Can you use CSG Subtract on terrain?

I know that terrain is a different thing than the models or blocks, however, I was trying to place some prefab huts and stuff on my level and noticed that I couldn't use the CSG subtract tool/option on the terrain model. I created the terrain using photoshop and it's just a random heightmap basically. It doesn't have places where I can conveniently fit building prefab models into the landscape. So I was going to cut holes for them and it didn't work. Maybe I'm doing something wrong? I don't have the terrain object locked or excluded or anything, I checked. I just wanted to confirm that I would be unable to use the CSG subtract feature on terrain models. Thank you in advance for any help .

It does not work for anything else than blocks.

but i can delete faces on you terrain ^^ if u want to make a cave or something...
or make a terrain whit brushes/blocks... then can u use the csg subtrack ^^
greetings: qwertzui.

or you could just paint at the heighmap, so that there is a solid colored area on it = flat

Thanks for your help. What I ended up doing was rebuilding the terrain with a big flat part in the middle and then repositioning all the buildings. Thanks for your help.

If you want to see the finished deal go here:
http://www.freewebs.com/therpgproject/apps/photos/photo.jsp?photoID=3833533

It's not much, but I'm just creating something to start designing a RPG with. I have another question now about using terrain objects but I think I'll just create another thread for that. Thanks again for your help.