Solo 401(k) vs SEP-IRA for Your Overemployed Side Income: Why Your Maxed Work Plan Picks the Winner.

An Overemployed engineer has already maxed the 401(k) at both of his W-2 jobs, and on the side he nets $100,000 from a corp-to-corp consulting contract. He wants to shelter some of that profit, and the internet hands him the usual answer: open a Solo 401(k), because it lets you contribute far more than a SEP-IRA. That advice is correct for a typical freelancer and quietly wrong for him, because the very feature that makes a Solo 401(k) bigger is the one feature he has already used up. For his exact situation, the two plans hold the same amount, and the choice turns on something else entirely.

A Solo 401(k) has two contribution lanes. As the employee, you can make an elective deferral, up to $24,500 for 2026, and as the employer you can make a profit-sharing contribution of roughly 20% of your net self-employment earnings, with the combined total capped at the $72,000 annual additions limit. A SEP-IRA has only the second lane: an employer contribution of about 20% of net earnings, with no deferral piece at all. For a freelancer earning a moderate amount, that deferral lane is decisive. On $60,000 of profit, a Solo 401(k) can absorb the full $24,500 deferral plus a profit-sharing piece, while a SEP is limited to roughly $12,000. The Solo 401(k) shelters two to three times as much. That is the comparison the standard advice is built on.

Here is the catch the standard advice never accounts for. The $24,500 elective deferral is a per-person limit under IRC §402(g), shared across every plan you touch in the year, and your two W-2 jobs have already consumed it. You cannot make any elective deferral into the Solo 401(k) without creating an excess deferral, so that entire advantage evaporates. What is left in the Solo 401(k) is only the employer profit-sharing lane, about 20% of your net self-employment earnings. And that is computed exactly the same way as the SEP-IRA's employer contribution. Same percentage, same earnings base, same $72,000 ceiling. The two plans now hold an identical amount, which is precisely the intuition a lot of Overemployed savers arrive at on their own, and it is correct. If that is your situation, book a free intro call and we will confirm it in a few minutes.

When the contribution room is identical, the tiebreaker should be simplicity, and on that score the SEP-IRA wins outright. A SEP never files a Form 5500, can be opened and funded as late as the extended due date of your return, and needs no plan document to maintain. A Solo 401(k) has to file a Form 5500-EZ once its assets pass $250,000 and carries a bit more administrative weight. So the default, for an Overemployed worker who has maxed the deferral at work, would be the SEP, unless one specific thing is true. For a lot of high earners, that one thing is true.

An Overemployed high earner is almost certainly over the Roth IRA income limit. For 2026 direct Roth contributions phase out between $153,000 and $168,000 of modified AGI for single filers and between $242,000 and $252,000 for joint filers, and two W-2 jobs blow past those figures easily. So the way these savers get money into a Roth is the backdoor: a nondeductible traditional IRA contribution converted to Roth. The problem is the pro-rata rule. A SEP-IRA is an IRA, and its entire balance is swept into the pro-rata calculation on Form 8606 that decides how much of any conversion is taxable. Park $19,700 a year in a SEP and your backdoor Roth conversions become mostly taxable, which defeats the whole point.

A Solo 401(k) is not an IRA, so it is invisible to the pro-rata rule. Your backdoor Roth conversions stay clean no matter how large the Solo 401(k) grows. That single difference is the reason a Solo 401(k) is the right answer for most Overemployed high earners despite the identical contribution and the extra paperwork: it protects the backdoor Roth that a SEP would poison. The Solo 401(k) also offers a Roth sub-account, a possible mega-backdoor-Roth lane if the plan document allows after-tax contributions, and the option of a participant loan, none of which a SEP provides. The rule of thumb is simple. If you do not and will not do a backdoor Roth, take the SEP for its simplicity. If you do, take the Solo 401(k) and keep the conversion clean.

This decision usually sits one step downstream of a bigger one. Once you have run the S-corp viability math and concluded that staying a sole proprietor is the right call, as it often is for an Overemployed worker whose wages already maxed Social Security, the consulting profit still needs somewhere to go, and this is that somewhere. It pairs with the same reasonable-compensation and payroll-tax questions I walk through in the S corp Medicare tax loophole, and with the deferral limits in the Overemployed 401(k) trap.