The Solo 401(k) is the default retirement plan for most small business owners — not because of a break-even income table, but because it is the natural output of a complete tax strategy. Once entity structure, deductions, and reasonable compensation are set, the Solo 401(k)'s flat-dollar $24,500 employee deferral plus 25% employer contribution shelters more than any competing plan at virtually every salary level. In 2026, combined contributions reach $72,000 base, $80,000 with the 50+ catch-up, or $83,250 with the SECURE 2.0 ages 60–63 super catch-up.

"The retirement contribution discussion is the last thing that happens after all other optimizations because it is so flexible and so dependent on the structure that we end up with. We layer in the retirement contribution once we have determined what the reasonable compensation is."

— Neal McSpadden, Founder, Tax Sherpa

Key Takeaways

• The Solo 401(k) is not chosen first — it is the output of a complete tax stack: entity structure, accountable plan, and reasonable comp analysis all happen before the retirement number is set.
• The $24,500 employee elective deferral (2026) has no W-2-percentage limitation — this is why the Solo 401(k) outperforms a SEP at almost every salary level.
• 2026 combined limits: $72,000 base; $80,000 with 50+ catch-up; $83,250 with the ages 60–63 super catch-up. Comp cap: $360,000.
• Catch-up contributions for earners over $145,000 in prior-year wages must be Roth effective January 1, 2026 — no opt-out.
• The 60–63 super catch-up is Roth-only with a 5-year seasoning rule; most high-income clients redirect that capital to a taxable brokerage instead.
• Dual-participant Solo 401(k) with an employed spouse can double household capacity — confirmed example: $800k pre-wage profit, two W-2s, ~$150k combined contributions.
• Fidelity's Solo 401(k) plan document does not support mega-backdoor Roth; Schwab and E*Trade do.
• Form 5500-EZ is required when plan assets exceed $250,000 at year-end. Penalty: $250/day.

Why Retirement Comes Last in the Tax Stack

Most retirement articles start with the wrong question: "Which plan fits my income?" The right question is: what does income look like after entity optimization and reasonable compensation — then which plan wins?

At Tax Sherpa, the planning sequence is fixed: (1) entity structure — S-Corp election creates a W-2 enabling the 25% employer contribution; (2) accountable plan — home office, vehicle, phone, and similar expenses reimbursed before income is recognized; (3) Summit Strategy Sessions — a proprietary deduction strategy, details are structure-specific; (4) reasonable compensation analysis — W-2 is set relative to distributions, optimizing QBI deduction and payroll tax; (5) retirement contribution — sized last, once W-2 is final.

By the time reasonable compensation is set, the Solo 401(k) wins almost automatically. The SEP can only match Solo 401(k) capacity at W-2 salaries near $350,000 — a threshold virtually no client in this planning profile carries.

The payroll tax math deserves honesty. The 15.3% payroll tax required to generate the 25% employer contribution creates only about a 10% net delta before the income tax deduction. Still worth doing — but it reinforces why retirement comes last, not first, in the planning stack.