Of course you can't do that in-game.
I'm lazy to build mods so this post shows existing mods but not how to make them.
Remodeling of homes:
It SEEMS they take some interiors from other places, then put it together to increase the size of some owned homes you have.
ALL IN ONE BASEMENT
http://tessource.com/files/cache/4326.html That is a wicked idea to add a common basement to any home you own. It says its common so, technically,when you enter that basement from any home in Cyrodyll, you reach your same basement. Fast travel enabled.
Benirus Basement Expanded
http://tessource.com/files/cache/1899.html Extends and renders Anvil's home basement useful. Has a secret passage that leads outside the town.
Nothing of outdoors modding of existing buildings is found.
Decoration:
I made some research just now and came up with only one thing (see bottom of this post). I remember of this I found on controlaltdel like a guide to interior decoration:
http://eqiiforums.soe.sony.com/eq2/board/m...message.id=3172but its not for TES. Well we can argue that TES looks better but its not customisable without mods.
WELL HERE IS the one thing I liked which allow many items to be materialized into stuff you can move around (candles, etc):
http://tessource.com/files/cache/4703.htmlDidnt have time to test it much.. let us know your findings and discuss further.