Straight to the point. While you are in housing editor mode, there are 2 options - normal mode (lists are contain real items, you own) and virtual mode (lists are contain ALL furniture in the game).
While in virtual mode, you can turn your fantasy to max mode and do whatever you want. then save it as template for future.
Notice, that list, containing all ingame furniture also has crown store furniture inside it (so this good point for Zenimax to make some more profit - ppl can really see and taste what they are going to buy, also they may find something new, they didnt know before).
Observing your newly created room, construction, whatever..., you will know what you need to make your plans real (furniture piece, or plan).
Such game option may be done by developers themselves or just by adding API, allowing to see virtual items, rotate them, etc...