In Canada there is some complicated formula for how much you can expense rent, based on the floor size of your office compared to that of the living space in your home and how much you use it for working etc.
When my partner applied for it, it ended up being a ridiculously small amount.
My accountant in the US always recommended against it because it is a red flag. And, yeah, it would be one room in my house and not dedicated for work purposes so it wouldn't be a big deduction. When I had a part-time software business it made more sense to just find reasonable deductions I could offset against my revenues.
When my partner applied for it, it ended up being a ridiculously small amount.