The foundation ..
Our new basement walls are practically adult Lego's. They are glued together, overlapping each other, to build a hollow wall. The blocks are very light since they are made of a type of Styrofoam with plastic ribs which hold the steel Reebar.
We did not make a door between the two basement areas since the heights are too different and it would have been awkward walking into the old one, you'd have been ducking to keep from cracking your head on the door-frame.
The new basement area will be our bar/pub/entertainment room.