New Empty Units

I've always wondered why cities didn't put an extra tax on empty apartments, in which case the whole thing becomes an easy win-win. 

I don't know if it's important to your argument or not, but you're not necessarily dealing with ordinary speculators.

Rumor has it that a lot of sales of vacant urban units, especially luxury units, are to (and/or among) money launderers. That means that (1) much of the value of a unit doesn't come from its utility as a place to live, but from its acting as a cover story and/or a quasi-currency-value-conduit, and (2) the buyers actually expect to take some losses. In fact, in some cases overpaying may be the whole point, because it lets the buyer transfer extra money to the seller in exchange for something outside of the visible real estate transaction. There don't have to be that many launderers out there for them to represent a big proportion of the liquidity in the market, because they tend to turn properties over a lot, and they either don't care so much about overpaying or they actively want to overpay (which then distorts the price signal for everybody else).

Even the launderers can only absorb so many units, but I imagine they may contribute to the markets staying more irrational than you'd expect for longer than you expect. Supposedly this also applies to fine art.

Matt Yglesias on this very topic (broadly):

• The “induced demand“ case against YIMBYism is wrong - Slow Boring

I really like this bit:

What YIMBY is

Last but by no means least, this entire conversation is based on a misconception about what the YIMBY proposal is.

The way high-cost metro areas currently work is this:

1. No building is allowed in desirable inner-ring suburbs.
2. No building is allowed in the most expensive parts of the city.
3. Along a relatively tiny gentrification frontier, a handful of politically well-connected developers can get permits for projects.
4. Because the permitting process is highly politicized, various activists, neighbors, and politicians try to shake the developer down for side-concessions.
5. Prices go up and up because this is just way too little new housing to meet demand.

Because leftists like “activists” and don’t like profit-seeking business people, they focus on (4) and see the YIMBYs as siding with the bad capitalists against the virtuous activists.

The actual YIMBY proposal, however, is to change steps 1-3 of the process so that by-right construction is possible throughout the in-demand parts of the city. Actual developers do not lobby for this change, because even though the developers like to beat the activists in step (4), the whole market niche the developers occupy is built around their expertise in navigating the permitting process. You don’t need a “developer” to convert your garage into an ADU, you need a contractor. And if you drive out to a rural area, there’s no such thing as a “developer;” there are just housebuilders.

The YIMBY is anti-anti-developer in the sense that he does not think that obsessively harping on the bad thing that Developer X did with regard to Project Y is a constructive approach to the housing supply. But YIMBY is not about individual projects, neighborhood-level price impacts, or siding with developers. It’s about a systematic approach to making decisions about housing policy, regional housing abundance, and creating a construction boom that makes The Developer a much less relevant character to metropolitan housing supply.

And last but by no means least, while YIMBY is about housing costs to the extent that it addresses the absurdity that middle-class teachers in Silicon Valley can’t afford to live anywhere near their schools, it’s not a comprehensive solution to poverty.

Not only is this obviously true, but you can do more direct analysis.  At a given price-point, a unit of housing is affordable to n residents in a given city, who demand m units of housing.

If there are greater than m units of housing in the city at that price point, that price is unsustainable.

Eventually reality will reassert itself, especially as empty units of housing have a cost of interest, maintenance, and property taxes.  An obvious counter-move to empty speculative units is to charge a greater property tax on them.  

To assert that a policy of 'build baby build' would not eventually result in cheaper and more abundant housing and office space is like stating that allowing the toilet paper manufacturers to produce vast amounts of new stock won't alleviate shortages and high prices.  

It's simple ignorance and a rational system of government wouldn't even consider this as a possible policy.

Robinson's example is off:

Ok, a tenant lives in a unit.
City A demolishes the old 30-unit building, builds a high-rise 100-unit building instead.

30 old unit tenants get evicted, since their original 30 units need to get demolished. 
20 new rich families move from their old units to new units from within the city.
Their 20 former units in the city will then be on the market again, available for someone else.
So far, the new building has caused -10 new available units to city-dwellers so far.

30 more people move into the city, but wouldn't have moved there, if this new building did not exist.
So we might say: they are not adding more unit-space for the poor local residents.
And in our little model it does not.
We are still at -10 units supply impact of this new development.

However, those 30 other tenants must have come from somewhere, let's call it city B.
City B will now have 30 more units on the market.
So the problem is now a prisoner's dilemma.
If city A does not impose any restrictions on doing the 30->100 switch and city B does not do it, 
then it'll all average out to +20 new units on the market, for each city.

Now for those remaining 50 units owned by people, who don't live in them.....
uhm..... yeah I dunno. 
The issue is framed as rich people creating an externality for financial gain.
But how could owning a unit, that is not lived in and not rented out to other tenants be profitable, 
if building supply in general is not restricted?
This sounds like the issue is only caused by restrictions like this in the first place.
Even if not, those same 50 rich people would presumably have a need for fifty units in city A.
So unless there is a law that prevents them from buying in units in city A, not building the 100 unit high-rise would still be worse, since they'd presumably just buy 50 already existing units instead which would be empty.