I think the ~30K number comes from the American Community Survey, run by the census department. At present, the most recent numbers are for 2017, and list 31K "vacant" units. https://factfinder.census.gov/bkmk/table/1.0/en/ACS/17_5YR/D...
Combining apartments not currently occupied (including just transiently between leases), condos which haven't sold, AirBnBs, etc, I don't think this number is crazy.
I'd be in favor of a tax on unoccupied units, and it should be painful. If we want to change policy to build more housing stock to house people, surely that policy choice is less effective every time some speculative investor buys a house or condo, and chooses not to rent it because tenants will drag down their resale price.
I'd be in favor of a tax on unoccupied units, and it should be painful. If we want to change policy to build more housing stock to house people, surely that policy choice is less effective every time some speculative investor buys a house or condo, and chooses not to rent it because tenants will drag down their resale price.