Solo 401(k) vs SEP IRA vs Defined Benefit Plan: Stacking Retirement Accounts for Business Owners

Start with the mechanics of each plan side by side. A SEP IRA is funded only by the employer: up to 25% of W-2 compensation for an S corp owner (roughly 20% of net self-employment earnings for a sole proprietor), with a 2026 cap of $72,000 and compensation counted only up to the $360,000 limit under Section 401(a)(17). A solo 401(k) has two buckets. The employee deferral of $24,500 is available at almost any income level, and the employer profit sharing contribution uses the same 25% formula as the SEP. Both buckets together are capped at $72,000, with the catch-up sitting on top. A cash balance plan is a defined benefit plan that looks like an account: each year the owner receives a pay credit and an interest credit, and the required contribution is set by an actuary based on age, pay, and the target benefit, up to the Section 415(b) limit of a $290,000 annual benefit at retirement.

Here is how the numbers stack for a 58-year-old S corp owner with $300,000 of W-2 wages and no employees. With a solo 401(k) alone, the owner defers $24,500, adds the $8,000 catch-up, and the corporation contributes $47,500 of profit sharing (25% of pay would be $75,000, but the $72,000 cap binds first). Total: $80,000. One wrinkle for 2026: because the owner's prior-year FICA wages exceed about $150,000, the $8,000 catch-up must go in as Roth under SECURE 2.0, so only $72,000 of that total is deductible. Add a cash balance plan, and an actuary might design an annual contribution somewhere around $150,000 to $225,000 at that age and pay. Because a defined benefit plan now exists, profit sharing drops to 6% of pay, or $18,000, for the reason explained below. Illustrative total: $24,500 deferral, $8,000 Roth catch-up, $18,000 profit sharing, and roughly $175,000 to cash balance, for about $225,000 a year, with the bulk of it deductible at the corporate level.

Why not just add a SEP on top of the solo 401(k) and double up? Because the tax code treats all defined contribution plans of the same employer as one plan for the $72,000 Section 415(c) limit. SEP contributions and 401(k) profit sharing come out of the same bucket, so stacking them adds paperwork without adding room. On top of that, the IRS model SEP document (Form 5305-SEP) cannot be used by an employer that maintains any other qualified plan. A SEP is a fine standalone plan for someone who wants simplicity and can hit the cap through the employer contribution alone, but for most owner-only businesses it is either the whole answer or the wrong layer. It is not a stacking tool.

The combined plan deduction limit is where many do-it-yourself designs go wrong. When one employer sponsors both a defined contribution plan and a defined benefit plan, Section 404(a)(7) generally limits the total deductible employer contribution to the greater of 25% of covered payroll or the amount needed to fund the defined benefit plan. The practical workaround is that employer contributions to the 401(k) up to 6% of pay are disregarded for this test, and employee deferrals never count. That is why a typical owner-only design pairs a cash balance plan with a 401(k) that includes the full deferral and catch-up but limits profit sharing to 6% of pay. Plans covered by the PBGC are treated differently, but a plan covering only the owner (and spouse) is generally not PBGC-covered, so the 6% design is the norm for this audience.

Deadlines differ by plan and matter more than people expect. A SEP can be established and funded up to the business's tax filing deadline, including extensions. For an S corp owner, solo 401(k) deferrals are withheld from W-2 pay, so the election and the payroll withholding have to happen by the last payroll of the year; the retroactive first-year deferral rule in SECURE 2.0 applies to sole proprietors, not to S corp W-2 wages. Profit sharing contributions can be made up to the corporate return deadline with extensions. Under the SECURE Act, a new qualified plan, including a cash balance plan, can be adopted after year end as long as it is in place by the employer's tax filing deadline including extensions, and the deductible contribution generally follows the same deadline.

The costs and obligations are what separate a cash balance plan from a 401(k). Expect roughly $2,000 to $5,000 a year for plan documents, actuarial valuations, and filings, more for plans with employees. The contribution is a funding obligation, not a preference: the actuary certifies a minimum required contribution each year, and while designs usually include a range, you cannot skip a year because business was slow. Solo 401(k) plans must file Form 5500-EZ once plan assets (across all one-participant plans) exceed $250,000 at year end, and a cash balance plan for an owner files the same form. Late filing penalties are steep, though the IRS offers a penalty relief program for late 5500-EZ filers.

Employees change everything. A solo 401(k) exists only while the business has no employees other than the owner and spouse. Once you have an eligible employee (generally age 21 with 1,000 hours, and long-term part-time rules now bring in some part-timers), coverage and nondiscrimination rules require that plans benefit a fair cross-section of staff. A SEP must contribute the same percentage of pay for every eligible employee. A cash balance plan can sometimes be designed with modest employee credits paired with a safe harbor 401(k), but the cost of covering staff has to be modeled before adopting the plan, not after the first hire.

If you are an owner-only business in your 50s or 60s and already maxing out a solo 401(k), the cash balance plan is the next question worth asking. It fits best when profits are steady, you expect to keep the business running at least three to five more years, and the deduction lands in the 24%, 32%, or 35% federal bracket plus Arizona's 2.5% flat tax. At those rates, a $175,000 deductible contribution can plausibly save $50,000 to $65,000 of combined income tax in the year it is made, though that tax comes back when the money is withdrawn in retirement.

The value of the deduction depends on the gap between today's marginal rate and your expected rate in retirement. A business owner who will sell the company, retire at 63, and live on a moderate draw with low taxable income in the years before Social Security and RMDs is the ideal candidate. An owner who will keep drawing large K-1 income into their 70s, or who already faces a large pre-tax balance and future RMD problem, may be better served by a smaller cash balance design and more Roth dollars.

Coordinate the design with your S corp salary and the QBI deduction. Higher W-2 pay raises the 25% profit sharing base and the cash balance formula, but it also raises payroll tax and shifts income out of QBI-eligible K-1 profit. Retirement contributions themselves reduce QBI. These three moving parts should be modeled together once a year, ideally in the fall, before the last payroll is run.

Plan the exit on day one. When you retire or sell the business, the cash balance plan is terminated, the actuary calculates final benefits, and the balance is typically rolled to an IRA or into the solo 401(k). Terminating after only a year or two can raise questions about whether the plan was intended to be permanent, and overfunding at termination can create excise tax problems. A clean exit is usually a decision made several years in advance, with contributions tapered as the plan approaches its funding target.

If you are about to hire, revisit the whole structure before the first employee becomes eligible. The cheapest time to redesign a plan is before someone else has a legal right to benefit from it.

• Opening a SEP IRA alongside a solo 401(k) for the same business, expecting two $72,000 limits, when both share one Section 415(c) limit and the model SEP form cannot be used with another plan.
• Keeping profit sharing at 25% of pay after adding a cash balance plan and running into the combined plan deduction limit, instead of capping profit sharing at 6% of pay.
• Missing the year-end payroll deadline for S corp solo 401(k) deferrals, assuming the retroactive first-year deferral rule for sole proprietors also applies to W-2 wages.
• Adopting a cash balance plan in a peak income year without the cash flow to fund the minimum required contribution in the following slower year.
• Forgetting that the 2026 catch-up must be Roth for owners whose prior-year FICA wages exceeded about $150,000, and overstating the deduction as a result.
• Failing to file Form 5500-EZ once one-participant plan assets pass $250,000, or forgetting the final filing in the year the plan is terminated.
• Hiring a first employee and continuing to contribute as if the solo 401(k) were still a one-participant plan.