I want to take down the walls of my house. I extruded them and saved, but now I can't control+z to fix\change anything. If I want to take all my planes down to zero as if they were never extruded, how do I go about doing that?
If you're just talking about ordinary planes being used for walls, or even 3D walls with thickness, select them each in turn, goto Edit mode, select the upper vertices ONLY and press X to delete them.
You might find it easier to switch the view to 'Wireframe' beforehand.
The floor from which they were extruded in the first place will remain.
If you're going to need to replace them again, just drag the vertices down to near 0. Their texture (if any) should return when you raise them back up again.