How should I detect horizontal and vertical surfaces for AR?

I just got the Vuforia plugin, which looked like a good place to start but it seems to not be exactly what I am trying to do. I don't want to recognize pictures and things from the video and place them in 3d, I just want to recognize any solid surface that is parallel or perpendicular to the ground by the distinctive way it moves.

I have a fuzzy idea in my head that starts with tracking 2d motion of lots of points in the camera view that are recognized as being the same from one frame to the next, and their screen space movement vector ( streaming video compression does this, doesn't it?) and then finding if any group of points are moving in the same way they would if they were on a level floor surface or 90º vertical flat surface. Then extend the planes to the corners where they all meet and eventually make some kind of voxel octree of the walkable volume of your house, that would get more accurate and filled-in each time you walk through it...

Is there already something I can find for free or cheap that does that- just the first part about plane detection from video? If not, what do I even start reading to understand how to code such a thing myself? I can already tell it's not going to be pretty.