Retirement rarely draws a clean line under earned income. A part-time consulting arrangement, a small crafts shop, a stream of freelance writing, or a few hours a week behind the wheel all generate self-employment earnings, and that income quietly reopens a tax door most older workers assume slammed shut at 65. A person already collecting Social Security can still open a Simplified Employee Pension or a one-participant 401(k), route a share of that side income into it, and deduct the contribution against the same year’s tax bill. The reward goes to whoever learns the rules before the filing deadline rather than after it has passed.
Why self-employment income is the trigger
The whole strategy hinges on one fact: the account holder must have net earnings from self-employment. Wages from a former employer do not count, and neither does a pension or an investment dividend, but profit from a business actively run in retirement does. The IRS lays out the menu of retirement plans for self-employed people, and neither a SEP nor a solo 401(k) carries an upper age limit. A 72-year-old with a landscaping side business and a 68-year-old bookkeeper working from home are both eligible, provided the venture actually turns a profit rather than merely running as a hobby.
Collecting Social Security does not disqualify the contribution either. Benefits and self-employment earnings coexist without conflict, and the retirement-plan deduction is calculated off the business profit, not off the benefit check. For someone past full retirement age, there is no earnings test to worry about at all, so the side income neither shrinks the monthly benefit nor blocks the deduction. That combination makes the late-career gig unusually tax-friendly compared with the same dollars earned as ordinary wages.
The deduction matters most for a retiree whose required minimum distributions or investment income have pushed taxable income higher than expected. Sheltering several thousand dollars of consulting profit can trim the marginal rate, soften the tax on a Roth conversion done the same year, or keep income just under a threshold that governs Medicare premium surcharges. The move converts otherwise fully taxed side income into tax-deferred savings, and it does so in a single filing without requiring the retiree to change how the business itself operates.
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SEP-IRA versus the one-participant 401(k)
A SEP-IRA is the simpler of the two structures. It has no annual filing requirement and can be funded with as much as 25 percent of net self-employment earnings, computed under the plan’s own formula. Setup takes minutes at most custodians and demands no ongoing administration, which makes it the natural default for a retiree with modest, irregular gig income who wants the paperwork kept to an absolute minimum and dislikes the idea of a separate plan tax return.
The one-participant 401(k), often marketed as a solo or individual 401(k), can shelter more at lower income levels because it layers an employee salary deferral on top of the employer contribution. A worker 50 or older can add a catch-up deferral on top of that, and the combined employee-and-employer total for a single plan reached 70,000 dollars for 2025, according to the IRS. The trade-off is more administration, including a Form 5500-EZ once plan assets cross a reporting threshold, so the larger deduction comes with modestly heavier upkeep.
The choice usually comes down to how much income the retiree wants to shelter and how much bookkeeping feels tolerable. A person netting a few thousand dollars a year leans toward the SEP, where the simplicity outweighs any lost capacity. Someone running a sustained five-figure consulting practice often captures a larger deduction through the solo 401(k), because the salary-deferral piece does not depend on a percentage of profit and can absorb nearly all of a small operation’s earnings in a lean revenue year.
Where the deduction actually lands
The deductible amount is not simply a flat quarter of gross receipts. A self-employed contributor must run a special computation that starts from net profit, subtracts the deductible half of self-employment tax, and then backs the contribution out of its own base. The result is an effective ceiling closer to 20 percent of net earnings for a sole proprietor, a distinction that routinely trips up first-time filers who assume the headline 25 percent applies to raw revenue and end up over-contributing.
Timing decides whether the deduction lands at all. A SEP can generally be established and funded up to the tax-return due date, including extensions, which lets a retiree open one after year-end once the year’s profit is actually known. A solo 401(k) has traditionally required the plan to exist by the last day of the business year, though the funding of employer contributions can follow later under more recent law. The mechanics, deadlines, and contribution formulas are all spelled out in Publication 560, the small-business retirement guide.
What remains unresolved for many older savers is whether the deferral is worth it when the money will be taxed again at withdrawal, possibly alongside future required distributions. For a retiree whose current bracket sits above the rate expected in later years, or who simply wants to shift a spike of taxable income out of one crowded year, the answer often tilts toward yes. The calculation turns less on age than on the gap between this year’s rate and the rate the eventual withdrawals will face.
This article was researched and drafted with the assistance of artificial intelligence.
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