Praise be to Allah.
It is permissible to pray in the house of a Christian or anyone else who does not follow the religion of Islam because of the general meaning of the words of the Prophet (blessings and peace of Allah be upon him): “… the earth has been made for me a place of prostration and a means of purification, so wherever a man of my ummah is when the time for prayer comes, let him pray...”
Narrated by al-Bukhaari, 323
As-Sindi (may Allah have mercy on him) said: What we learn from the hadeeth is that the earth itself in its entirety is a place for prayer, unless there is any impediment in the presence of which prayer is makrooh or invalid, in which case the ruling is connected to that impediment.
End quote from Haashiyat as-Sindi, commentary on Saheeh al-Bukhaari, 1/140
Al-Bukhaari gave a chapter in his Saheeh the title “Chapter on prayer in churches”, in which he narrated that ‘Umar (may Allah be pleased with him) said: We do not enter churches because of the statues on which there are images. And Ibn ‘Abbaas used to pray in churches except a church in which there were statues.
Al-Haafiz ibn Hajar (may Allah have mercy on him) said: With regard to the fact that he said “Chapter on praying in churches”, the author of al-Muhkam said: The ruling on churches applies to churches, synagogues, monks’ cells, temples in which there are idols, fire temples and so on.
End quote from Fath al-Baari, 1/531. See also al-Insaaf by al-Mirdaawi, 1/496
If prayers offered in churches are valid, then it is more appropriate that they should be valid if offered in the house of a Christian or other non-Muslim.
The scholars of the Standing Committee (6/270) were asked: Sometimes the time for prayer comes when I am in the house of a Christian, so I take my prayer mat and pray in front of them. Is my prayer is valid, because I am in one of their houses?
Yes, your prayer is valid; may Allah increase you in keenness to obey Him, especially by offering the five daily prayers on time. It is obligatory to try to offer them in congregation, frequenting the mosques as much as possible. End quote.
Standing Committee for Academic Research and Issuing Fatwas.
And Allah knows best.