I manage to do this without cheats, and pretty quickly, and I don't consider myself to be talented builder. It might be a problem to built this in already built home.
First, I lower the terrain in about half level of walls, placed walls and floor tiles on top. Then, at the edge of lower part I put another wall and floor tiles. Then connected those 2 with stairs:
Once you put floor tiles you can erase walls and flatten the terrain back, I did that and add a foundation part next to it and got this: