Add a stairway from the floor below

[Vuemme43]

My apartment is on the top floor, how do I enter the stairway from the floor below? Is it possible to add a level below the zero level?
Thank you

[bdfd]

My apartment is on the top floor, how do I enter the stairway from the floor below? Is it possible to add a level below the zero level?
Thank you

two choices with same action, 'add new level' :
1. top floor is on the ground and floor is under the ground with negative elevation (like cellar or a garage) => 0 & -1.
2. top floor is in height and below is at same level of the ground, so you must elevate the top floor => 0 & 1.

[:)]