Is it permissible to pray in a room located beneath an upstairs bathroom in a reinforced concrete building, knowing that the ceiling of the room is the floor of the bathroom, and there is no direct contact between them?

There is no impediment to praying in the aforementioned room as long as the worshipper is not touching impurity. If there is a barrier between him and the impurity, then there is no harm. It is disliked to establish a mosque beneath a restroom due to the disrespect it entails towards prayer.