Buford was a factory town before it was anything else. The Bona Allen tannery ran here for more than a century, and at its peak it turned out saddles and harness good enough that Hollywood sent horses to Buford to be fitted.

When the leather operation finally closed in the eighties, what was left was a town full of people who knew how to make things and work for themselves. That is still the character of the place. Buford straddles the Gwinnett and Hall county line, sits on Lake Lanier, and runs on owner-operators: contractors, marine and dock services, landscaping outfits, specialty trades, small distribution companies, and a long list of one-truck-and-a-crew operations that grew into real businesses without ever adding an HR department.

For those owners, the retirement plan question almost always arrives in the same form. The CPA says it is time to put money away. Somebody mentions a SEP-IRA. Somebody else mentions a solo 401(k). The two get compared on a single number, whichever one lets the owner put more in, and a plan gets opened before anyone asks what else it touches.

The Question That Actually Decides It

Before income enters the conversation, one fact narrows the field: does the business have employees other than the owner and the owner’s spouse?

A solo 401(k) is built for an owner-only business. The owner and a spouse working in the business can both participate. Once the company has an eligible employee who is not the owner or spouse, the plan no longer fits its intended design, and it either has to become a conventional 401(k) with testing and administration, or come to an end.

A SEP-IRA works with or without employees. But it comes with a rule that changes the arithmetic completely: the employer generally has to contribute the same percentage of compensation for every eligible employee that the owner takes for themselves. An owner funding a large percentage for their own account is committing to that same percentage across the eligible payroll.

• Owner-only, no plans to hire. Both plans are available. This is where the real comparison happens.
• Owner-only, hiring within a year or two. The decision should be made with the future payroll in view, not the current one.
• Already has eligible employees. The solo 401(k) is off the table, and a SEP-IRA needs to be priced across the whole census before anyone commits to a percentage.

How the Two Plans Actually Fund

This is where most side-by-side comparisons go wrong, because the two plans do not fill up the same way.

A SEP-IRA has one funding source. The employer makes a contribution calculated as a percentage of compensation. There is no employee deferral. If the business has a modest year, the percentage applies to a modest number, and the contribution is modest with it.

A solo 401(k) has two. The owner can make an employee deferral, which is a dollar amount rather than a percentage, and the business can make an employer profit-sharing contribution calculated as a percentage of compensation on top of it.

The practical consequence shows up at moderate profit. Because the deferral bucket is a dollar figure rather than a slice of the year’s earnings, a solo 401(k) can often accept meaningfully more than a SEP-IRA at the same income. As profit climbs, the two structures converge toward the same overall annual ceiling, and the funding advantage narrows.

Which means the owner earning a very high profit may see little difference in total contribution, while the owner in the middle sees a large one. Current-year limits are indexed and change, so the actual figures should come from the CPA for the year in question rather than from an article.

Where the SEP-IRA Still Wins

The solo 401(k) gets most of the attention, and the SEP-IRA gets treated as the beginner option. That is not quite fair.

It is genuinely simple

A SEP-IRA can be established with a short form. There is no ongoing plan document to amend, no annual return in most cases, and no deferral elections to track. For an owner who wants to make one decision a year and move on, that has real value.

The deadline is forgiving

A SEP-IRA can generally be established and funded up to the business tax filing deadline, including extensions. That matters more than it sounds. Plenty of owners do not know what the year actually produced until the CPA finishes the return in March or September. A SEP-IRA lets the decision be made with the final number in hand.

It rewards uneven income

Contributions are discretionary. A contractor who has a strong year followed by a slow one can fund heavily and then fund nothing, with no ongoing commitment and no plan to maintain in the meantime.

Where the Solo 401(k) Pulls Ahead

The solo 401(k) is a real retirement plan with a plan document, and the extra structure buys options the SEP-IRA does not have.

