Legal Question in Family Law in California

If they are there.

It depends on what you are serving and and what point in the court case. Summons and Petition? It has to be in person. As Mr. Roach says, if your process server finds them there and hands it to them, then fine. But no, it's not okay to hand it to a random person at that "last known address" and call it good. Are you the Petitioner in a divorce and you're simply serving financial disclosures? Arguably, you can serve at the last known address. There are several variables here. Talk to an attorney or the Facilitator's Office.