Doesn't really help getting more affordable flats on the market.
I'm not sure how familiar with the London property market. London is currently seeing a large amount of real estate being built, mostly studio, 1, 2 and 3 bedroom. Hardly luxurious. Prices are commensurate with London income levels. Otherwise people wouldn't live in London.
You also can't arbitrarily increase density of a city as you need infrastructure to go along with all those people. Not to mention that Berlin has many old buildings that are under protection, so you can't just demolish buildings and replace them with skyscrapers.
I feel like most people are moving to the outer zones. Buying flats in zone 1-3 is obviously out of reach for most people, even for most developers not working in The City. From what I've seen, even 1-bedroom flats cost at least £310k near Seven Sisters (Zone 3).
I have to admit though that I was mostly looking for flats to rent, not for sale in the last few months so I'm not an expert.
I don't really think this has much significant impact on the housing shortage and pricing there, but some part of the housing stock there is owned by people who don't actually live there, and who can afford to keep it vacant just in case they need to move in one day.
Most buyers of such property (whether private or investment funds like Deka-Immobilien or BlackRock Global Property or Brookfield Property Partner or one of the many others) do this as an investment vehicle with a particular risk/reward structure. They will absolutely have to rent out these properties, and indeed, when they fund property construction projects, as they have been on a massive scale in London in the last decade, they do so to cash in on rent.
£310k near Seven Sisters
That's not expensive relative to London income levels, given that property is typically financed over 25 years or so, basically over the most productive phase of one's working life, so it's paid off roughly when one retires. Most young people don't think long-term: they just look at the price and their current annual salary, and see a mismatch.Maybe it's good this way, because unless you realise that a property is a serious long-term investment, and that the target is to save for retirement, they probably don't have the financial maturity to make such a large purchase and should stick to renting.
Note that property always reflects income-levels of the working population. That's because they are the ones that buy / rent property. Rent and property prices are strongly correlated, because it's fundamentally a risk/reward trade-off: when you buy you take on a fair amount of risk: heating may break down, the roof may leak, you might not find tenants in time, tenants may ruin the property, not pay rent etc etc (From bitter experience I can tell you that "may" really means "will"). Tenants don't carry this risk, hence need to compensate for this risk transfer. That's why renting appears to be more expensive.
Focussing on foreign billionaires who buy-to-leave-empty distorts how the property market works. The real question is: what percentage of their income is the average person willing to invest in property / rent. This is remarkably constant.
I would imagine that a much bigger issue is old people who are in a care home or a hospital, unable to live in their old place, but not willing/able to give it away either.
At least over here (Finland) this is the major reason for "empty" apartments in central areas of cities.