secondary residence; a house owned in addition to one's main home (e.g., a getaway house, second home, or residence near work).
彼は都内にマンションを所有し、郊外に別宅も持っている。
He owns an apartment in the city and also has a secondary residence in the suburbs.
週末は別宅で過ごすつもりだ。
I'm planning to spend the weekend at my second home.
別宅を改装してゲストハウスにした。
I renovated the secondary residence and turned it into a guest house.