After they had a second child, they upgraded to a four-bedroom.
Tips
usage
Real-estate shorthand. The 居 here means 'bedroom unit' (居室, jūshì). Pattern: 一居 (1BR), 两居 (2BR), 三居 (3BR), 四居 (4BR). Used interchangeably with the 室 system but 居 is more colloquial in casual conversation about apartments.