Ok. If you select the worldspace where you are trying to put your building, there's a World->World Spaces... dialogue. Look at your settings here:
This may be what you're looking for. Let me know if it isn't. I have another idea but I'm not sure how complex it will be. Essentially, I'm thinking that if you increase the elevation of your building overall, you can put transition pieces that will "collide" with the ground but your interior will be lifted so the grass bits aren't poking through.
Anyway, let me know. I'm still looking