Thank you all and thanks for the code
But I can't really use it, because my game is a FPS and strategy mix and the only camera option is first person and there's no cursour
But this is not my problem. In my game, the building should not be at the mouse cursour befor you build it, but it is in front of the player I also can check if the building is in another building and it becomes red then.
My only problem now is that i have not really an idea how i could check if the building is in the map(wmb file, i don't have any terrain in it yet). How could that be done?

Edit: Oh, there was a german word