How do I put a house on a rooftop?

@shrimpworm Unfortunately a few things come into play in what you want that are barriers...

• You cannot place anything from the gallery on the rooftop except a room... Unless you are placing a penthouse, as it places like a regular lot - so you can either place a home there or a room. A penthouse is treated like a house.
• If you place a house or a build classified as a lot it will replace the entire lot. 
• You can only place, edit or save as rooms in apartments in the city.

Now I have done this the opposite to get a goal a bit like you are aiming for... If you place the home you want on top you can then do a few things to get it on the roof:

• Place the home in a large lot and move it to one side. Build a simple framework of the other floors you want to add. Then on the top floor move the home room by room like a puzzle to the top floor(s). (Not sure if the first floor ones will need to be saved as rooms first then moved in - I would have to experiment about that...)
• Do as above but save each room from the home you want on top into your library and re-assemble it later like a puzzle on the roof.

Afterward you can move  to a smaller lot if you like. Other methods would be more complicated to explain. Let me know how it goes.