C Corporations: The Extended California Form 100 Is Due Mid-November 2026

A calendar-year C corporation that extended its 2025 California return has until the 15th day of the 11th month after year-end to file Form 100. That's November 15, 2026, which is a Sunday, so the return can be filed on the next business day, Monday, November 16. The extension covered filing only. Any tax was due April 15, 2026.

The dates

The FTB set out the rule in its October 2026 Tax News: the extended due date for Form 100 is "the 15th day of the 11th month after the close of a C corporation's taxable year - November 15, 2026, for calendar year filers." The original due date was the 15th day of the fourth month, April 15, 2026.

The FTB grants this extension automatically. Its extension page says it "automatically allow[s] a 7-month extension for businesses to file Form 100 that are not suspended" (FTB, Extension to file). No form is needed to get the extra time.

November 15, 2026, falls on a Sunday. The FTB's business due-dates page says: "If the due date falls on a weekend or holiday, you have until the next business day to file and pay." I'd still plan around the 15th and treat Monday the 16th as the cushion, not the target.

The federal Form 1120 for a calendar-year 2025 corporation was due on extension October 15, 2026, because the federal corporate extension for that year was five months, not six (IRC §6081(b)). So the federal return should already be filed. California is the trailing return.

What the extension didn't cover

The FTB is direct about this: "An extension to file your tax return is not an extension to pay" (FTB, Extension to file). For a C corporation, the total tax was due on the original date, April 15, 2026 (FTB, Due dates for businesses). That includes the $800 minimum franchise tax for a corporation with little or no income.

Two penalties come into play if things slip:

• Late payment. If the tax wasn't paid by April 15, the penalty is 5% of the unpaid tax, plus 0.5% for each month or part of a month it stays unpaid, for up to 40 months (R&TC §19132; FTB, Penalties and interest).
• Late filing. If the return isn't filed by the extended due date, the extension falls away, and the delinquent filing penalty is figured from the original due date: 5% of the tax still unpaid for each month or part of a month, up to 25% (R&TC §19131). The FTB's Tax News says it plainly: "Late filing penalties are calculated from the original due date if the return isn't filed by the extended due date."

That second point is the expensive one. Missing November 16 by a day can mean a penalty measured back to April, not to November.

Why the California return gets attention from lenders and buyers

For a company with a bank loan, an investor or a sale in progress, the filed California return is often on someone's checklist. A lender's covenant package, a buyer's due diligence request list, or an auditor's tax provision workpapers will usually ask for filed returns and proof of payment.

What I'd have ready alongside the filed Form 100:

• The federal Form 1120 as filed, so the starting point for California matches.
• A reconciliation of federal to California income. California doesn't follow every federal rule, so book, federal and California figures rarely agree without explanation.
• Proof of payment for the April 15 balance and the 2026 estimated payments.
• The tax accounts in the financial statements (income tax payable, deferred taxes) tied to the returns as filed.

When those four pieces agree, a buyer's or lender's tax questions usually get short.

What I'd do now

• Confirm the federal 2025 Form 1120 is filed.
• Confirm the California balance was paid by April 15, 2026, and, if not, pay it now to stop the monthly penalty from growing.
• File Form 100 by November 15, 2026, with Monday the 16th as the cushion.
• Tie the filed returns to the financial statements before anyone outside the company asks for them.

I don't prepare C corporation returns, but I do build the reporting that sits around them: financial statements, tax account reconciliations and packages for lenders and buyers. If you need that work, see reporting and modeling or send an inquiry.

General information, not advice for your situation. The dates here are for calendar-year 2025 California corporation returns, as checked on the date above.