Solo 401(k), SEP, or Cash Balance: The Owner's Largest Tax Lever

By MercConsulting · Published 2026-08-26 · Updated 2026-09-02

A qualified retirement plan is usually a profitable owner's largest deduction. The plan ladder by profit level, how cash balance plans stack on a 401(k), TPA and actuary roles, and timing.

For a profitable owner, a qualified retirement plan is usually the largest deduction available, because every dollar contributed is deducted from taxable income while remaining your money. The right plan depends on profit level, age, and whether you have employees: a SEP or Solo 401(k) at modest, variable profit; a Solo 401(k) with employer contributions as profit grows; and a defined-benefit or cash balance plan stacked on top of a 401(k) for owners past their mid-forties with consistent profit who want to set aside several times what a 401(k) alone allows. Each step up adds administrative cost and, if you have staff, a required contribution for them.

This guide lays out the plan ladder, how a cash balance plan sits on top of a 401(k), what a third-party administrator and actuary do, the timing rules that trip owners up, and what changes once employees are in the picture. All contribution limits change annually; verify current-year figures before deciding anything.

"I'd been putting a little into a SEP for years and calling it a plan. When someone finally ran the numbers on a cash balance plan on top of a 401(k), the deduction was in a different league — and so was the paperwork. Both were worth it."

Why Retirement Plans Are the Owner's Largest Lever

Most deductions require spending money on something the business needs. A retirement contribution is different: the deduction is real, the money stays invested in an account you control, growth inside the plan is not taxed until withdrawal, and qualified plan assets carry strong creditor protection under federal law. For an owner whose profit has outgrown the business's reinvestment needs — the situation described in our guide to what to do with excess business profits — the plan is often the first place to look.

The trade-offs are liquidity and administration. Money in a plan is committed until retirement age, with penalties for early withdrawal outside narrow exceptions, and the larger plans require annual filings and professional administration.

The Plan Ladder by Profit Level

Rung one: SEP IRA

A SEP allows employer-only contributions calculated as a percentage of compensation, up to the current annual limit. It is simple to open, has no annual filing, and can be established and funded as late as the return due date including extensions. Its weakness is that it has no employee-deferral component, so at lower profit levels it allows less than a Solo 401(k), and any eligible employee must receive the same contribution percentage the owner receives.

Rung two: Solo 401(k)

Available to an owner with no full-time employees other than a spouse, a Solo 401(k) combines an employee deferral, which is a fixed amount regardless of profit, with an employer contribution calculated as a percentage of compensation. At modest profit the deferral alone beats a SEP; at higher profit the two pieces together reach the same overall limit. It can also offer Roth deferrals, participant loans, and, if the plan document allows, after-tax contributions.

Rung three: SIMPLE IRA

A SIMPLE fits a small team that wants low administration. The employer must either match employee deferrals or make a flat contribution for everyone, the limits are lower than a 401(k), and a business generally cannot maintain another plan in the same year.

Rung four: 401(k) with profit sharing

Once there are employees, a full 401(k) with a profit-sharing feature becomes the workhorse. A safe harbor design avoids most annual testing in exchange for a required employer contribution, and a cross-tested profit-sharing formula can direct larger allocations to owners while giving employees a minimum contribution the rules require.

Rung five: defined-benefit or cash balance plan

Covered below: this is where the contribution moves to a multiple of the 401(k) limit, and where the administrative commitment becomes real.

How a Cash Balance Plan Stacks on a 401(k)

A defined-benefit plan promises a specific benefit at retirement rather than a contribution today, and an actuary calculates each year what must be contributed to fund that promise. Because an older owner has fewer years until retirement, the required contribution to fund a given benefit is larger, which is why the deduction grows with age. A cash balance plan is a defined-benefit plan that expresses the benefit as a hypothetical account balance, which is easier to understand.

The stacking works like this: the owner keeps the 401(k), including the employee deferral and a reduced employer profit-sharing contribution — the profit-sharing piece is limited when a defined-benefit plan is also maintained — and adds the cash balance contribution on top. For an owner in their fifties with strong profit, the combined deduction can be several times what the 401(k) allows alone. The candidates who fit:

• Age roughly forty-five and up. Younger owners can use the plan, but the actuarial math favors those closer to retirement.
• Consistent profit for at least three to five years ahead. The contribution is not optional once the plan exists; there is a required minimum each year, with a range above it.
• Cash flow to fund it without straining operations. The deduction is only useful if the money is genuinely available.
• Few employees, or employees you are willing to fund. Staff must receive meaningful contributions, and the cost is part of the design.

