I get this question frequently. The answer: Sure, you can ask (you can always merely ask) the court to award you temporary custody of the child(ren) under such circumstances, but whether the court will grant your request under such circumstances is a separate question.
Call me old fashioned, but the “my fiancé” term is, in child custody and family law settings, usually followed by words like “of 5 years” or “with whom I have 3 kids,” so if you are in such a situation, then use of “my fiancé” doesn’t give you, your “fiancé” or your circumstances the appearance of greater legitimacy or stability than shacking up. Asserting, “I have an apartment, but I’m not on the lease” means “my residential circumstances are insecure and unstable” and “I can be kicked out of where I currently live at any moment.”
Likewise, claiming “my child(ren) has/have his/her/their own room in someone else’s apartment that I don’t lease” means “my child(ren)’s residential circumstances are insecure and unstable too.”
But if the other parent’s circumstances are even less secure and less stable than yours, living with your boyfriend/girlfriend in his/her apartment at his/her pleasure may lead the court to award you temporary custody in the absence of any better alternative.
Utah Family Law, LC | divorceutah.com | 801-466-9277