Praise be to Allah.
If when this hall was built, your intention was that it was not part of the mosque, and that it was rather for celebrations or gatherings and seminars, then it does not come under the rulings on mosques, so it is permissible for disbelievers to enter it.
But if this hall was built on the basis that it was to be part of the mosque and was to be used for these celebrations and for prayer when needed, then it is part of the mosque, and it comes under the rulings on mosques.
Similarly, it comes under the rulings on mosques if you did not intend anything in particular when building it, but it was part of the structure of the mosque, or there was a wall enclosing it and the mosque.
The scholars of the Standing Committee for Issuing Fatwas said:
Whatever is within the wall of the mosque is part of the mosque and comes under the same rulings as the mosque. So the yard of the mosque is part of the mosque and the library of the mosque is part of the mosque, if each of them is enclosed within the wall of the mosque. End quote.
Shaykh ‘Abd al-‘Azeez ibn Baaz, Shaykh ‘Abd al-‘Azeez Aal ash-Shaykh, Shaykh ‘Abdullah ibn Ghadyaan, Shaykh Saalih al-Fawzaan, Shaykh Bakr Abu Zayd
Fataawa al-Lajnah ad-Daa’imah, vol. 2 (5/234)
For more information, please see the answer to question no. 103136
It is not permissible for a disbeliever to enter the mosque unless there is a need or an interest to be served by his entering it, such as if he enters it to listen to a lecture, or to drink water, and the like. This has been explained previously in the answers to questions no. 2192 and 9444.
If the disbeliever is going to enter the mosque and commit some reprehensible actions, such as reckless mixing between men and women, or if women who are uncovered and adorned are going to enter, then it is not appropriate to be hesitant about prohibiting that.
If this hall comes under the same rulings as the mosque, then it is not permissible for the disbelievers to enter it in order to celebrate their festival, because there is no shar‘i purpose to be served by that.
It is not permissible to rent it to them or to anyone else, because mosques are not built for that purpose; rather they are built for the remembrance of Allah and the establishment of prayer. This is a second reason to disallow rental of the place to them.
Ibn al-Qaasim (may Allah have mercy on him) was asked: What do you think, if a man builds a mosque and rents it out to someone who will pray there? He said: This is not right in my opinion, because the mosques were not built to be rented out.
There is a third reason which dictates that it is not allowed for these people to enter this hall, whether it is part of the mosque or not, which is that they are entering it only for the purpose of celebrating their festival, and the festivals of the polytheists are false and invalid. It is not permissible for the Muslims to participate with them in that or to help them in doing so, because Allah, may He be exalted, says (interpretation of the meaning):
“Help you one another in Al‑Birr and At‑Taqwa (virtue, righteousness and piety); but do not help one another in sin and transgression”
Al-Bayhaqi (19334) narrated from ‘Umar ibn al-Khattaab (may Allah be pleased with him) that he said: “Avoid the enemies of Allah during their festivals.”
Some of the early generation interpreted the words of Allah, may He be exalted, describing the slaves of the Most Gracious (interpretation of the meaning) “And those who do not witness falsehood” [al-Furqaan 25:72] as referring to the festivals of the mushrikeen.
See: Tafseer Ibn Katheer (3/435).
Undoubtedly the festivals of the mushrikeen come under the heading of falsehood that we are instructed not to attend. The verse is general in meaning and indicates that we should not witness falsehood, which includes all haraam words and deeds, including the festivals of the mushrikeen.
Tafseer as-Sa‘di (p. 686)
For more information, please see the answer to question no. 11427
And Allah knows best.