Is it permissible to perform the Sunnah of Salaah whilst sitting down?

It is permissible to perform all the Sunnah (Sunnah-Muakkadah & Sunnah Ghair-Muakkadah) and all Nafl Salaah whilst sitting down (on the ground or on the chair) even if one has the ability to stand and perform the Sunnah and Nafl Salaah. It is also permissible to start the Sunnah/Nafl of Salaah standing and complete it after sitting down during Salaah.

However, by sitting and performing the Sunnah/Nafl of Salaah, one will receive half the reward of a person standing and performing Salaah unless one is sitting and performing Salaah due to a valid reason. If one sits and performs the Sunnah/Nafl of Salaah due a valid reason eg. he/she is unable to perform the Sajdahs of Salaah, then one will receive the full reward like one standing and performing Salaah. (Tahtaawi Ala Maraaqil Falaah 1/402/403)

Note: In the case of the Five Fardh (obligatory) Salaah, it will not be permissible to perform them whilst sitting down without a valid reason.

And Allah Ta’ala Knows Best