Even married couples can file as single if it benefits them in their returns. It doesn’t impact their marriage but their tax returns are separated. The CRA doesn’t care if your common law or married and it is only an issue if you file as common law/married when you aren’t.

It really depends on your situation and how much it benefits you either way.

Fill out one of the many online filing programs as common law and as single to see which benefits you more. They are usually free to use but pay to submit so feel free to play with both options, if you qualify that is.

You can submit adjustments to past filings if it becomes worth the effort. They are a pain in the arse though, just a heads up.