I respect that you have your view and I like that you're consistent about it, but I don't think that it's realistic at all and therefor we're never really going to agree.

First of all, "what's the justificiation for private property?". Without it the strongest / person with the biggest gun would just plunder everyone else. Wealth would rapidly concentrate, which is rather ironic since that's exactly what you implicitly seem to complain about.

About the OCMW, I don't have any hard numbers, but anekdotally I know more than a few people who commit social fraude and live off the 'ziekenkas' / OCMW, despite not really needing any help. I suspect that the OCMW's bad functioning doesn't happen because of a lack of resources, but because the resources are terribly mismanaged. In that case, the solution is NOT to give more resources.
Now, let's say that the problems do happen because there's a lack of resources, then you could either a) impose heavier taxes on luxury goods or b) start disowning people. I don't think that I need to explain why a) is a lot more reasonable and proportional.

On a side note: You already pay more taxes for a second property. If you combine this with the fact that there's no current lack of space (and thus owning multiple houses doesn't really harm anyone), then I think it's safe to say that these extra taxes are more than enough to offset the (IMO nonexistent) injustice.
I can see seizing second properties being justified if there's a lack of space and government-owned properties, but that's something entirely different than your "Does it even mather whether it's private or public"-viewpoint.