Question: We are moving into a brand new house which we just purchased in approximately one month and we have already affixed the Mezuzot. Was it correct to do so?
Answer: The obligation of the Mitzvah of Mezuzah rests upon one who resides in the house. However, if one has a completely empty house which he does not reside in, one need not affix a Mezuzah to it. Our Sages (Baba Metzia 101b and Rashi’s commentary on Pesachim 4a) derive this from that which is states in the verse, “The doorposts of your home”-only a doorpost which you go in and out of. The Rambam (end of Chapter 5 of Hilchot Tefillin) rules likewise.
Maran Ha’Chida therefore writes in his Sefer Birkei Yosef (Orach Chaim, Chapter 19) that if one is about to move into a new home, whether it is rented or purchased, one should not affix the Mezuzot before one begins residing in the home, for one is not yet obligated to do so. When one comes to live in the house, one should immediately recite the blessing of “Likbo’a Mezuzah” and affix the Mezuzah.
Based on the words of Maran Ha’Chida, the Mishnah Berura (Chapter 19, Subsection 4) writes that one should only recite the blessing on affixing the Mezuzah immediately before beginning to live in the house, just as one recites a blessing on a Tallit only immediately before wrapping one’s self in it.
On the other hand, Maran Rabbeinu Ovadia Yosef zt”l (in his Halichot Olam, Volume 8, page 218) disagrees with the Mishnah Berura and writes that one need not affix the Mezuzah specifically before beginning to live in the house; rather, one may affix the Mezuzah when one is preparing the house to begin living in it, such as by bringing some furniture, bedding, and other appliances into the house, for one becomes obligated in the Mitzvah of Mezuzah from then just as one who has ten houses must affix Mezuzot on each of them although he only enters the house once a year and does not live there steadily. He quotes most of the great Acharonim as ruling likewise.
Summary: When one is about to move into a new residence, one must affix a Mezuzah. The Mezuzah should be affixed when one begins bringing in some household items, such as furniture, into the house. Nevertheless, one should not affix a Mezuzah when the home is completely empty and one only plans on living there at a later time.