"Some larger layouts are only available after rebuilding a home from another layout, such as layouts including a second floor or basement. Make sure to keep picking the ?A big house? option until you see the layout you want. You may need to rebuild multiple times through increasingly larger layouts.
Below is a list of all the room layouts that we?ve seen in the game. Again, you may need to rebuild multiple times using the ?A big house? option to access the larger ones."
You can also unlock the ability to choose your floorplan by studying the "handbook." When you sit down at your desk to write your report, you should specify that you want to read the handbook. There should be a lesson called "Choosing your Layout" or something like that, and it costs 5 PlayCoins.