When we first began joint custody, my husband's ex lived with her sister who wouldn't allow my husband in her house... And things were pretty awful back then, in general, with both working very hard at being civil to each other. The precedent was set at that point, I guess, that the picking-up parent called from the car when they arrived and the exchange happened in the driveway or on the sidewalk. It's become routine for us, and I don't think any of us see it as something hostile at this point, it's just what we've always done. I think we both value the privacy of our own family and own space, and neither party really wants to give that up. It also allowed us all to be much more relaxed and take our time with pick-up/drop-off, which I think is really positive overall.
At one point, my step-daughter wanted to show us something in her room at mom's and mom let my husband and me come in to see it. My husband returned the favor/goodwill by inviting her in one time. I'm fairly certain that is the only time either of us has been in the other one's house.
So, no, neither of us goes into the other person's house, but it isn't something that isn't allowed, it's just a respectful boundary that we all seem to have agreed to and that we are all interested in maintaining. At this point, all exchanges happen at school or at the airport, so it's not really an issue anyway...
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