depends on which country you are living
Sometimes ground floor == first floor.
Anyway, the last floor is where the bedrooms are (Sophia's balcony), the ground floor is where the kitchen and other rooms are.
The floor in the middle, the one with the balcony and the stairs giving on the pool should be the one left unused.
I think in the game he's considering 1st floor as the ground floor. I don't know what's the convention in Germany or if he is using the US convention. I always get confused!
When I was dreaming of the ladder thing I was pointing at the middle floor