Let's say you buy a house at 500K with a 400K mortgage. House prices fall 50%, you owe 400K on a 250K house. Banks would repossess as the loan value can't be guaranteed by the asset. Separately, you could default on the loan if laws allow it, losing much of your wealth and making it impossible for you to get a loan for the 150K to move to another house. You could keep paying the loan, spending many years of income for exactly nothing in return. People tend to buy another house immediately, so there isn't that much "freed" money. So either banks lose a huge lot of money, or people do. Either way, massive financial crisis. That's an extreme case, adjust for lower fluctuations. I can be, and probably am, completely wrong about this.
In which country can Banks reposess if market value falls? In Germany that risk is 100% on the bank. No reprocessing without you failing to pay anyway.
I misspoke - you would not be able to remortgage and would need to repay the loan when the term expires, or be repossessed. In the UK, most people are on a 2 or 5 years fixed-rate mortgage, after which you don't need to remortgage but really want to, because the variable rate is typically a lot more expensive - so you're throwing even more money into the hole.
