It's early November, and three business owners have each decided this is the year to open a Solo 401(k). They'll sign up for the same plan, with the same $24,500 deferral limit for 2026. Then each will learn that the date their own money has to be deposited depends on details they hadn't thought to ask about. One is working against the last paycheck of December. One has until next October. One won't get a plan set up until January, and will only have until April. The plan is the same for all three, yet the deadlines are months apart. What separates them is how each owner gets paid, and whether the plan is in place by December 31.
A Solo 401(k) takes two kinds of money. The first is employer profit-sharing, contributed by the business and due by the business's tax filing deadline, including extensions. That one is simple and is the same for all three. The second is the owner's own pay set aside, called an employee deferral, and its deadline is the one that changes from owner to owner.
Layla pays herself a salary
Layla owns an S corporation and pays herself $5,000 a month through payroll, with taxes withheld like any employee. A deferral works the way it does at a large employer: it comes out of each paycheck. A paycheck can only be reduced before it is paid, so Layla's last payroll of the year is her last chance. Her plan has to exist by December 31 for the deferral to count toward this year.
Suppose she opens the plan on December 20 and wants to defer the full $24,500. Her election applies only to pay she has not yet received, so the earlier months are out of reach. Her December paycheck is $5,000, and a deferral can't be larger than the pay it comes from. She can't take her usual $5,000 at the end of the month and defer $24,500 out of it. What she can do is run a larger final payroll, such as a year-end bonus, and defer from that. A bonus is wages, so the cash has to leave the business as pay, and payroll taxes apply to it. Whether that makes business sense is a question for her CPA. The deferral limit tells Layla the most she can set aside. The amount she pays herself after her plan exists decides how close she can get to that limit.
Arjun takes owner draws
Arjun is a freelance designer. He reports his business income on a Schedule C, the form sole proprietors and single-member LLC owners file with their personal tax return. He draws money from the business account to himself when cash allows. There is no payroll, so there is no paycheck to reduce. What counts as his pay for deferral purposes is the business's net profit, which is figured at tax time, not the draws he took.
If Arjun's plan is in place by December 31, he can fund his 2026 deferral any time up to his tax filing deadline, including extensions, which is generally October 15 of next year. The extra months give him time to save for a bigger deferral. The cost is that money added the following October spends most of a year outside the plan and not invested. Which matters more depends on his cash flow.
Danielle waits for January
Danielle files a Schedule C too. She decides this month to open a plan, then gets pulled into year-end work and doesn't finish until January. For a long time, that would have meant giving up the 2026 deferral. A federal law passed in 2022, known as SECURE 2.0, changed that for sole proprietors and single-member LLC owners. A plan adopted after year end can still take deferrals for the year that just closed.
The window is shorter, though. It closes at the tax filing deadline without extensions, generally April 15. Compared with Arjun's October 15, Danielle gets about six months less. If she adopts the plan after April 15 but within her extended filing period, it can accept only employer profit-sharing contributions for 2026.
The deadlines side by side
| How the owner is paid | Plan must exist by | Deferral must be funded by |
|---|---|---|
| Salary through W-2 payroll | December 31 | December 31, through the last payroll of the year |
| Schedule C draws, plan in place by year end | December 31 | Tax filing deadline, including extensions |
| Schedule C draws, plan adopted after year end | Tax filing deadline, without extensions | Same date |
| Employer profit-sharing, any structure | Tax filing deadline, including extensions | Same date |
December 31 is the date that keeps the most options open. For Layla, it's the only date that works. For Arjun, it unlocks the longer window. For Danielle, missing it means a shorter clock. Year end is less than two months away, and it's worth leaving at least two weeks to get the plan in place and open a bank account that can accept contributions.
Three questions are worth answering before the year ends, ideally with a CPA who knows the business's numbers. How will the business pay me in December? Could it support a final payroll large enough to carry the deferral I have in mind? And if I take draws, would I rather fund early or use the full window? The answers show which of these three owners you most resemble, and therefore which deadline is yours.