Only if you're into incest. Or if your grandma ate you. Both options are rather a bit too sick to think of.
Yeah, that would mean that you are literally inside your grandma. But the form "babci" would stay, that's true ;)
Well, you could say "Jestem w domu/mieszkaniu babci" (I am in grandma's house/apartment).
Spanish speaker here, I was expecting for a complement to know grandma's "what", since its impossible to leave a owner/posseser without a thing to posses, I already see it could mean the owned or leased priopiety. But please let me know: is this an usual saying in polish? Tbh I barely heard this in english until today.
OK, true, but I believe that it would be fine with "at grandma's". I mean, English usually needs the possessive, but "grandma" would be among few exceptions for family members so close that it's used like that, especially that the word itself is less formal than "grandmother".
So far I'm changing the main translation to "at grandma's".