If your S corporation or partnership timely requested an extension for its 2025 tax return, one important date is approaching: September 15, 2026.
For calendar-year S corporations and partnerships that filed a valid six-month extension, September 15 is generally the deadline to file the extended return.
For business owners, this deadline matters for more than simply getting the return filed. The information reported by the business can also affect the individual tax returns of shareholders and partners.
What Is the Extended Deadline?
Calendar-year partnerships generally file Form 1065, while S corporations generally file Form 1120-S.
The regular filing deadline for calendar-year partnerships and S corporations is generally March 15. For 2025 returns, however, March 15, 2026 fell on a Sunday, making March 16, 2026 the regular filing deadline.
Businesses that properly requested a six-month extension generally have until September 15, 2026, to file their 2025 returns.
Why Is the September Deadline So Important?
Partnerships and S corporations are generally pass-through entities. Instead of the entity paying federal income tax in the same manner as a C corporation, income and other tax items generally pass through to owners.
Those items are reported to owners on Schedule K-1.
If the business return is delayed, the owners may not have the final information they need to complete their personal returns.
That becomes particularly important because the extended individual tax filing deadline is October 15, 2026, giving individuals only about one month to incorporate finalized business information into their personal returns.
What Should Businesses Have Ready?
Waiting until September to start gathering information can create unnecessary pressure.
Business owners should work with their accounting team to make sure financial records are complete and accurate. That may include bank and credit card reconciliations, payroll information, fixed asset purchases, loan activity, owner contributions and distributions, and other relevant financial records.
If your books are incomplete, let your CPA know early. Identifying missing information ahead of the deadline can make the filing process much smoother.
Does an Extension Mean I Had More Time to Pay?
This is an important distinction.
A tax filing extension generally gives additional time to file, not additional time to pay any taxes that are due. While partnerships generally pass income through to their partners, S corporations may have certain tax obligations at the entity level. Owners may also have personal tax obligations related to pass-through income.
This is why business and personal tax planning should not always be viewed separately.
What Happens After the Business Return Is Filed?
Once the S corporation or partnership return is complete, shareholders or partners generally receive their Schedule K-1 and, when applicable, Schedule K-3.
That information is then used to complete the owner’s individual income tax return.
If you also filed an individual extension, your CPA can use the finalized business information to complete your personal return ahead of the October deadline.
September can therefore become a key transition point between business return preparation and individual tax filing.
Frequently Asked Questions
When is the extended S corporation deadline in 2026?
For a calendar-year S corporation with a valid extension, the 2025 return is generally due September 15, 2026.
When is the extended partnership deadline?
Calendar-year partnerships that received a valid six-month extension generally have the same September 15, 2026 deadline.
What form does an S corporation file?
S corporations generally file Form 1120-S.
What form does a partnership file?
Partnerships generally file Form 1065.
What is a Schedule K-1?
A Schedule K-1 reports an owner’s share of certain income, deductions, credits, and other tax items from a pass-through entity.
What is Schedule K-3?
Schedule K-3 provides additional information about certain items of international tax relevance that may be reported to partners or shareholders when applicable.
Need help getting your extended business return across the finish line? Contact DDC to prepare for the September 15 deadline.