Is i’tikaaf in musallas and Islamic centres valid?
Question

Is it permissible to observe i’tikaaf in musallas or Islamic centres (in which the five daily prayers and Jumu’ah prayer are offered), or is it permissible only in masjids? May Allaah reward you with good.
Praise be to Allah.It is only valid to observe i’tikaaf in masjids, because Allaah says (interpretation of the meaning): “And do not have sexual relations with them (your wives) while you are in I‘tikaaf (i.e. confining oneself in a mosque for prayers and invocations leaving the worldly activities) in the mosques” [al-Baqarah 2:187]. 
Al-Haafiz Ibn Hajar (may Allaah have mercy on him) said: 
The evidence in this verse indicates that if it (i’tikaaf) were valid in places other than the mosque, the prohibition on intimacy with one’s wife would not be singled out for prohibition there because intercourse is contrary to i’tikaaf according to scholarly consensus. So it is known, from mention of the mosques, that what is meant is that i’tikaaf can only be in the mosques. Ibn al-Mundhir narrated that there was scholarly consensus that what is meant by “sexual relations” in this verse is intercourse. Al-Tabari and others narrated via Qataadah that the reason for revelation of this verse was that if they were in i’tikaaf, and a man went for some need and met his wife, he had intercourse with her if he wanted, so this verse was revealed. End quote. 
Ibn Qudaamah (may Allaah have mercy on him) said in al-Mughni (3/65): 
I’tikaaf is not permissible except in a mosque in which prayers in congregation are held. 
I’tikaaf is not valid anywhere except in a mosque if the person observing i’tikaaf is a man. We do not know of any difference of opinion among the scholars concerning this. The basic principle concerning that is the words of Allaah (interpretation of the meaning): “And do not have sexual relations with them (your wives) while you are in I‘tikaaf (i.e. confining oneself in a mosque for prayers and invocations leaving the worldly activities) in the mosques” [al-Baqarah 2:187]. End quote. 
Al-Nawawi said in al-Majmoo’ (6/505): 
I’tikaaf on the part of a man or of a woman is not valid anywhere except in the mosque. It is not valid in the prayer-place of a woman’s house or the prayer-place of a man’s house, which is a separate area set aside for prayer. End quote. 
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said: 
Shar’i i’tikaaf must be in the mosques, because Allaah says (interpretation of the meaning): (interpretation of the meaning): “… while you are in I‘tikaaf (i.e. confining oneself in a mosque for prayers and invocations leaving the worldly activities) in the mosques” [al-Baqarah 2:187]. 
End quote from Fataawa Noor ‘ala al-Darb (8/176) 
See also the answer to question no. 48985 
Based on this, i’tikaaf in musallas or Islamic centres is not valid. 
And Allaah knows best.