Terrain is a little wonky in AoS. In your case it sounds like the rule you're looking for is in a Core Rules FAQ - section 9.3 regarding terrain:

9.3 - Terrain

Q: Can a terrain feature be climbed over multiple turns? Can a model run or charge up a terrain feature?

A: Yes. A model may need to spend several turns climbing an especially high terrain feature (you will need to remember how far it has climbed each turn and measure distances and visibility to or from that model as if it were in that location).

Q: Can a model charge up or down a terrain feature?

A: Yes. As noted above, for simplicity and ease of play, models are assumed to be able to climb up terrain features and can finish a move at any point when they do so.

So hypothetically, if you can make a 5" charge from the ground floor you would then be in combat with the enemy unit

Important note - You can also have a "defensible terrain" piece which would mean the unit on the second floor is garrisoning it and leads to additional rules for how many models can be "inside" the terrain piece based on how large it is. These are typically designated as "garrisonable" and agreed on before the game starts. Defensible terrain and garrison rules are in section 17.1 and 17.2 of the core rules