Where I'm staying in Osaka has an internal space of around 28 square meters, I estimate, which is 300 square feet. This isn't counting the not-very-usable stepped entranceway (where you'd leave your shoes) but is otherwise an overestimate (I treat my armspan as 2 meters, it's probably a bit less. I've been here almost five weeks so far, out of seven scheduled, and am considering extending my stay. The biggest problem is that the stairs are more like a ship ladder, so it would be annoying to haul stuff like books up to the bedroom/storage room. As a traveler in an era when a whole library fits on my phone, that's not much of an issue for me.
And it actually is a tiny house, just barely detached from anything else.
(It's slightly more than one armspan wide at the widest, and around 3.5 armspans long; I'm 5'10".)
The lot is basically the size of the house; Japan doesn't seem to require setbacks or yards. You can *have* a yard, but zoning doesn't hide the opportunity cost of having a yard instead of another house.