How to make basement of structure spawn under the surface part of the stucture

I had a question yesterday with size i solved it. how ever there is another problem. I need to tackle the problem of how I'm going to make it so the part of the structure on top of the ground and the basement/ground is underground for example if you have a house with a basement and make it a structure how would you make it so the house part is on top and the basement is bellow.