outside to inside level design

what is the best way to optimize a level…i was thinking of creating exterior levels just the outside of buildings then having a trigger to transport you to interior level basicly a whole new level for outside and then a new level for the inside…or would it be better to create a building model with exterior and interior as one model…i only use unity not the pro version my pc is quite powerfull…but just need some advice…thanks

derpends on how long your in the building but i would make a 3d model of a building so you can walk thougth it and out in the same level but like i said it derpends on how big the building is