If you own a calendar-year S-corp and you extended your 2025 Form 1120-S, three deadlines land on the same day this year: September 15, 2026. The return itself is due, and a late 1120-S carries a per-shareholder, per-month penalty for up to 12 months even when no tax is owed (IRC §6699). Confirm the current inflation-adjusted per-shareholder monthly amount for returns filed in 2026 before relying on a figure. Your deductible employer retirement contribution for 2025 closes. And your third-quarter 2026 individual estimated payment is due. One date, three obligations, and they pull on the same cash.
Here is what is still open, what closed at year-end, and the actions to take this week.
What is still open. The employer side of your retirement plan. Under IRC §404(a)(6), an employer contribution to a qualified plan is deductible for 2025 if it is paid by the return's due date including extensions and designated on account of 2025. For a SEP, the same timing applies under §404(h), and a SEP can still be established for 2025 up to that date. Your employer profit-sharing dollars are live until September 15.
What closed December 31. The employee elective deferral. For an S-corp owner-employee, that deferral had to be withheld from your 2025 W-2 wages during 2025. Once wages are paid and currently available, you cannot go back and elect to defer them (Reg. §1.401(k)-1(a)(3)). That door shut on December 31, 2025.
The worked example. Maria is the sole owner of a calendar-year S-corp in Denver and extended her 2025 Form 1120-S. She took $150,000 in W-2 wages from the corporation in 2025 and has a solo 401(k). The deferral and catch-up figures below are 2025 plan-year limits — the year the contribution is on account of.
Her employee deferral for 2025 (up to $23,500, or $31,000 with the $7,500 catch-up at 50 and over, per IRS Notice 2024-80) had to come out of her 2025 paychecks. Whatever she deferred is set. There is $0 left to add now.
Her employer profit-sharing contribution is different. Its maximum is 25% of her $150,000 in W-2 wages, or $37,500, before applying combined limits. Check it against the §415(c) annual additions limit of $70,000 for 2025. The catch-up is excluded from §415(c) under §414(v)(3)(A)(ii), so it is not counted here. Her non-catch-up annual additions are the $23,500 elective deferral plus the $37,500 employer contribution, or $61,000 — under the $70,000 limit. The $7,500 catch-up sits on top of that and does not reduce the room, so the full $37,500 employer contribution is available. If the corporation funds that $37,500 by September 15, 2026, the S-corp deducts it on Form 1120-S, Line 17 ("Pension, profit-sharing, etc., plans"), and it does not show up as wages on her W-2.
Same day, her Q3 2026 individual estimate is due under IRC §6654(c)(2). That is real cash competing with the $37,500. This is an illustration, not a promise; her actual numbers depend on her full return.
What to do this week. Move 1: confirm the return is on track to file by September 15, not on it. Move 2: decide the contribution-versus-estimate cash question before the date, and run the two together, since funding the employer share reduces the S-corp's income and can change what your estimate needs to be. On the estimate, the safe harbor is generally the lesser of 90% of current-year tax or 100% of prior-year tax, rising to 110% if your prior-year AGI exceeded $150,000 (IRC §6654(d)(1)(C)). States do not always mirror the federal safe harbor.
The next marker. October 15, 2026 is the extended due date for your 2025 individual Form 1040 under IRC §6081 and Reg. §1.6081-4. One deadline handled, one more on the calendar.
Run the two numbers side by side before the 15th, not after.
Book an exploratory conversation — bring the wage figure and we will help you frame the cash decision. gnomontax.com
General tax information, not personalized advice. Talk with us before acting. Full disclosures at gnomontax.com.
General information, not advice about your specific situation, and it may not reflect current law. Nothing here is investment advice and no securities are recommended.