住宅 is formal and refers to residential property. 房子 (fángzi) is the everyday word for 'house'. 家 (jiā) means 'home'.
usage
宅 alone has taken on the internet slang meaning of 'homebody/shut-in' (from Japanese 'otaku'): 宅男 (zhái nán, male homebody), 宅女 (zhái nǚ, female homebody).
