Do not succum to my number one pet peave for building interior areas; clown-car syndrome. It is rampant in BGTSCC. I did it too when I first began tinkering in the Toolset. This is my own personal preference too, so I do not intend to offend. Anyway, here goes:
Persons typically lay out some tiles then fill them with stuff to fit their vision of the area. They do not seem to keep in mind the appearance of the exterior of the building while creating its interior. Also, just because there are 10000 placeables in the Toolset does not mean you should put 3/4ths of them into an area. Also, avoid repetition unless there is a point to it.
VFX's greatly increase frame rate. So limit those too. Try to keep frame rates less than 2000. It is not so important in non-combat zones, but it is a good rule of thumb. And Tfunke agrees with me.
Some "clown car" examples:
The original CK Library interior with its exterior placeable overlaying the interior tiles.

As you can see, the interior size and shape does not at all correlate with the exterior. Inside is far too large compared to the outside.
I re-worked the Library to be this:
There are actually 3-floors now with nearly identical layout. They all "fit" within the footprint of their external placeable building's footprint.The hallways are now far closer together like any real-life library:

It is OK to be repetative in someplace like the Library. It is full of books. Everywhere. Not many interiors should be similar to this, though. As always, keep the purpose (theme) of the tile in-mind first and foremost. Not the whole area. Each tile. Tile by tile. Less is often more.
Another clown car example, The Shelves of Many Books in BG:

The exterior of this building is equivalent to a 1x2 tile interior with two floors. As you see, the 7x7 interior could never reasonably fit into its exterior. Hence, clown-car syndrome.
Many interiors are multiple-floor buildings all within one area file. Far better to go big by going up and down than spread out. So, how to separate interior floors all on the same area? Blank tiles between floors. Use at least 2, preferably 3 blank tiles around separating the interior floors. 2+ blank tiles also prevent overhearing conversations in adjacent areas which should reasonably be on different floors of the building.
Next, to go from floor-to-floor, you do not need to use the stair tiles. They can be useful, but they use a lot of floor space. Use them sparingly. Instead, simply use doors and label them "stairs up" or "stairs down". Visit CK Inn or Beregost Inn to see first hand. Also, visit the revised BG Temples in the East Gate to see how multiple interiors can be packed into one overall interior file. Consolidating areas descreases overall server file size and file count.
I always try to align (within 45-degrees) the interior with the exterior's facing. Doors are located to be about where the interior matches to the exterior. More or less. The tiles are a fixed size, so we must work with this limitation.
Tiles are ~ 30' x 30' with often overly thick walls (for wood structures at least). They are actually too big for a reasonable single room. You can partially divide tiles by using the wooden wall in the RWS Ship placeables. The back rooms in the Ilmater Temple in BG are similarly divided with makeshift walls and curtains.
You need to balance the room's purpose (its "theme") with playability. So crowded may be more realistic, but becomes difficult to manuver, the walls obstruct the view, etc. Avoid the tile sets with very high walls for compact interiors. Experiement. Have fun.
I hope this is helpful. D47