One of the things we have been doing with the Station 42 project is that we are making all of the buildings as playable as we can. That is we have buildings that can be opened up and furnished so we can put miniatures inside. We've found this is extremely helpful in making the terrain more useful for different types of games.
There are a lot of excellent terrain products and scratch built projects that look amazing but are not very playable. For some items that is ok but for larger buildings especially that can make a beautiful piece of terrain far less useful. I've got a number of buildings on my terrain shelves that look cool but aren't that fun to game on because they just become big blocking pieces of terrain. Like square cliffs.
Big non-playable buildings are especially frustrating when you are playing skirmish games or role playing games. Having interiors can be a lot more dynamic and fun when you are doing small scale battles. If the heroes duck inside a building you can just open it up and continue to play rather than move to a map or try and just position the minis on top of the building.
However even in larger scale battle having buildings that can open up for play can provide for my interesting situations for snipers, scouts, and skirmishers.
There are some things to keep in mind when designing playable buildings. For one thing you need to make sure you can get enough miniatures in them and that the interior details and furnishings allow enough room for a reasonable number of miniatures to move around in. A good thing to do is to keep most of the furniture up against the wall in smaller buildings. Doorways should be larger than is realistic to better accommodate based models although sometimes you will just have to pick a model up and place them on the other side of the door if the have too large a base.
Multi story buildings get pretty complicated. The upper floors need to be more simply furnished than the lower floors because you will have to move them around more. Also on very large buildings you may want to only make the lower and upper floors playable or perhaps every few floors. It can be assumed that the floors in between are of a similar layout.
Remember to keep your construction simple and durable. Playable buildings are going to be handled a lot more and they aren't going to be as supported as a non-playable building. Furnishings and interior details need to be small enough that they don't hinder play. If you keep your interiors simple you can change out finishing to change the look of the building. Wall details can be held in place with sticky tack as well as floor details like rugs.