• More funding at moderate income. The deferral bucket does not depend on a percentage of the year’s profit, so a middling year can still be funded well.
• A Roth option. If the plan document permits it, deferrals can be made on a Roth basis. For an owner who expects higher rates later, or who is building a tax-diversified retirement, that is a lever the SEP-IRA does not offer.
• Catch-up contributions at 50 and over. Relevant for owners funding late, which describes a great many business owners.
• A loan provision. Available if the plan document allows it. Rarely the reason to choose the plan, occasionally the reason an owner is glad they did.
• It does not interfere with a backdoor Roth. The reason for this is worth its own section.

The Pro-Rata Problem

This is the piece owners most often find out about after the fact.

An owner whose income is too high to contribute to a Roth IRA directly may use a backdoor Roth contribution, funding a non-deductible traditional IRA and converting it. Whether that conversion is mostly tax-free depends on the pro-rata rule, which looks at the owner’s total pre-tax IRA balances at year end.

Owners who chose a SEP-IRA for its simplicity and later decided to pursue a backdoor Roth often end up rolling the SEP balance into a 401(k) to clear the way, which is a solvable problem but an avoidable one. If a Roth strategy is anywhere in the picture, the plan choice should be made with that in view rather than corrected around it later.

Deadlines, Filings, and the Paperwork Nobody Mentions

The two plans do not run on the same calendar, and the difference has cost owners real money.

A SEP-IRA can generally be established and funded up to the tax filing deadline including extensions, which means a decision made in the spring can still apply to the prior year.

A solo 401(k) is more nuanced. Rule changes in recent years made it possible for a sole proprietor to establish a plan after year end and still make an employer profit-sharing contribution for that prior year. The employee deferral is the part that trips people up, because a deferral generally depends on an election made during the year it applies to.

On filings, a solo 401(k) generally requires an annual Form 5500-EZ once plan assets pass the IRS filing threshold, currently $250,000. It is not a difficult return, but it is a return, and it is the kind of obligation that gets missed by owners who assumed a solo 401(k) was as hands-off as a SEP. A SEP-IRA generally has no comparable annual filing.

What Happens When You Hire

Around Buford this is not a hypothetical. An owner-operator adds a crew, then a second truck, then somebody in the office, and the retirement plan chosen three years ago is quietly no longer the right one.

If the plan is a solo 401(k), an employee who meets the plan’s eligibility conditions means the plan can no longer operate as an owner-only arrangement. It has to be converted into a conventional 401(k), with the testing, notices, and administration that come with it, or terminated.

If the plan is a SEP-IRA, it continues, but the uniform-percentage rule takes hold. An owner contributing a high percentage for themselves is committing to that same percentage across every eligible employee, which can turn a comfortable owner contribution into a payroll expense the business did not plan for.

Neither outcome is a disaster. Both are much easier to handle when the plan was selected with the hiring plan on the table.

Why Your Salary Sets the Ceiling

Both plans calculate the employer contribution off compensation, and for an S corporation owner, compensation means W-2 wages, not distributions.

That creates a collision most owners never see coming. A salary set as low as defensible to reduce payroll taxes also lowers the compensation figure the retirement plan runs on. The owner saves on payroll tax and loses funding capacity in the same move, and because the two decisions are usually made by two different people at two different times of year, nobody is holding both.

This is not an argument for a higher salary. It is an argument for making the compensation decision and the retirement plan decision in the same conversation, with the CPA and the advisor both looking at the same numbers.

How to Actually Decide

When Both Plans Stop Being Enough

There is a point where an owner is funding a solo 401(k) to the limit every year, the business is still consistently profitable well beyond what the household needs, and a large amount of income continues to land on the tax return.

That is usually where the conversation moves past defined contribution plans entirely. A cash balance plan can allow substantially more tax-deferred funding, but it is a defined benefit pension plan with actuarial funding requirements, employee considerations, annual administration, and investment risk that stays with the employer. It is a different kind of commitment, and it should not be evaluated until the simpler options have genuinely been used.

---

Frequently Asked Questions