If you're renting the room at fair market value, you must declare the income as rental income. You could then also claim rental expenses.
If your tenant isn't paying fair market value, you're in a "cost-sharing" scenario. In a cost-sharing scenario, you don't have to declare the payments you receive from the tenant. But you also can't claim any rental expenses, nor can the tenant claim any benefit from rent paid, like the Ontario Trillium Benefit or other benefits that consider rent payments.
