Is it "in" or "at"?

My colleague was asking me today, which one is correct? "Restaurant A is now in Publika or Restaurant A is now at Publika?"

I told her it should be "at" but she thought otherwise and went on saying "but the restaurant is in Publika building"?

I goggled and found this on "allexperts".

In many cases "in" or "at" can be used interchangeably with no difference in meaning, especially with "the office." The way you use "in the office" in this example is fine, and your reason for using it makes sense. However, you could just as well say "at the office" in that situation. "At" is often used to a something as a place, a location, perhaps some sort of institution, such as at the theater, at the library, at the restaurant, at the office. You could use "in" with most of these nouns, too, to convey the idea of being INSIDE that place.

There are more subtle idiomatic expressions and situations for each of these nouns than I could begin to explain. ("I was at the library when I found the book" "The book was in the library" "We met at the library" "We met in the library" are all correct and just a few examples.) But I think, on the whole, more often than not, you might use either "at" or "in" in most situations involving these sorts of nouns. It depends somewhat on a person's preference and habit, but you probably have some sense of when you are talking about something "in" (inside) a place or "at" a place ("at" a location).

However, the second sentence, the woman's response, is one case where I think there would be a preference; it would be preferable and more natural to say "at the office" in this case because of "welcome." There's a sense that it's "the office" that is "welcoming" you, and so it makes sense to refer the office as a place, a location, using "at," rather than to think of it as a place to be "inside." On the other hand, one might say "You are not welcome in this house." There's a territorial feeling about this expression: within these boundaries. But one might be at least as likely to say "You ARE welcome AT this house."

My colleague looked confused and asked "So, which is correct in my scenario? Restaurant A is now in Publika or Restaurant A is now at Publika?" I told her, for your scenario, both are correct but "at" is more appropriate to inform that Restaurant A is now at Publika.