This is how you change your Z-axis:
First you find some item that you can walk when it is on the ground, but when you put like 5 on top of each you can't walk over it.
Then you start dropping one by one until you can't walk over it, when you reach that number ( probably 3 ) , you remove one and step over it, you have your first step of a ladder. You Z axis may still be the same, thats why you keep doing it until your Z axis is high enough... inside a house you can test it using the leveling tool....
My memory fails here, but the first item I would try would be Damaged Books ( Doom Rare )
Regards,
Mugs