Can confirm. I live in a 450 square foot studio apartment and I work from home every day. I would love a separate room to just be my "office," but my only room is already my kitchen, bedroom and living room too...
Alternatives I've heard from others are things like dressing up for work. I had a colleague that wore a suit to work every day (at a tech startup) so that when he got home he could change into something else. That helped him define a boundary.
Find what works for you, it doesn't have to be physical space.
That's actually a great point - the days that I put on a button-up and nice jeans as if I was going into an office are days that I'm way more productive, as opposed to wearing pajamas all day.
Also my kids whom are stuck at home at the moment recognise the difference and "somewhat" tries to disturb me less if I got a work shirt on.
Make sure you've got everything ready to go when you 'arrive'. Don't do chores during the day -- but if you have a partner, discuss your reasoning for this with them beforehand.