What an extension changes about your filing date

Every spring we file extensions for clients who are waiting on a Schedule K-1, a corrected 1099 or a brokerage statement that arrives in March. An extension is a routine form. It moves one date and leaves the others where they are, and most of the questions we hear about extensions come from mixing those dates up.

The date that moves

For individuals, Form 4868 gives an automatic six-month extension of time to file Form 1040. For a 2025 return, the original due date is April 15, 2026 and the extended date is October 15, 2026. You don't give the IRS a reason, and the IRS contacts you only if it denies the request. You can also get the extension without filing the form: make an electronic payment and mark it as a payment for an extension.

Business returns use Form 7004. Partnerships and S corporations on a calendar year file by the 15th day of the third month, and the extension adds six months. Trusts and estates get five and a half months. The dates for 2025 returns look like this:

March 15, 2026 fell on a Sunday, so partnership and S corporation returns were due the next business day. When a due date lands on a weekend or a legal holiday, it moves to the next business day.

The dates that stay

The extension applies only to the return. Any balance you owe is still due on the original date, and the IRS charges interest on unpaid tax from that date until you pay it. Interest applies even when you had a good reason for paying late.

The late-payment penalty is usually half of one percent of the unpaid tax for each month or part of a month it stays unpaid, up to 25%. For individuals, there is a useful rule on Form 4868: you are treated as having reasonable cause for the extension period if at least 90% of your total tax is paid by the original due date, through withholding, estimated payments or a payment with the extension, and you pay the rest with the return. That is why we ask for a rough number before we file the extension. A good estimate in April keeps the penalty off the table in October.

Your first estimated payment for the current year is also due on the original filing date. For 2026, that is April 15, 2026, the same day as the 2025 balance. An extension for last year's return leaves this year's first installment where it is.

What happens after the extended date

A return filed after the extended date is late. The late-filing penalty is usually 5% of the unpaid tax for each month or part of a month the return is late, up to 25%. When both penalties apply for the same month, the late-filing penalty is reduced by the late-payment amount. The late-filing penalty reaches its maximum after five months, and the late-payment penalty keeps running.

If you owe nothing, the penalty math is small. That changes quickly when a K-1 shows more income than expected, which is the usual reason the return was extended in the first place.

State returns follow their own rules

Each state sets its own extension rules. Some accept the federal extension automatically. Others require their own form or a payment by the original date to grant the extension. If you file in more than one state, each one gets its own check. We track these on the same calendar as the federal dates so the state returns don't fall behind the federal one.

How we use an extension

We treat an extension as a scheduling tool. In March we list every return still missing a document, estimate the tax on each with what we have, and send the payment figure to the client with the extension. The return then goes out when the last document arrives, reviewed at the same pace as a return filed in April.

A few habits help:

• Send us the documents you have now, even if one is missing. Most of the return can be prepared and reviewed before the late form arrives.

• Pay the estimate with the extension. The 90% rule works in your favor only if the money reaches the IRS by the original date.

• Keep the confirmation of your payment. Electronic payments give you a confirmation number; save it with the return.

• Put the extended date on your own calendar. October 15 comes around faster than it sounds in April.

If a partnership or trust you invest in routinely sends its K-1 late, plan on an extension every year and set your April payment to cover it. That makes the extension a planned step each year.