Designing enterable buildings for multiplayer game

I’m working on an open world multiplayer game, I’m wondering what is the best approach to model enterable buildings, should I model interior and exterior separately? In case of modeling them separately, for bigger buildings, should I make the whole interior as 1 mesh or should I model each floor/room separately.
Modeling the whole thing as 1 mesh will reduce draw calls, but modeling them separately will reduce polygon count and will handle better LODs.
Windows will be transparent and players will be able to shoot other players through them