Hi, I am about to build a wall in the living room to extend the door into the hall. I don't want to build a stud I want to build it out of block. Can I just remove the 2-3 inch of screed and the small sheet of polystyrene and then lay the new block work directly on top of the sub floor? I was wondering if any wall needs a foundation underneath other than the sub floor. If not it will have to be a stud wall.