If you do not file your extension form by the due date, the IRS will usually charge you a late filing penalty. The penalty is usually 5% of the unpaid taxes for each month (or partial month) that the return is late, up to a maximum of 25%. For example, if you owe $1,000 in taxes and you file your return two months late, the penalty will be 10% of the unpaid taxes, or $100.