I'm dinking around with making a house and I am considering using an existing in-game house, editing the contents, and then saving that as a new house (and then setting a new location for it, of course).

Obtuse question: can I do this? For instance, if I wanted to make an Ebony Claymore, the simplest thing to do would be to edit the Umbra Sword file and save that as a new object. I know houses are a bit more complex, due to all the objects inside of them, but does the same basic rule apply, or should I just stick to making one from scratch/copying an existing one piece by piece?

Yes you can do this.
Just remember that copy/paste objects retain any scripts and ownership that they might have, so you'll want to remove them after you paste the objects.

Gotcha, thanks