If we do not file on the extension due date (October 15, 2024), is a penalty imposed? If yes, is there still a penalty if we are due a refund (i.e., we have overpaid our taxes through estimated payments)?
If you have a valid extension and file your tax return by the extended due date, you will not be assessed a Failure to File penalty. However, if the tax return shows a balance due, the IRS can assess a Failure to Pay penalty (and interest), as an extension only grants additional time to file the tax return; and is not an extension to pay the balance due. Taxes must be paid by the original due date of the return to avoid penalties and interest on the amount owed after that date.
If you do not file by the extended due date and owe taxes, you can be assessed Failure to File and Failure to Pay penalties.
However, note that all penalties and interest are based on the balance due to the IRS. As such, if you file late and you are receiving a refund, there is no penalty or interest assessed.