Place a staircase before creating the second or third floor.

Once you’ve created the first floor and started filling it with furniture and other interior items you can place a staircase along with other objects. That way you’ll be sure you have spared a place for the staircase and won’t be needing to free the place for it on later stages.

In the Objects menu choose Construction -> Stairs. Pick whichever you like the most and apply it to your plan. You can change the height of the staircase in the object's settings by clicking on it. That would allow you to adjust the staircase height to the ceiling height you’ve set up.

Check out the video below how that can be done: