Thank you! This is such a gorgeous house and I absolutely adore the sloped roof

Very nice job.

What a lovely house!!! Thank you for making this

Hey, this is a ridiculous question, but where did you learn how to do split-level stairs? I've been playing the game for ~seven years but I'm finding no good (updated, from this century) tutorials and lots of dead sites.

It is not that hard actually. I have kind of taught myself though, and I'm no good at explaining things... Its all about the "boolProp constrainFloorElevation true/false" cheat code.
As you can see in this house, the walls around the stairs are sloped. It is because the walls on the very first story (where the side door is) are considered at the same level as the basement, so they have to be "linked" (hence the sloping : you have one side of the wall at basement height and the other at normal height, to be able to place the door). When on the first story (where all the rooms are), you can see the split level of the stairs. Actually, it is considered by the game to be on the same floorplan as the regular floor (eg. in the rooms), but is is distorted. If you had put floor tiles, they would follow the walls and be sloped the same way.
And finally, the small portion of walls on the split level (where the stairs "turn") is considered to be on the same floorplan as the walls in the other rooms. Thing is, it has been flattened (with the same tool used to modify the ground) and reduced to the other normal walls' height (otherwise, the top of it would be sloped just like the walls on the very first story).
I hope I did not say things you already knew..