Halacha requires searching one’s home for Hametz on the night of the 14th of Nissan. One is obligated to check in all areas of the house where there is a reasonable possibility of Hametz being found.
Nowadays, of course, most people’s homes have been thoroughly cleaned and checked for Hametz before the night of the 14th of Nissan. Already for several days or weeks, we clean the house to ensure that all Hametz is eliminated. The question thus becomes as to whether a thorough search must be done again on the night of the 14th, given that the house had already been thoroughly cleaned.
The Shulhan Aruch (Orah Haim 433:11) writes explicitly that rooms which had been cleaned before the night of the 14th, even if care had been taken to ensure that no Hametz was brought there afterward, must nevertheless be searched on the night of the 14th of Nissan. This ruling is quoted from the Rishonim (specifically, the Mordechi and the Terumat Ha’deshen). He explains that since the Sages enacted an obligation to search on the night of the 14th, this search must be conducted even if rooms were cleaned previously. Secondly, it is possible that the rooms were not cleaned properly, and thus a final search must be done on the night of the 14th.
By contrast, the Hid"a (Rav Haim Yosef David Azulai, 1724-1806), in his work Mahazik Beracha (listen to audio recording for precise citation), writes that some have the custom to conduct a very perfunctory search on the night of 14th of Nissan ("Ha’abara Be’alma"), relying on the fact that the house had already been cleaned of Hametz. The Hid"a writes that such people have a basis on which to rely. This ruling (which is brought by the Sha’areh Teshuba 433:2) appears to stand in contrast to the ruling of the Shulhan Aruch.
Hacham Bension Abba Shaul (1924-1998), in Or Le’sion (vol. 3, note to 7:15), suggests reconciling these two rulings. He explains that the Shulhan Aruch referred to a case where the house had been cleaned in a general sense, but not with the intention of searching for Hametz. Therefore, there is reason for concern that some Hametz remains, and thus searching is required on the night of the 14th. The Hid"a, however, speaks of those who clean specifically for Pesach in the days and weeks before the holiday, and for this reason, he justifies the practice to conduct a quick, ceremonial search. And although the Sages enacted searching specifically on the night of the 14th of Nissan, this obligation can be fulfilled by checking a room that had not yet been thoroughly cleaned.
Nevertheless, as a practical matter, it would seem that one should make a point of conducting a serious search for Hametz on the night of the 14th of Nissan, even though the house had already been cleaned. One should not simply ceremonially "march" through the house with the candle, but should instead get down on his hands and knees, and look underneath pieces of furniture and in corners of rooms to check if there might still be Hametz. Bedikat Hametz (searching for Hametz) might constitute a Biblical Misva on this night (given that the "Bittul" proclamation is not made until the following day), and so one should not belittle the importance of this Misva, and conduct a serious search.
Summary: Although our homes have been thoroughly cleaned in the days and weeks before the 14th of Nissan, nevertheless, it is proper to conduct a serious search when performing Bedikat Hametz on the night of the 14th. Rather than casually walking through the house as though conducting a formal ceremony, one should look around in each room to see where they might still be Hametz.
