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E-Filer Attachments

Income Tax Return Supporting Documentation

Supporting documentation must be submitted to the Department in order to prove eligibility for credits and/or subtractions claimed on the Colorado Income Tax Return. For information about specific documentation requirements, refer to the Credits and/or Subtractions web page(s).

Using Revenue Online, the Department's free, online tax filing and account management portal, ensures the documentation is immediately available to the Department in your account while reviewing the income tax credit claims.

Do not use this service to respond to a letter or bill. To file a protest or respond to a letter or bill, go to Revenue Online and click “Protest a Notice or Letter” in the “Additional Services” box.

Income Tax Returns Filed on Paper

If you file your return on paper, the documentation to prove eligibility for credits and/or subtractions claimed should be included with your income tax return submission. 

If required documentation is not provided when filing by paper, uploading an electronic copy of the supporting documentation through Revenue Online is another method of submitting these documents. Follow the instructions below to submit your supporting documentation.

Income Tax Returns Filed Electronically

Supporting documentation must be submitted to prove eligibility for credits and/or subtractions claimed. If your tax software does not support PDF attachments, Revenue Online is the preferred method of submitting these documents because the copy will be added to your account. Supporting documentation for electronically filed returns is called an e-filer attachment.

Using Revenue Online to submit your e-filer attachments allows for quicker processing than sending it via standard mail. You do not need to log in or create an account to upload e-filer attachments via Revenue Online for your individual income tax return. Follow the instructions below to submit your e-filer attachments.

If using Revenue Online is not an option, an alternative method is to send your e-filer attachment supporting documentation via USPS standard mail. Be sure to include a completed E-Filer Attachment Form (DR 1778) with your e-filer attachment supporting documentation. The DR 1778 serves as a kind of "cover letter" for the documents when they arrive at the Department by mail and ensures that your supporting documentation is matched up with your return.

How to Submit Income Tax Return Supporting Documentation Using Revenue Online

  1. Go to Revenue Online, but do not log in.
  2. Click on "Submit an e-Filer Attachment" in the "File a Return or PTC Form" box on the right-hand side of the Revenue Online home page.
  3. Read the instructions and click on the "Next" button.
  4. Follow the steps to enter the Taxpayer Information, Spouse Information (if applicable), and Mailing Address. Then, click "Next."
  5. Enter the Filing Period and select the tax type. The filing period is the last day of the month, quarter, or year you are filing for. 
  6. Select the Document(s) Submitted. Then, click "Next."
  7. Click on the "Attach a File" button on the far right side of the screen.
  8. Enter a description of the file to be submitted.
  9. Click the "Choose File" button to select the electronic file to submit.
  10. Click the "Open" button to attach the document. The document will be listed in the "Attach Documents" section. To remove a file, click on "Remove" on the far right.
  11. Optional: Click on the "Attach a File" button on the far right side of the screen to upload any additional attachments.
  12. Once all attachments have been added and appear in the list, click on the "Next" button.
  13. A pop-up box will appear to confirm that the attachment(s) should be submitted. After reviewing all the information to ensure it is correct, click "OK."
  14. Your confirmation number and the following message will display on your screen, “Your request for e-Filer Attachments has been sent to the Department of Revenue.” Keep a copy of your confirmation number for your tax records.

The document(s) should be added to your tax account by the next business day, so that the Department can begin to process your supporting documentation and tax return. However, because no Login ID is required to submit an e-filer attachment, the submission will not display in your Revenue Online account.