Retire Richer. Retire Faster.

The short answer: at almost every income level below roughly $360,000, a Solo 401(k) lets you shelter more money than a SEP IRA — often more than twice as much. If you are self-employed with no full-time employees, it is usually the right plan.

For decades, CPAs have put self-employed clients into a SEP IRA because it's easy paperwork. But the SEP IRA has a major mathematical flaw: no "employee" contribution. In a SEP, you can only contribute ~20% of your net income. To max out the $72,000 limit, you need to earn nearly $360,000.

In a Solo 401(k), you wear two hats:

• The Employee Hat — you can contribute your first $24,500 of earnings dollar-for-dollar (a 100% contribution rate).
• The Employer Hat — THEN you add 20–25% of profits on top of that.

That first hat is the whole game. The SEP starts every calculation at zero; the Solo 401(k) starts at $24,500 and builds from there.

Here is the comparison for a self-employed owner with $100,000 of net income:

At a 32% combined marginal rate, that extra $24,500 of deduction is roughly $7,800 of tax saved in a single year — from filling out a different plan document, not from earning a dollar more.

If you run an S-Corp, the math keys off your W-2 salary instead of net profit: the employee deferral comes out of your wages, and the employer piece is up to 25% of those wages. This is one of the quiet synergies of the S-Corp election — your "reasonable salary" becomes the base for the retirement plan. Set the salary with both goals in view, not just payroll tax savings. (Run the structure side through our S-Corp savings calculator.)

A "self-directed" Solo 401(k) lets you invest the account well beyond a brokerage menu:

• Real estate — buy a rental property, a commercial building, or raw land. You write the check directly from the 401(k) bank account, and all rent checks flow back into the account tax-free.
• Private lending — act as the bank. Lend money to house flippers or other investors at 10–12% interest; the interest payments flow directly into your retirement account with no taxes due.
• Alternative assets — invest in startups, precious metals (physical gold), or cryptocurrency. You hold the keys and control the allocation.

One bright line to respect: prohibited transactions. The plan cannot buy property from you, rent to your family, or pay you to manage its assets — self-dealing rules can disqualify the entire account. Checkbook control means you write the checks; it does not mean the money is yours yet.

Normally, hiring employees disqualifies you from a Solo 401(k). There is one exception: your spouse.

• Hire your spouse. If your spouse performs legitimate work for the business (admin, scheduling, bookkeeping), you can pay them a salary.
• Separate $72,000 limit. They get their own contribution limit. This effectively doubles your household's tax-advantaged space to $144,000/year — plus catch-up contributions if you're 50 or older.
• Family wealth transfer. This builds retirement assets in both names, providing robust asset protection and estate planning benefits.

The word "legitimate" is doing real work in that first bullet. The salary has to match actual duties at a market rate, run through real payroll, and be documented like any other employee's. A no-show paycheck sized to max a retirement plan is exactly the pattern examiners look for.

Most Solo 401(k) documents let you make the employee deferral as a Roth contribution instead of pre-tax: no deduction today, but qualified withdrawals — including decades of growth — come out tax-free. The choice is a bet on your tax brackets. In a peak earning year, the pre-tax deduction usually wins; in a low-income year (a startup year, a sabbatical, early retirement before Social Security), Roth dollars are often the better buy.

The advanced move is the mega backdoor Roth: plan documents that permit after-tax contributions above the normal deferral, paired with in-plan Roth conversions, can push total annual Roth funding far beyond the standard limits — up to the overall plan ceiling. It requires the right plan document and clean execution, but for high savers it is one of the largest Roth on-ramps in the code.

Either way, the decision should sit inside a broader plan — bracket management, Roth conversion timing, and your other deductions all interact. That's the case for treating this as year-round planning, not an account you open and forget. If you're weighing plan types entirely, our defined benefit vs. defined contribution guide covers the next tier up.

The Solo 401(k)'s only real downside versus a SEP is that it rewards acting early. Establish the plan by December 31 to contribute for that tax year; the money itself can generally follow as late as your filing deadline with extensions.

Two more dates worth knowing: employee deferrals should be formally elected by year-end (S-Corp owners need them set up through payroll before the final paycheck), and once plan assets exceed $250,000, an annual Form 5500-EZ filing kicks in — a short form, but an expensive one to forget.