We didn't want to spend the hundreds of $$ on the expansion floor, and were lucky enough to find a "parts" mansion for cheap, so here's what we came up with:
Instead of a "door to nowhere" we have a "bare spot in our lattice" which I hope to repair soon. (The piano makes a nice temporary cover.)
Our kitchen is now twice as big as the standard 5300's, our sun room is now moved to the second floor (more open lattice that will need filled or covered) and our third floor is a nice open "ballroom" or party room.
We really like the extra space and the openness of the third floor.
As one can usually find (beat-up) mansions for much cheaper than expansion floors, this seemed a unique and less expensive solution. (And the left over parts made a decent guest cottage.)
doug