As-salaamu `alaykum,
Ahl us-Sunnah believe in affirm in the nuzool (lowering) of Allaah in the last third/part of the night. It is known. How it occurs is unknown; it occurs in a manner befitting Allaah's Majesty. Belief in it is compulsary and questioning it is an innovation [paraphrased from the words of Imaam Maalik Ibn Anas regarding Allaah rising over his `Arsh (throne)]. We treat the nuzool of Allaah in the same way we treat the other attributes of Allaah, such as His hands, His ascending over the Throne, His eyes and the rest of them. We leave them as they are and accept them without attaching to them metaphorical meanings that have not been substantiated by the Qur'aan, Sunnah and understanding of the salaf (the first generations of Muslims).
Al-Waleed Ibn Muslim said, "I asked Maalik, Al-Awzaa`ee, Layth Ibn Sa`d and Sufyaan Ath-Thawree, may Allaah heave mercy upon them, concerning the reports related about the Attributes, so they said, 'Leave them as they are without asking 'How?''" [Al-Bayhaqee; Asmaa was-Sifaat. The chain is Hasan (acceptable)]
For more information regarding Ahl us-Sunnah's beliefs regarding Allaah's attributes, please refer to the books Lum`at ul-I`tiqaad (Sufficiency in Creed) by the great scholar, Muwaffaq ud-Deen Ibn Qudaamah Al-Maqdisee, who was from the students of Shaykh `Abdul-Qaadir Al-Jaylaanee. Also see Ibn Taymiyyah's `Aqeedah al-Waasitiyah and his `Aqeedah al-Hamawiyyah. I would recommend reading these books with their explanations by the scholars.