Sitting at Uva Letzion

Although one may recite the Kedusha of Uva Letzion (also known as Kedusha DSidra) without a Minyan, the Mishna Berura (O.H. 132:3) writes that if one is praying with a Minyan and they have arrived at the Kedusha before him, one should skip Ashre, Ya’ancha Hashem and Uva Letzion and recite the actual Kedusha DSidra with the congregation. After the Kedusha, one may return and repeat those preceding portions which were skipped. Rabbi Haim Kanievsky (Ishe Israel § 234) concurs with this approach.

The Rambam (Shu”t HaRambam [Mekitze Nirdamim Edition] § 33) writes that the proper practice is for Kedusha DSidra to be recited while sitting down, and derides those that tried to change it to standing up as ignoramuses. Additionally, the Arizal says that, on a Kabbalistic level, it is proper to sit during the Kedusha of the Keriat Shema and of Uva Letzion. He explains that during the Kedusha of the Amida, we stand in unison with angels who are also reciting it, whereas the Kedusha of Uva Letzion is only recited by people, and may be done sitting. Furthermore, he says that if one is standing during the Kedusha DSidra, one should make a point of sitting down.

Summary: If one is trailing behind and the congregation has reached the Kedusha DSidra, one should skip Ashre, etc. and recite it with the congregation. Kedusha DSidra is recited while sitting down.