So you mean plop a plan down and then edit before build? I don’t think you can do that, you can only add items such as windows, doors, and beds. I think once you have it down, since you didn’t add walls and roof you cant remove those. You can change the color and style of them, that’s it. Maybe later in Alpha Builds you will be able to do that.
So make sure to build a custom building the way you want it before you save it. Or you could just build a floor plan and save that.
I know what your wanting to do, and I get it. Like build this hallway and then maybe a room. Then add the two together. maybe later when they might add that.