[close04]

But if "empty living space", space that's reserved but unused, is immoral would you extend it to any such reserved but unused living space?

Is an empty room in your house immoral? Or massively oversized rooms even if you live in them? In the end you're still blocking a lot of "empty space" that someone else could use if only there were smaller but more living units. Same applies when you live in a detached house and "blocking" any potential living space that could have existed on higher floors of a tall apartment building. Zoning laws can also be an issue but the question stands.

To put in in practical terms, one person having two 50sqm/550sqft apartments is immoral. One person having one 150sqm/1500sqft apartment is fine? Where is the line and how arbitrary do you want it to be?

[foldr]

It’s not clear exactly where to draw the line, but that’s true of all kinds of moral distinctions. So I don’t think that kind of slippery slope argument is very persuasive.

[close04]

I didn't mean it as a slippery slope argument, rather an explanation of why calling "this in particular" immoral is probably just one person drawing the line in such a way that what they need/want is perfectly covered. It's more likely that their changing needs/wants move that bar, than that the moving bar changes their needs/wants.

So "buying a house and purposefully leaving it empty is immoral" is bound to change the moment they purchase a second, empty house.

Years ago I hosted some African refugees in my home for a short time while more appropriate accommodation was in the works. I cannot describe the feeling I had seeing one of the children understand how a modern toilet works: we do our business in a bunch of clean water, and then dump a bunch more clean water to take it away. Given their circumstances this was probably the most immoral thing we could have done in the modern world.

[foldr]

>It's more likely that their changing needs/wants move that bar, than that the moving bar changes their needs/wants.

This seems to be an unsupported speculation. And indeed it cuts both ways. Maybe you only think the second house is ok because that covers your needs and wants (either present or future anticipated). If you leave out the psychologizing, there's nothing to your argument beyond the slippery slope. Even if the exact location where individual people draw the line is psychologically explicable based on self-interest and their particular life circumstances, it still doesn't follow that a moral distinction cannot possibly be made.

More generally, it's fairly obvious that people who believe in strong property rights will tend to be people who have lots of property and people who don't will tend to be people who don't. That doesn't invalidate the arguments of either side; these have to be evaluated on their merits. You can say "I bet you'll believe in strong property rights once you own a house!", while a homeless person might equally say "I bet you'll have more sympathy for squatters once you've lived on the streets for a year!" Those sorts of examples tell us that people aren't perfectly disinterested when forming their moral outlook, but it doesn't tell us much about where moral distinctions can or can't be drawn.