Transparent/Disappearing Walls When Entering Room

I'm in the basic planning stages of a 3D isometric survival/stealth game and I wish to have a system whereupon the player enters a room/house and the walls disappear to about 1/4 height.

I was also hoping to have this be room dependent such that the player can be in a house, but one (or more) rooms that have not been 'discovered' yet are blacked out with the walls still intact until the player enters that room.

One more thing is that I would like to have multiple-storey buildings if possible, but I can see how this would be more difficult.

So my question is, how do I go about this? At this stage I mainly just want to build the environment so that this would be possible later. I was thinking that I would have to make the walls in two sections. For example, I would have an upper and lower section of the wall that are both fully rendered when the player is outside. Then when the player enters the house, the upper section of the walls would be rendered transparent.

I should also note that I'm new to Unity, but I have some experience with C#

Here's a basic drawing of what I wish to accomplish (please don't make judgements on my artistic abilities based on this =P)