Ah, I understand. The more info you provide the better we can try to help you out 
So, what you try to get is a modular building? You determine the size by the means of e.g. a bounding box and the blueprint populates that with meshes. At first only wall meshes but in the editor you want to be able to go to each mesh and say: Hey, you wall, I want you to be a door. Or a window. Or something like that.
Ooooor do you want to get the building generated completely procedurally? So that the doors and windows are being spawned at reasonable positions automatically?