1. We need a house which we can live in.
2. We need a house we can live in.
3. We need a house to live in.
4. We need a house in which we can live.
5. We need a house in which to live.
(Are they all grammatical? What about #3 and #5?)
6. We need a house to live.
7. We need a house in order to live.
(Can we use these sentences?)
All of 1 - 5 are grammatically correct. Sentences 2 and 3 are the most commonly used in the way they're phrased.
6 and 7 imply that if we don't have a house, we'll die! I'm sure that's not what is meant!!