It depends on the context. If you want to stress it that you saw this specific house, you might use "цей" ("this") or "той" ("that"). In this case, you would say:
Я бачив цей / той дім. — I saw this / that house.
However, it is true that we don't use definite and indefinite articles (at least not that I have heard of.) So, you will have to use "цей" ("this") or "той" ("that") if you need to stress it.
Other than that, the sentence "Я бачив дім" can mean both "I saw a house," and "I saw the house."
-- Ти колись бачив дім у своєму житті? (Have you ever seen a house in your life?)
-- Так, я бачив дім (Yes, I have seen a house.).
-- Ти бачив дім, про який я казав минулої ночі? (Have you seen the house I was talking about the other night?)
-- Так, я бачив дім (Yes, I have seen the house.).