frequently compared. The biggest practical difference is in the contribution structure. A SEP IRA only allows employer-style contributions of up to 25% of net self-employment earnings. It has no employee deferral component. That means if your income is modest, say $60,000, a SEP IRA maxes out around $11,145 in employer contributions, while a solo 401(k) lets you defer an additional $23,000 on the employee side, potentially tripling your total contribution.
At very high income levels the combined caps converge, so the advantage of the solo 401(k) is most pronounced for people earning under roughly $275,000. The solo 401(k) also offers the Roth option and the ability to take plan loans, neither of which a SEP IRA provides. The tradeoff is slightly more paperwork: adopting a plan document upfront and filing Form 5500-EZ once assets grow large enough.
Who Benefits Most
A solo 401(k) is especially useful for self-employed people who want to maximize retirement savings and reduce their current tax bill. If you’re a freelancer earning $50,000 to $150,000, the employee deferral alone can shelter a meaningful chunk of your income that a SEP IRA wouldn’t cover. If you value flexibility, the loan provision and Roth option add tools you won’t find in simpler plans. And if your spouse works in the business, contributing to both accounts can shelter a substantial portion of household income.
The plan stays simple as long as you remain a one-person (or one-couple) operation. The moment you’re ready to hire, you’ll need to either expand the plan to cover employees or transition to a different retirement plan structure.