When do you use in and when at the flat, in English? Can I say both
I live in the flat.
I live at the flat.
Here's what prompted this question; a passage from a textbook goes like this: A girl arrived in a city where she was going to live for a while, in a new flat. Her friend picked her up at the airport.
She: ´How is your wife?´
He: ´She is fine, she is at your new flat.´
Which means, if somebody stays in a flat which is not his/hers, then he/she is AT someone´s flat, not IN someone´s flat?