But when you have a base with several buildings on and the movement trays don't fit in the gaps between the buildings then it can get tricky.
Yes. That's when I would say the unit loses cohesion. In a block game, the unit would not enter. In Warhammer, you can pretty easily move individual models and play with them in skirmish manner. No rank bonuses, but otherwise the rules bend easily. (Think 6th ed skirmish.)
Also the scale issue. Let us assume you have a building and a movement tray both the same size.
But the unit on the tray represents a regiment of 500 troops, even though they both occupy the same footprint there is no realistic way that small cottage takes up the area of a regiment of men.
So do we assume the cottage is in fact a BUA of a village and we just use a single building to represent the whole BUA.
Personally, I think one model is roughly ten combatants. This has been stated as a design principle from the 3rd ed to the 6th ed, I think.
Then it logically follows that everything on the 28mm "scale" gaming table actually contains ten of what is present. So even a small cottage can be a hamlet, a farm, etc.
Or, if you imagine it's 1:1 scale, then each house is one house.
Both are fine.
Or as many people do, should we use smaller scale buildings so we can have what looks like a village taking up the same space as a regiment.
It all comes down to aesthetics. In wargames, buildings tend to be small and cramped compared to the figurines. Sometimes the "footprint" of the house is small but the height of the house (and the doors and windows) is in scale with the 28mm figurines, so that you get very thin and tall houses. It's also a fantasy art style. Makes good photos, both from the bird's eye and from the level of the 28mm eye. It's not very realistic, but works well enough.
Hope that makes sense?
Yes, all of it makes perfect sense. I guess it's your aesthetic taste and level of realism which must decide the issue in the end. I wouldn't have a problem either way (28mm buildings or smaller scale BUA's), as long as the interpretation/rules are stated beforehand.
-Z