It sounds nice, but the real question is why are people letting these building sit empty. There is probably a good reason for it. Maybe some renter protection laws that went too far?
* Real estate investors can borrow against the long term income of their properties, and often value that above the short term rents. They also tend to be highly leveraged to build the next project. This creates situations where high vacancies and poor returns are worthwhile because they can pretend they'll fill the building at their price eventually, and therefore borrow against the building as though it would return that price X all their units X however many years.
* A lot of people seem to think (I haven't dug into the true scale of this) that enormous amounts of money from China is being parked in Western real estate as some sort of protection against seizure by the government.
Therefore, the owners have little imperative to offer the properties for rent, with all the associated hassles, to meet their limited expenses. The properties are often poorly maintained for this reason.
They can instead treat them as stores of value, which they might even be able to leverage into cash flows via other financial instruments.
I think this is more the case for commercial properties than residential, but the problem exists to a degree in the residential market also.
In SF, rent control resets to market rates automatically when a tenant moves out, and doesn't apply to single family homes or condominiums.
An unoccupied apartment can be leased at market rates.
Here in Spain it takes months, even a year to get a judge to evict a tenant that has stopped to pay rent. In the meantime, you need to keep paying for utilities or you will be fined, not the person not paying their rent. Even when you get them out, you could find a destroyed property, just for the sake of it, and you have to pay for the repair work. So just one stroke of bad luck with a tenant will make you lose years of profits.
The laws are extremely weighted in favour of the tenant: there is a minimun contract of five years, that the tenant can broke just telling the renter a month before, but the renter can't. Just a few weeks ago, the government approved a new law so if the renter is a company and not an individual, the minimum contract period is 7 years, and in this time you can only rise your price to keep with the official inflation index. So basically investors are tying themselves for seven years with these contracts. Of course they have the option to sell, but selling a property with a tenant that has the right to live there for years with a fixed price is difficult, so you'll need to sell for a lower price.
At the end all of this regulations makes the renters very careful, so they prefer to have their properties empty or wait months to find a suitable tenant. And that means that the price is higher and only people who can show they have an stable high paying job are able to rent.
The agency parasites are the worst - charging 10% of the annual rent just for giving you the keys and printing a copy of the contract.
I was fortunate enough to be able to rent my flat from a family friend, who was afraid of okupas if it was left empty.
That said I live far-out from the centre (which I prefer) and it is still much more expensive than when I arrived just 4 years ago.
The real reason is to economically disencentive land speculators holding uninhabitable land, who were planning on flipping it without developing it after several years.
The real reason buildings would stay vacant is either ownership attempting to charge above market for rent, or land speculators who are looking to hold and flip, without developing it enough so it can be used in the meantime.
B) Buy another property: sit it empty
C) First property rent is higher than the rent of 2 properties
The useful information that high priced vacant housing communicates to me is that there is an anticipation of significantly higher future demand that isn't satisfied by current supply. That should signal for others to build more housing, as there is a lot of money to potentially be made. Or it could be signaling that there are too many things hampering construction of new housing.