You have several buildings on the same plan with a different number of floors.
1st building: 2 floors + roof
2nd building: 1 floor + roof
Go to 'Floors settings' to add a floor to the 2 buildings:
Create a floor:
To then add a 2nd floor to the 1st building, you have to create a floor by unticking "duplicate walls and windows":
Draw the walls above the existing walls of your 1st building by going to the "Build" menu:
The plan now has 2 buildings with different heights:
To add the roofing, you simply need to position yourself on the 2nd floor to create the roofing of the 1st building and on the 1st floor to create the roofing of the 2nd building.