However it also reduces the incentive to reapay mortgage capital at all (is that bad?)
Currently if you have £500k in cash, you can either
1) Buy a house and pay no rent
2) Pay £1500 a month in rent
Why would you pay £1500 a month in rent (which at a marginal rate of 40% means having to earn £3,750 a month, or £45k a year, extra in income.
Effectively you get a 9% roi on that £500k by buying the house (ignoring house price inflation). Someone with £500k in cash and £50k a year is better off than someone with £0 in cash and £90k a year, despite the latter being nearly twice as valuable to the economy.
Now in a situation where £1500 a month is tax free, you'd have to earn £18k extra a year, and that roi on house ownership drops to 3.6%, and that's good. Housing should be somewhere to live, not an investment.
Of course on the flip side, the extra money available would likely push private rents and house prices up anyway. Perhaps the solution would be to ringfence the interest into some form of long term savings (either to use on buying a house, or to fund a pension).
The flip side might be true, but I think it's not as easy. I have a meeting now, so I can't go into detail (and not think about the details; ergo you might still be 100% right!). But if private buyers - those who would rent - can more easily compete with companies on buying, companies need to make renting more attractive. But maybe that effect is negligible.
I want to see this equalized - either allow humans to pay tax on profits only (i.e., they can deduct all expenses of their living), or tax a company's revenue like how a human is taxed.
60% of Australian landlords claimed a rental loss in FY17/18 and that's not an outlier. [0]
In jurisdictions where rental losses can be written off against ordinary income, landlords are happy to collect the tax benefit and just do the capital play.
[0] https://www.theguardian.com/business/grogonomics/2020/jul/21...
Although I don't know about Germany specifically, I suspect this is exactly the position you'd be in if you followed through with this plan - the company would have to pay tax on the rent. And you wouldn't be able to get around this by reducing the rent say to zero because it's a related party so the company would likely be taxed on an imputed market rent.
Now of course if the company pays e.g. 1000 Euro per month for the loan(s), it has to charge the tenants (=me & my SO) more than that plus taxes. The question is now if the effective tax rate can be pushed down to zero% by deducing the interest. As for the companies earnings, maybe that money could be "reinvested" into the house (better WiFi), so that MIGHT just be as good as putting some money aside. Or you actually buy another house for renting out. Obviously it's a good idea to talk about this to a tax advisor. For me this is only a Gedankenexperiment ;-)
If you buy another house to rent out that might prevent this. But the other thing as well is who benefits from the houses? Because now your friend will own the house you like and vice versa. You probably cannot put in any provisions to ensure your friend will at some point get ownership of his house.