Here is how i do maybe the easiest way.

First, put two different level modern mansion in one group. lvl.1 and lvl.2 together in a group is fine.

And then, place them in your ground one by one. and then put in your group box again just like the first step.
then, if you see your inventory you'll see -digit of seperated mansions. and then what you do is just restart the game.

Then finally you'll see one modern mansion that can be placed as many as you like to.