Praise be to Allah.
It says in al-Mawsoo’ah al-Fiqhiyyah (5/225):
With regard to the addition of the mosque, which is an area that has been added to the mosque to make it larger, if it is encompassed with a stone wall, what may be understood from the comments of the Hanafis, Maalikis and Hanbalis according to the correct view in the madhhab, is that it is not part of the mosque, as opposed to the other view (of the Hanbalis) that it is part of the mosque. Abu Ya’la reconciled the two views by noting that if this addition is surrounded by walls and has a door, then it is part of the mosque. The Shaafa’is were of the view that the addition to the mosque is part of the mosque, and if a person observes i’tikaaf there, his i’tikaaf is valid.
Shaykh ‘Abd al-‘Azeez Aal al-Shaykh said:
Whatever is encompassed by the wall of the mosque and included in the mosque is part of the mosque, and whatever is outside the mosque enclosure is outside the mosque.
Majallat al-Buhooth al-Islamiyyah, 59/81.