Multi-part buildings

Let’s say I wanted to make a building with multiple parts to it, like the Hospital or Mansion. How would I go about doing it?
I’m already familiar with how to add a single building to CDDA, I’m just not too clear on how I’m supposed to go about making sure they spawn adjacent to one another. I’ve searched around some, with no luck. Any advice?

I’m afraid we don’t have support for this in JSON yet, you have to manipulate some stuff in the mapgen code to make it happen at the moment. Specifically overmap.cpp generate()