"Do you have a house anywhere in the South?"
Translation:У вас є будинок де-небудь на півдні?
Somehow replacing де-небудь with будь-де in this question is wrong... Even though both mean "anywhere" in the sense of "doesn't matter where"...
Де-небудь means anywhere, as in, somewhere, but I don't care where, so we can use it in a question.
Будь-де means more like "at a random location", so it sounds really off asking "Do you have a house at a random location".
"Somewhere" and "anywhere" are often interchangeable in English as well, yet they are different words. We have decided to accept десь=somewhere and де-небудь=anywhere, and same for someone/anyone, so that learners get the idea of which one is which, and so that we can test whether they understood that the difference between these two words is the same as "somewhere" and "anywhere" in English. There are cases when they are not interchangeable, e.g. "He can sleep somewhere" and "He can sleep anywhere".