It is not buildings, but apartments. And you completely la-la-ed your way around what I wrote and considered them ALL as long-term speculatively withheld apartments, while only about a THIRD of them are. Which means that only about two years' growth could be accomodated like this. Then, what? Given that the city has spent all its money on buying out old stock?

Zero unoccupied housing is unachievable anywhere but in a dilapidated slum, because apartments must get remodeled from time to time. If you remodel an apartment every 15 years and it takes three months (a rather optimistic timeline), you already have 1,6 per cent of the housing stock out of the market inevitably, because 3 months is 1/60th of 15 years. If anything, only 2 per cent of the Berlin apartments being out of the market indicates that they already don't get enough maintenance!

Then you have pauses between tenants. If an average tenant stays for 8 years and it takes about a month to fix the most basic things once he moves out, and find new people, then you have 1 per cent of the housing stock momentarily unoccupied just due to this. Again, Berlin having only 2 per cent of the stock out of the market indicates that the changing of tenants is already overoptimized, with not enough time to vet new ones and fix smaller things: one moves in on the heels of another.

Then you have a certain small percentage of apartments being part of litigations, either commercial or inheritance-related. Inevitable. Companies split, people die, and not everyone has their papers in Ordnung.

You seem to be claiming that all those mechanisms can be compressed to zero, and that all the apartments that are unoccupied due to them can be put on the market by fiat. How? Will you ban apartment remodeling? That's the way to produce - fully occupied - ghettoes.