Solo 401(k) Contribution Deadlines by Business Type

The solo 401(k) contribution deadline is your business’s federal income tax return due date for the year you’re funding, including any extension you file. For a sole proprietor or single-member LLC, that means April 15, or October 15 with an extension. An S-corporation owner works from a March 15 baseline (September 15 extended), and a C-corporation owner from April 15 (October 15 extended). A contribution deposited by that deadline counts as if it were made on the last day of the prior tax year, so you can still deduct it on that year’s return.¹

One catch changes the picture for anyone with an existing plan: the employee deferral election is due by December 31, even though the money can arrive later. More on that below.

Deadlines by Business Type

Sole Proprietors and Single-Member LLCs

If you file Schedule C with your Form 1040, both halves of your solo 401(k) contribution ride your personal tax return. The employee elective deferral and the employer profit-sharing contribution are both due by April 15 of the following year. File a Form 4868 extension and both deadlines move to October 15.²

S-Corporations

S-corps file Form 1120-S by March 15, and that is the baseline deposit deadline for both the withheld employee deferral and the employer profit-sharing contribution. A Form 7004 extension pushes the return, and the contribution deadline, to September 15.³

There is a separate rule for the deferral itself. Because the employee portion of a corporate solo 401(k) comes out of W-2 wages, the amount has to be withheld from pay issued by December 31. The withholding is a payroll event; the transfer of that withheld money into the plan trust can happen later, up to the filing deadline. The employer profit-sharing side is capped at 25% of the W-2 compensation the corporation paid you during the year.⁴

C-Corporations

C-corps file Form 1120 by April 15, giving those owners a month more than S-corp owners at the baseline. A Form 7004 extension moves the return, and the contribution deadline, to October 15. The same year-end W-2 withholding rule applies to the employee deferral portion.

If total employer contributions exceed what is deductible for the year, the excess triggers a 10% excise tax under Section 4972 of the Internal Revenue Code, paid by the employer and applied each year the excess sits in the plan.⁵

The December 31 Election on Existing Plans

If your solo 401(k) already exists, you have to make a written election by December 31 stating your intent to defer part of that year’s earnings. The election is separate from the deposit. Missing it means you cannot make employee deferrals for that year at all, no matter how much time is left on your filing calendar. You can still make employer profit-sharing contributions by the filing deadline, but the deferral bucket is closed.

For an S-corp or C-corp owner, the parallel act is running the deferral through payroll by year-end so it shows on the W-2. For a sole proprietor with an existing plan, it is a signed election in your records.

How Filing Extensions Push Back the Deposit

Filing an extension moves your return, and with it your contribution deposit deadline. Individuals use Form 4868 for an automatic extension to October 15. Corporations use Form 7004 for six additional months, so the S-corp deadline moves from March 15 to September 15 and the C-corp deadline from April 15 to October 15.²

Two limits on that. The extension has to be filed by the original tax deadline. And an extension of time to file is not an extension of time to pay: estimated taxes owed are still due by the original date.²

Setting Up a New Plan After Year-End

You do not have to open a solo 401(k) before December 31 to fund it for that year. Under changes from the SECURE Act and SECURE 2.0, an employer can adopt a 401(k) after the tax year ends and treat the plan as if it existed on the last day of the prior year, provided the plan is established by the tax return due date including extensions.⁶

The important limit for solo filers: a sole proprietor setting up a brand-new plan can make a retroactive first-year deferral election, but both the election and the deposit of those deferrals have to be completed by the original tax return due date, without regard to extensions.⁶ For a calendar-year sole proprietor, that means April 15. Filing a 4868 extension does not buy more time for the first-year employee deferral. Employer profit-sharing contributions to the new plan can still be made by the extended deadline.

Depositing the Money and Proving the Date

You can fund a solo 401(k) by ACH, wire, or check into the plan’s trust account. Label the transaction for the tax year you intend, for example “2025 Prior Year Contribution,” so the custodian codes it correctly. A wrong year designation can produce errors on Form 5498, the custodian’s annual contribution report to the IRS.⁷

Keep the confirmation from the custodian and the bank record showing the date funds left your business account. If the IRS ever questions whether you funded on time, those records are your evidence. Hold them with your signed plan document, deferral elections, and tax returns for as long as the plan exists.

If You Miss the Deadline

A missed deposit is not automatically fatal to the plan. Late or missed deposits of employee elective deferrals can create two separate problems: an operational failure, because the plan did not run according to its terms, and a prohibited transaction, because plan assets were not properly segregated.⁸

The operational failure is corrected through the IRS Employee Plans Compliance Resolution System (EPCRS), which offers three routes:

Self-Correction Program, available without an IRS filing if you meet the eligibility conditions and have compliance procedures in place. No user fee.
Voluntary Correction Program, available if the plan is not under audit. You submit through Pay.gov and pay a user fee scaled to plan assets.
Audit Closing Agreement Program, used when the IRS finds the issue on audit. Correction and a financial sanction are negotiated with the agency.

Whichever route you use, the fix is to deposit the missed deferrals plus lost earnings. The prohibited-transaction side is handled separately through the Department of Labor’s Voluntary Fiduciary Correction Program, because EPCRS does not resolve prohibited transactions on its own.⁸

1
Office of the Law Revision Counsel. 26 U.S. Code 404 – Deduction for Contributions of an Employer to an Employees Trust or Annuity Plan and Compensation Under a Deferred-Payment Plan
2
Internal Revenue Service. Get an Extension to File Your Tax Return
3
Internal Revenue Service. Publication 509 (2026), Tax Calendars
4
Internal Revenue Service. Retirement Plan FAQs Regarding Contributions – S Corporation
5
Office of the Law Revision Counsel. 26 USC 4972 – Tax on Nondeductible Contributions to Qualified Employer Plans
6
Internal Revenue Service. Issue Snapshot – Deductibility of Employer Contributions to a 401(k) Plan Made After the End of the Tax Year
7
Internal Revenue Service. Form 5498 – Asset Information Reporting Codes and Common Errors
8
Internal Revenue Service. 401(k) Plan Fix-It Guide – You Haven’t Timely Deposited Employee Elective Deferrals