People keep asking me when they have to file as common law for tax purposes. Today I called CRA and this is what the friendly phone person told me. You have to file as common law when you have been living together for 12 months or when a couple (who share a home) have a baby together, or share custody of a child.
If you get married or become common law during the year, and you receive UCCB or GST/HST Credits, be sure to let the tax man know or expect to be asked to pay back over-payments.
Your EI premiums will be calculated based on your income tax and benefit return for the year in which you apply. For example, if you register in 2012 , your EI premiums will be calculated based on your 2012 income tax and benefit return and will be payable by April 30, 2013.
Regardless of when you register during a given year, EI premiums are payable based on your self-employed income for the entire year. See calculation of premiums page for more information.
See my blog post on EI for the self-employed for more information.