Earlier quoted context omitted.
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…
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.
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.