Subsequent demand for securities by the bank?

Keyword: "significant deterioration of financial circumstances"

This refers not only to income but also to existing assets, for example, real estate.

what exactly is a significant deterioration of financial circumstances? even if everyone in the house receives Hartz 4 benefits, the bank will not notice as long as the payment is made on time. this will hardly be a reason for termination either, since the bank cannot prove this significant deterioration. the borrower simply presents their proof of timely payment and states that nothing significant has deteriorated. end of story.
otherwise, the bank could kick anyone out of their house...

what is a significant deterioration of the financial circumstances?

If the value of the underlying property drops so much that even through realization of the security itself it no longer covers the remaining debt. This is derived from the Building Code 490.

Of course, Hartz 4 alone is not enough; as long as you are servicing the installments. But if the property value falls and you have a relatively high loan-to-value ratio, the situation looks completely different. Then the world looks very different.

I do not believe that the real estate market in Germany will collapse nationwide in the next few years; but if it does happen, it will be exciting for full financing and beyond.

Since banks have been burdened with Basel and other regulatory nonsense, they automatically have this on their radar in case of emergency.

A significant deterioration does not necessarily have to affect income, as already mentioned. It can also concern assets.

And here specifically the market value of the house (or other assets, e.g. securities, stocks, etc.) as long as it is a substantial part of the total assets.

If your assets amount to, for example, 600,000 euros, of which 500,000 euros is the house, and now the market value drops by 20%, then you can just about offset the loss in value with the remaining assets.

At 30%, I would speak of substantial.

"Significant asset deterioration" is an indefinite legal term; percentages are not found in the literature. But I would say: If there is not just a temporary noticeable loss in the total asset value and an immediate repayment (through liquidation of the total assets) is no longer guaranteed.

And whether the bank becomes aware of it or not... the borrower is obligated to report this if that is the case.

And @TE
You don’t need to read the contracts. This will be stated in every standard loan agreement or the T&Cs of any bank operating in Germany. You can save yourself the trouble.

Since banks have been blessed with Basel and other regulatory cheese,...

I am currently writing a thesis about that. About the regulatory cheese.

and what is the additional security supposed to look like if suddenly all houses in Germany are only worth half?! high loan-to-value ratios and large stock portfolios don't really go together. there may be some exceptions, but the majority are virtually bankrupt and cannot offer additional collateral. guarantors and the like usually won't be found (or want to be). so banks would practically have to auction off the houses forcibly, which would probably increase the loss even more. conclusion: in the event of a massive loss in value, very likely nothing will happen, since the safely paid installment is better than the immense loss through the sale.