Does that mean an authority would have to exist to prevent people from doing so or would this just be a culturally reinforced social norm (i.e no violence or coercion needed to enforce it)?
If no authority existed, you wouldn't be able to keep steady control over (several) houses you don't live in. The social norm would be the definition of self defense, is someone moving into a house you don't personally use threatening you enough to exercise physical force over him? And of course there needs to be an agreement about the difference between abandoning and leaving for a while.
are we imagining the abolishment of money as well?
When you plan to move out of your awesome house, and someone else wants to live in there, you can of course ask for an exchange. Money is a good tool for that. And if he doesn't have the cash, you're free to make a deal with him, this involves third parties and mortgages.
No, voluntary exchange is great. But you wouldn't take a mortgage for a house you wouldn't use, because there would be no authority helping you to throw out potential squatters :P
The services a good landlord provides can probably, with minor modifications, be a business of their own.