Individuals, corporations, pass-through entities, and fiduciaries can file income tax returns up to six months from the original due date with a valid extension.

A valid extension means you can avoid a penalty for filing late. There aren’t any extensions allowing you to pay late. You’ll owe interest on any tax not paid by the original due date.

Even with a valid extension, you could owe penalties. Some common situations when penalties apply are if:

  • You file on or after the original due date and pay tax due after you file.
  • You file after the extended due date.
  • You pay after the extended due date.

See the bottom of this page and the Income Tax Interest and Penalties page for more information.

Ensuring you have a valid extension

To avoid the late filing penalty, your prepayments (e.g., withholding, estimated payments or payments applied from previous refunds) must equal either:

  • 80% of the estimated tax due for your current return
  • 100% of the income tax reported on your return for the previous year

Keep in mind:

  • If you didn’t file the previous year, you must pay 80% of the current year’s liability. 
  • If you filed a return the previous year and the “tax due” (immediately following the “Payments and Other Credits” section) was zero, you don’t need to make a payment to get the extension. 
  • If the necessary payment would be $50 or less, you don’t have to pay it to get the extension.

We have two worksheets to help you make sure you have a valid extension to file late:

After you finish the worksheet, do one of the following:

  • If you need to make a payment, you can use our free, secure QuickPay service online to authorize a payment from your bank account. Or, mail the form at the bottom of the worksheet with your check.
  • If you don’t need to make a payment, you don’t have to send us Form 51 or Form 41ES.

If you qualify for a valid extension, be sure to file your income tax return and pay any tax due by the extended due date. File and pay at the same time to avoid a penalty.

If you don’t have a valid extension

If it’s after the original due date and you don’t have a valid extension:

  • File your income tax return as soon as possible.
  • Pay as much as you can now. (Pay the entire amount due, if possible.) See the E‑Pay page for options.

These steps can greatly reduce the amount of interest and penalties you owe.

More about interest and penalty

You’ll owe interest and possibly penalties on all overdue tax payments. Idaho law requires us to charge interest.

In addition to the penalties mentioned above, other penalties can apply even if you have a valid extension. Penalties can vary from 0.5% per month to a maximum of 50% of the tax due. See the Interest and Penalties page for the different kinds of penalties and how we calculate them.