Retirement Plan Contributions Extension Filers Can Still Fund by October 15
One of the more useful, and least discussed, features of filing on extension is the extra runway it gives certain retirement plan contributions. For a business owner who filed Form 4868 or Form 7004, October 15 isn't just a filing deadline — it's also, for some plans, a funding deadline that wouldn't otherwise exist.
SEP-IRA Contributions Follow the Extended Due Date
A SEP-IRA can generally be established and funded for a prior tax year up until the due date of the employer's tax return, including extensions. For a sole proprietor or single-member LLC on a valid extension, that means SEP-IRA contributions for the prior year can still be made through October 15, 2026 — six months later than a filer who didn't extend.
Solo 401(k) Employer Contributions Follow the Same Rule
The employer (profit-sharing) contribution to a one-participant 401(k) generally follows the same extended-due-date rule as a SEP-IRA. The employee elective deferral portion is more time-sensitive: for a sole proprietor with no common-law employees, the deferral election for a given year generally needs to have been made before the end of that calendar year, since it's treated as an election over compensation for that year, even though the actual deposit can occur later. This is a detail worth confirming against the specific plan document rather than assuming the full extended deadline applies to every component of a solo 401(k).
This Doesn't Extend Traditional or Roth IRA Contributions
It's a common point of confusion: the extended due date that helps a SEP-IRA or solo 401(k) employer contribution does not apply to a traditional or Roth IRA contribution. Those follow the unextended filing deadline — April 15 — regardless of whether a return is later extended. A taxpayer who files an extension in April does not get extra time to fund a traditional or Roth IRA for the prior year.
Why This Matters for Business Owners Specifically
For a profitable year where the final numbers weren't clear until closer to the extended deadline, this gap gives real flexibility: the SEP-IRA or solo 401(k) employer contribution amount can be calculated based on the actual net income for the year, once the return is essentially finished, rather than estimated months earlier under pressure to meet an April deadline.
Before October 15
If a SEP-IRA or solo 401(k) contribution for the prior year hasn't been made yet, confirm the plan was established by the applicable deadline (a SEP can generally be adopted as late as the funding deadline itself; a solo 401(k) generally needs to have been established earlier, with limited exceptions), calculate the contribution once the return's net income is finalized, and make the deposit before the extended filing deadline closes the window.
This information is educational and describes the general funding-deadline rules for SEP-IRAs and one-participant 401(k) plans under current law. It does not determine contribution limits or deadlines for a specific plan or taxpayer, and plan establishment deadlines in particular depend on the plan document and entity type. If you're weighing a retirement plan contribution before the extended deadline, our tax planning team can help you calculate the right number.
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