That's just the thing, you can't see the actual boundary of structures from the build preview. Often times I want to make a barricade so that it mostly blocks a door (and appears to completely block it) but I can still squeeze past. Since the bounding box is larger than the build preview I often have to destroy and rebuild the structure until it's right. I know this is pretty minor, but if the build preview included the actual boundary (just as a flat 2d square), or you could cancel a structure before it's completely built (without the timer penalty) then it'd be a lot less annoying.
If not, I guess I'll just get used to it.