I'm pretty far in the game and have only been able two make two room houses.
And I think once you finish the handbook you stop being able to unlock new features.
Thats just my experience so far though.
To get three rooms you have to go back and redecorate the villagers house by selecting "new house", which then gives you the option to add/expand rooms. When you're designing a house for the first time you can only have up to 2 rooms unfortunately.
Three is the maximum by the way with the 3rd being a basement or attic. After that you can't add anymore rooms no matter how many times you redo the house for that villager.