Answer
It is disliked; because what is above it takes its ruling, and it is not disliked to urinate in a house that has a designated place for prayer and has a mihrab, nor above it; because it has not been given the ruling of a mosque, and it appears that there is no dislike to urinate in the mosque of the house as well; because it is not a mosque to the extent that it can be sold, thus it does not have the sanctity of a mosque. See: Fath Bab al-'Inayah 1: 313, and 'Umdat al-Ri'ayah 1: 198.