A while back I saw some 400 square feet houses in Panama City. They are still recovering from Hurricane Michael.
They were cute and also had smaller but still usable appliances. They were technically mobile homes but I think they were intended to be placed permanently. They looked like regular houses.
I think a young couple could live fine in one at least as a first home. I have claustrophobia so count me out.
Do you know about the Sears houses? Tiny, efficient cottages, You could buy them as a kit from the Sears catalog in the early 20th century. And now the few that are left are worth the big bucks!