It's supposed to be good news, but it's not exactly fully good news. For Jimmy, the church agreeing to give him the usage rights is already quite a gain, considering it's not easy to snatch anything from the church. Yet, since it's just usage rights, ensuring secrecy about the door inside that house is difficult, as he can't resist if the church reclaims the house.
However, the church would need a reason to reclaim the house. For someone like Jimmy, an outsider in the exorcism priest hierarchy who draws a lot of attention, they shouldn't randomly come up with fabricated reasons to target him. He might be able to maintain this secret for a while.
Jimmy decided not to inform Nia about his plans for the house yet. He cared deeply about the matter, and wanted to wait until he had a detailed plan before telling her. Besides, he couldn't tell her the main reason. The environment around the house is quite nice, which is a sufficient overt reason to handle any queries Nia might have.
