Residential rent for a period of a month or more is generally exempt from GST/HST. This is the default a lot of operators mentally apply everywhere — which is exactly what causes trouble once the same portfolio also runs nightly stays.
The CRA treats a rental period under 30 continuous days, at a daily rate over $20, as a commercial supply — the same tax treatment as a hotel room, not as residential rent. Run any nightly-stay inventory in Ontario and that income counts toward HST registration, even inside a portfolio whose other units are all long-term leases.
You're required to register for GST/HST once your total taxable revenue exceeds $30,000 CAD across four consecutive calendar quarters — and if you operate multiple properties, their short-term income is combined to test that threshold, not assessed unit by unit. Once you cross it, you have 29 days to register.
Ontario's HST rate is 13%. If a booking platform is collecting and remitting on your behalf while you're unregistered, that doesn't remove the reporting obligation — the income still has to appear on your return.
The operators most likely to get this wrong aren't dedicated Airbnb hosts — they're long-term-lease portfolios that quietly picked up a few nightly-stay units to cover vacancy. If your books don't already separate short-term from long-term income per unit, you won't notice you've crossed the threshold until a filing deadline forces the question.