Blog · 14 October 2026

October 15, 2026 Extension Deadline: A Final-Day Checklist for US CPA Firms

By Keshav Bansal, CA

The October 15 extension deadline is the last big date of the 2025 filing season, and it covers more than Form 1040. For calendar-year individuals who filed Form 4868, Thursday, October 15, 2026 is the final day to file their 2025 return. Several other deadlines land on the same day, from FBAR and Form 3520 to retirement-plan funding and a §163(j) election window. This checklist pulls them together for US CPA firms, with each date attributed to the source we took it from.

One point is worth repeating to every client: an extension gives more time to file, not more time to pay. Late filing and late payment are separate penalties.

Why clients still get this wrong

A September NATP poll reported by CPA Practice Advisor (Isaac M. O'Bannon, October 7, 2026) found that 90% of surveyed tax pros say client misunderstandings about extensions are very or somewhat common. In that poll, 48% said clients think an extension also extends the time to pay, and 23% said clients think penalties and interest don't apply.

The same poll points to what actually stalls extended returns. Missing income documents (30%) and incomplete business, rental or side-income records (29%) made up 59% of the answers. In practice, that often means a Schedule K-1 that arrived late in the summer, a corrected 1099, or side-income records the client never organized. If that sounds familiar, our QuickBooks Online cleanup handoff checklist covers how to get messy records into shape before they block a return.

The October 15, 2026 checklist

1. Extended Form 1040 and calendar-year Form 1120

CrossLink Tax (Mark Castro, CPA, October 7, 2026) lists Form 1040 and calendar-year Form 1120 as due Thursday, October 15. Form 1041 was already due September 30.

The Journal of Accountancy's October 2026 tax column also recapped Karp v. United States (Fed. Cl., May 21, 2026). There, the IRS treated a Form 4868 extension as void because the couple wrote $0 as their estimated tax. The court held the estimate was bona fide and reasonable because the government wasn't left short, so the extension stood. The lesson for firms: Form 4868 needs a good-faith, reasonable estimate of tax, and the workpapers should show how you got it.

2. FBAR (FinCEN Form 114)

According to the KLR tax blog and the IRS FBAR page, October 15, 2026 is the automatic extended FBAR deadline for 2025 foreign accounts. US persons whose foreign financial accounts added up to more than $10,000 at any time in 2025 must e-file with FinCEN, not with Form 1040. No separate extension request is needed. Form 8938 is a different report that goes with the tax return, and filing one does not replace the other.

3. Form 3520 for foreign gifts and trusts

The IRS fourth-quarter tax calendar lists October 15, 2026 as the extended Form 3520 deadline for calendar-year filers who filed Form 4868. Form 3520 is mailed separately to Ogden rather than attached to Form 1040. Per the IRS page on gifts from foreign persons, gifts from nonresident aliens or foreign estates over $100,000, or from foreign corporations or partnerships over $20,116 for 2025, must be reported even though foreign gifts are generally not taxable income. A late Part IV can cost 5% of the gift per month, up to 25%. Ask every extended client about foreign gifts and inheritances, not only about foreign bank accounts.

4. SEP contributions

CPA Practice Advisor (September 30, 2026) reminds that October 15 is the final deadline to set up and fund a SEP for the 2025 tax year if the client filed Form 4868. IRA contributions for 2025 closed on April 15, 2026, with no extension.

5. Solo 401(k) employer contributions

Kiplinger (Kate Schubel, updated October 6, 2026) notes that for sole proprietors and C corps that timely filed Form 4868, October 15 is the last day to make employer Solo 401(k) profit-sharing contributions for 2025. S corps and partnerships already hit their September 15 date. Kiplinger gives the 2025 overall limit as $70,000, plus a $7,500 catch-up at 50 or older, or $11,250 at ages 60 to 63. Employee elective deferrals generally had to be elected by December 31, 2025.

6. Excess IRA contributions

The same Kiplinger piece notes that October 15 is the last day for extension filers to withdraw excess 2025 IRA contributions and avoid the 6% tax. It gives the 2025 limit as $7,000, or $8,000 at 50 or older, or the person's compensation if less. If a client over-contributed, deal with it before the day ends.

7. Withdrawing certain §163(j) elections

Under Rev. Proc. 2026-17 (March 18, 2026), businesses have until October 15, 2026, or the earlier statute-of-limitations date, to withdraw §163(j)(7) real property, farming or utility elections made for the 2022 to 2024 tax years. The withdrawal is made on an amended return marked "FILED PURSUANT TO REV. PROC. 2026-17". Business clients who elected out in those years should have heard from you already. If not, review them now.

Check for disaster postponements first

Not every client is due on October 15. CrossLink Tax notes that disaster relief in Wisconsin, Louisiana, Michigan and Mississippi pushes some taxpayers to November 2, and others in Hawaii, South Dakota, Indiana, Washington, Nebraska, West Virginia and Mississippi (Tropical Storm Arthur) to February 1, 2027. Confirm each client's county against the IRS announcement before you assume a date. A governor's declaration alone doesn't move the deadline. Our post on IRS disaster relief under §7508A explains how those postponements work.

If an e-filed return is rejected on the day

CrossLink Tax describes the IRS perfection period. If an individual return is transmitted on time on October 15 but rejected, it still counts as timely if it is corrected, retransmitted and accepted by October 20, which is five calendar days later. A calendar-year Form 1120 gets 10 calendar days, ending October 25. The perfection period is not a second extension, and it doesn't cover a return first sent after the deadline. Check for acceptance, not just "transmitted", on every return sent this week.

After October 15: fix the cause, not just the symptom

Once the deadline passes, look at which returns went to the wire and why. The NATP numbers suggest the answer will usually be documents, not tax technicalities. Three changes help most firms:

• Earlier document requests. Send K-1, 1099 and side-income requests to extended clients in spring, with a follow-up schedule.
• Books closed monthly. Clients whose bookkeeping is current produce business and rental figures in days, not weeks.
• A written extension memo. Every client who extends should receive, in writing, the reminder that payment was still due in April.

The same thinking applies to year-end information returns. Our note on the 1099-NEC threshold change for 2026 payments covers what to update before January.

If your firm wants extra capacity for bookkeeping, cleanup and tax prep support before next season, see how we work with US CPA firms or email info@taxbrewglobal.com.

Check current IRS guidance: this article summarizes sources as of 14 October 2026 and is general information, not tax advice. Rules, dates and figures can change. Confirm them against the cited IRS release, publication or source and current IRS.gov guidance before acting.