I think if you kill someone, everything that they owned becomes yours. I would like considerable consequences for this as well, though. But, yeah, I don't want to kill someone and then have this abandoned home just around.
This.
Every unowned house can be claimed. Some claims would be easy, some would be even free, others would be contested (You want to claim the king's castle just because someone slaughtered everyone inside one dark night? Well, there's a bunch of nobles and their armies who have a different opinion ...). How and when the houses become unowned is not important here.
Also, new houses should be able to be built, and old ones destroyed.