3a] Ruling on observing i‘tikaaf in a room that is separate from the mosque

the Messenger (blessings and peace of Allah be upon him), because the
houses of theMessenger had doors that opened into the mosque, but
despite that they were houses and the Messenger (blessings and peace
of Allah be upon him) did not go out to them [i.e., during i'tikaaf].
So to be on the safe side, the one who is observing i'tikaaf should
not do it there. But the custom of people nowadays is that rooms that
are in the mosques are regarded as being part of the mosque.
End quote from Sharh al-Kaafi.

And Allah knows best.