What the Third-Party Administrator and Actuary Do

A 401(k) with employees and every defined-benefit plan require a third-party administrator, and a defined-benefit plan requires an enrolled actuary. The TPA drafts the plan document, runs the non-discrimination tests, prepares the annual Form 5500, tracks eligibility and vesting, and keeps the document updated as the law changes. The actuary certifies the funding each year, sets the required contribution range, and signs the schedule that accompanies the return.

The plan's investment policy also matters: a defined-benefit plan that earns far more or far less than the actuarial assumption creates over- or under-funding that the actuary must correct through future contributions, so these plans are usually invested more conservatively than a 401(k).

When Employees Are in the Picture: Non-Discrimination

Qualified plans earn their tax treatment by covering rank-and-file employees on terms that do not unduly favor owners. Several tests enforce that: coverage rules that require a sufficient share of eligible employees to participate, tests comparing what highly compensated and other employees defer and receive, and top-heavy rules that impose a minimum contribution for employees when owners hold most of the plan's assets.

Design handles most of this. A safe harbor contribution removes the deferral tests. A cross-tested profit-sharing formula lets owners receive a larger share while employees receive a required gateway contribution. In a cash balance plan, employees typically receive a contribution expressed as a percentage of pay. Model the employee cost honestly: the owner's deduction net of employee contributions and administration is the real number.

Timing: The Deadlines That Cost Owners a Year

• Establishing the plan. A 401(k) must generally exist before the end of the year for employee deferrals to be made for that year; recent law changes have loosened the rules for employer contributions and for first-year sole proprietors, so confirm the current deadline with your provider.
• Electing deferrals. For an owner on payroll, deferrals come out of pay during the year and must be elected in advance; a year-end lump sum does not work.
• Funding employer contributions. Employer and profit-sharing contributions can generally be made up to the return due date including extensions.
• Cash balance contributions. Due by the return due date, but the plan must be adopted, and the actuarial work done, in time to know the required amount.

The practical rule: decide on the plan in the third quarter, when profit for the year can be forecast, so the document is in place and payroll is set up before December. Beneficiary designations, plan documents, and the way plan assets interact with the rest of the estate belong in the review described in our estate planning basics for business owners.

Roth Versus Pre-Tax Inside the Plan

Pre-tax contributions reduce this year's taxable income; qualified Roth withdrawals are not taxed later. The choice turns on your bracket now versus your expected bracket in retirement, the value of current cash flow, and whether you want a pool of retirement money that will not add to taxable income when withdrawn.

A retirement plan is one element of a broader approach to minimizing taxation that should be coordinated with entity choice, salary design, and cash flow rather than chosen in isolation. MercConsulting is a business consulting firm, not a law firm or CPA firm; strategies are general information, depend on your facts and current law, and are implemented with licensed tax and legal professionals — no tax outcome is guaranteed.

Frequently Asked Questions

Which is better for a business owner, a SEP IRA or a Solo 401(k)?

For most owners without employees, a Solo 401(k) allows larger contributions at lower profit levels because it adds an employee deferral to the employer contribution, and it can offer Roth deferrals and loans. A SEP is simpler and can be opened after year end, which makes it useful when a strong year is discovered late.

What is a cash balance plan and who should consider one?

A cash balance plan is a defined-benefit plan that promises a stated retirement benefit expressed as an account balance, with an actuary setting the required annual contribution. It suits owners roughly forty-five and older with consistent profit who want to contribute well beyond 401(k) limits, and it is typically stacked on top of an existing 401(k).

Can I have a 401(k) and a defined-benefit plan at the same time?

Yes. Combining a 401(k) with profit sharing and a cash balance plan is a standard design. The profit-sharing portion is reduced when a defined-benefit plan is also maintained, but the employee deferral and the cash balance contribution stack on top of each other.

Do I have to contribute for my employees?

In most qualified plans, yes. SEP contributions must be made at the same percentage for eligible employees, SIMPLE plans require a match or flat contribution, and 401(k) and cash balance plans must pass non-discrimination tests that generally require meaningful employee contributions.

When do I need to set up a retirement plan to deduct contributions this year?

It depends on the plan type and the contribution. Employee deferrals generally require the plan to exist and the election to be made before year end, while employer contributions can often be funded up to the return due date including extensions. Recent law changes have adjusted some of these deadlines, so confirm the current rules with your plan provider.

This article is for educational purposes only and is not legal, tax, or investment advice. Consult qualified professionals about your specific situation.