User blog comment:Mathmagician/Patch 1.1 - Testing NPC Housing/@comment-4694163-20111220065225/@comment-4674838-20111221041555

I take back what I said earlier =P. I just did a more thorough check and it turns out that inactive stone blocks DO NOT count as walls or entrances, like I originally tested in 1.1. To verify this, I made a new world and made a single NPC house out of wood, but trying to use 2 Inactive Stone Blocks in the wall. I got the message "This is not valid housing." Replacing them with wood platforms made the house suitable and NPC-valid.

The reason it appears that Inactive Stone Blocks can be used in a valid house has to do with the rest of your house. For example, if you're in a large mansion (many NPC rooms, possibly some hallways, etc.) and you just replace a door with some Inactive Stone Blocks, it can still be valid because something else is counting as the walls and entrance now.