When I moved in the main center area had old rotten plywood down, but the slatted ceiling areas on both sides were wide open with only old blown in insulation in between the main floor ceiling joists.  One my first tasks in the attic was to replace the rotten center flooring and then add flooring in the entire attic.  The areas under the slatted ceiling on both sides were then used for storage.  I had some day planned to finish the attic and attain a view, but thought the basement had a bigger space to work with, so I started down there.