You're only gonna get that big impact one time, but it goes to show that the tax was worth doing.
Doesn't solve the problem by itself, but the tax will help.
The amount of units that suddenly appeared showed that there was a small percent of units "leaking" from every new build that never hit the market.
Does SF have homestead property tax exemptions? Where I live property taxes are quite high, unless the property is owner occupied. The second homes and rentals subsidized the taxes of those who live here.
Any property tax, from vacant lot to strip mall to vacation home to industrial plant, can not rise above 2% per year. Meanwhile property values rise 5%-10% per year. The only way for property to get reassessed at market value is to build on it or sell it.
This subsidizes speculation. If you own a vacant lot since 1970, and pay an effective tax rate of 0.05%, and building on it raises your taxes to 1%, you're taking a huge risk by deciding to enter the market now. Similarly, that super old 6-unit apartment building from 1930 has a ridiculously low tax basis; upgrading to 20 units hugely increases taxes.
Presumably, the building company would be the owners for all the empty apartments which would make it easy to find and tax. It would be more difficult to detect someone buying a home/apartment as an "investment".
For example, obviously a smoke alarm is something that takes 15 minutes to install, so that wouldn't be when you start the clock.
You might be able to base it on when any single unit is sold in the block. That is "You sell 1 apartment, you've got a year to sell to rest or you are going to start seeing vacancy taxes".
That isn't the reason rent is high in SF.
See ‘homestead exemption’. Other states use this term, but it means different things.
WV, for instance, only allows the exemption for people above a certain age.
Further, the revenue from that tax could be used to fund public housing to further drive down housing costs.
[1] https://www.bookweb.org/news/san-francisco-approves-vacant-p...