Is it permissible to perform I'tikaf in a newly built musalla after a mosque was demolished, knowing that it is closed off like a mosque, and is it permissible to perform the 'Tahiyyat al-Masjid' (greeting of the mosque) prayer therein?

If the new building is endowed as a mosque (waqf), then it takes on the rulings of a mosque, such as the validity of i'tikaf (spiritual retreat) within it and the desirability of performing "Tahiyyat al-Masjid" (two rak'ahs upon entering a mosque). However, if it is merely a temporary prayer area and has not been endowed as a mosque, then i'tikaf is not valid in it, nor is "Tahiyyat al-Masjid" legislated for it. This is because i'tikaf is only valid in a mosque, based on the Almighty's saying: "And do not have sexual relations with them while you are in i'tikaf in the mosques."