Election to defer the cap gn when changing rental prop to princ res Appears a letter is required attached to the tax return, which means I can't Netfile. Is this true?

When you need to file a letter or election with your income tax return and choose to NetFile rather than "paper file", CRA will allow you to send the election separately. 

"The Income Tax Act provides for these various elections to be made. Some are made on authorized Canada Revenue Agency (CRA) forms, while others are made by providing specific information in a letter/note format. All elections, including supporting documentation, must be submitted to us in writing for electronically filed returns unless otherwise indicated. For the election to be considered valid, it must be submitted by the due date established in the Income Tax Act." Which means you should send it in by the April 30th deadline if you choose to NetFile.

When submitting this documentation:

• clearly state your full name, address, and social insurance number on all the election forms and letters/notes; and
• clearly indicate in the covering letter that this documentation is being submitted in support of your electronically or NetFiled return.