Reverting an extruded plane

I’m trying to build my house in Blender as a learning experience. I’ve extruded my planes on the Z axis but mis-selected a few items. Unfortunately I’ve since saved, closed, and re-opened the project. How do I take a plane back down to nothing?

Select one of the top faces in face select mode, shift+G -> co-planar. Then either view further away and mouse cursor way below the whole thing, G twice to vertex slide, or select one additional face from the target level, active element pivot (alt+period), and scale to 0 along Z (S, Z, 0, enter).

Select all, W -> remove doubles.

Or:
select plane
“CTRL +” to select neighboring faces
press “x” and select “Dissolve Faces”

Thanks, JA12’s worked for me. Motimo’s probably would have also worked but I didn’t see his reply until too late! :smiley: