"Where do you live?"
"Oh, a mansion in Nakano."
You spend the next four months quietly assuming she is extremely wealthy and slightly humble about it. She is neither. She is paying 95,000 yen a month for a room you could vacuum without unplugging anything.
In Japanese, マンション means an apartment in a reinforced-concrete building, usually with an elevator, usually newer. The word was chosen by developers in the 1960s precisely because it sounded expensive — the same instinct that names British housing estates "The Gables".
The distinction that actually matters here is マンション versus アパート (apaato): concrete versus wooden, elevator versus stairs, more expensive versus less. Nobody is thinking about a driveway.
This is not Japanese people misunderstanding English. It is a loan word doing what loan words do — arriving, dropping its old baggage, and taking a job in the local economy. English did the same thing to French for six hundred years and nobody sends English an apology.
The joke is not that Japanese got "mansion" wrong. The joke is that a 1960s developer got it exactly right: sell a small concrete box with an aspirational foreign word on it, and it works so well that sixty years later 30 million people describe their ordinary flat with a word that used to mean a stately home. That is not a language error. That is the greatest branding win in Japanese real estate history.