Chaz, I need to give you your house key. You'll need to be online at the same time as me, though!
The new system (selling ready-made houses via the player vendor) works much better, because I can just leave the key with the vendor and if someone's interested in buying the house, they have it within minutes.
As for expanding houses - no; you'd need to find a larger property that was for sale, buy that and (if you want to) sell your old house.
Who was it that bought Elendiel's old house, by the way? I'll need to change the sign outside it!