For many South Florida business owners, Tuesday, September 15, 2026, packs two important IRS obligations into a single day. It’s the extended filing deadline for calendar-year partnerships and S corporations that submitted Form 7004 by March 16, 2026, and it’s also the due date for the third quarter estimated tax payment. Missing either one can trigger unnecessary costs, but a little planning goes a long way.

What’s due on September 15, 2026?

This date applies specifically to pass-through entities that operate on a calendar year:

  • Form 1065 (U.S. Return of Partnership Income)
  • Form 1120-S (U.S. Income Tax Return for an S Corporation)

These returns were originally due March 16, 2026. If you filed a timely extension (Form 7004), September 15 is your new filing deadline. It is not the deadline for C corporations (Form 1120) or individual returns (Form 1040) — those remain on extension until October 15, 2026.

September 15 also marks the third quarterly estimated tax payment for 2026. That payment applies to individuals (including pass-through owners) and to calendar-year corporations. Pay any required federal estimated tax by this date to avoid underpayment penalties.

Extension is time to file, not time to pay

One of the most misunderstood rules is that an extension only postpones the filing of the return, not the payment of any tax due. If your partnership or S corporation owes tax — though uncommon for most pass-throughs — interest will have been accruing since the original due date. More importantly, the late-filing penalty for partnerships and S corporations can be severe even if no tax is owed.

The real cost of a late 1065 or 1120-S

For each month (or part of a month) that a partnership or S corporation return is late, the IRS charges $255 per partner or shareholder. This penalty applies for up to 12 months and can multiply quickly. A three-partner partnership that files six months late, for example, faces a penalty of $4,590. That’s money better spent on growing your business.

First Time Abate: a possible remedy

The IRS may remove a late-filing penalty through First Time Abate if the same return type was timely for the prior three years and the account is otherwise clean. Whether it applies depends on your facts. If a notice arrives — or you want to know where you stand before Tuesday — we can review whether First Time Abate fits your situation.

K-1s and the October 15 individual deadline

Partnerships and S corporations don’t pay income tax at the entity level; instead, they issue Schedules K-1 to owners, who report their share of income on their individual returns. If the entity return isn’t filed by September 15, K-1s are delayed, and owners may struggle to complete their personal returns accurately by the October 15 extension deadline. Filing the entity return on time keeps the entire tax picture moving smoothly.

Estimated taxes: don’t forget the owners

While the entity return is being finalized, individual owners must still make their Q3 estimated tax payment by September 15. This applies to any owner who expects to owe $1,000 or more in tax for 2026. Because Florida has no state personal income tax, the focus stays on your federal obligation. A missed estimated payment can lead to an underpayment penalty, even if you eventually pay in full by April 15, 2027.

No Florida disaster postponement for this date

As of now, there is no IRS disaster-related postponement affecting the September 15, 2026 deadline for South Florida filers. Unless a new declaration is issued, the date stands. Relying on a potential extension is risky; the safest approach is to prepare as if the deadline is firm.

Three steps to take now

  • Confirm whether your partnership or S corporation filed Form 7004 by March 16, 2026. If not, the return was due in March and is already late.
  • Gather all financial records needed to complete the return. Even if you’re working with a professional, timely delivery of information is essential.
  • Review your personal estimated tax situation and make the Q3 payment if required.

How Andean Consultants can help

We work with South Florida business owners every day to meet these deadlines without last-minute stress. Our tax preparation and advisory services cover Form 1065, Form 1120-S, estimated tax calculations, and penalty abatement requests. If you’re unsure where you stand or have received an IRS notice, reach out to us for a practical review.

You can also call or send a WhatsApp message to (561) 674-4461. A clear conversation about your timeline and obligations can save you far more than the cost of a penalty.