This notice is issued to inform the taxpayer that the IRS has adjusted their tax return and there is a balance due because of the adjustment. The notice will explain the changes to the tax return and request payment of the balance due.
This must be done within 60 days of the notice date. If not, you will lose your right to appeal the IRS’s decision. Be sure to attach documentation supporting your claim. Include your phone number (with the area code) and the best time of day to call. Be sure to write your SSN, the tax year, and form number (i.e. 1040, or 1040-A) on each page you’re attaching to the response form. Before mailing your response to the IRS, make a copy of the entire correspondence for your records. If possible, it is suggested you send the response through Certified Mail with Return Receipt requested. The IRS will then review the information and either reverse the proposed changes or forward your case for audit.
Sample CP 11: PDF Pages 1-5
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.