How to make airport building on ground poly
From FSDeveloper Wiki

Hi,
Once you are done with the ground poly by this tutorial. You might wonder what is the next step. Yes! you need to build the buildings in the airport such as terminals, hangar. I think, personally, we can use the same file for the whole process. This tutorial is how to build the building using your reference plane and ground poly as the reference. I have made Beijing International Airport using this method and I think it is quite good. I would like to share this idea to anyone who wants to make big airport. It is also applicable to airport at any size.
Here are the tutorials.
How to make airport buildings.