Construction costs are currently skyrocketing

Of course, it's a conflict. On the one hand, you want to save CO2, on the other hand, it shouldn't cost anyone anything ;)
Personally, I also think the mandatory renovation in old buildings is nonsense. The same money is better invested in an energy renovation and continued operation of a gas burner in case of doubt.

Personally, I don't mind if it costs me something. We have also accepted the other measures and have already spent tens of thousands of euros on them. None of the measures would have been absolutely necessary. But we are talking about large investments here; it's not done with 5,000–10,000 EUR. As I said, in principle, I am absolutely for climate protection, I vote accordingly and live by it. But it has to be somewhat feasible.

Of course it's a conflict. On the one hand, you want to save CO2, on the other hand, it shouldn't cost anyone anything

I wouldn't say for myself that it shouldn't cost anything. As I said, the conversion was planned anyway, but I would like to have the option to possibly wait 2-3 years longer. For example, because there is another child on the way and therefore the savings rate is low in between Or because a car suddenly breaks down and money is needed accordingly Or because something else happens. It's simply unpleasant when you (yes, even as heirs possibly) are obligated, just a few years after buying the house, to put a lot of money in again, even though the existing heating system is still running completely smoothly and the chimney sweep had absolutely nothing to complain about.

By the way, in this context: what happens if one could only cover the costs through a loan, but does not get a loan, for whatever reasons? For houses that are paid off, the value should regularly be high enough that even retirees get a loan, but what about houses that were bought expensively during the low interest rate phase and fully/almost fully financed? There are certainly some cases where an increase of the loan to such an extent is not easily approved by the bank?!

but when the heating needs to be replaced after 20 years, they won’t be fully financed anymore either.

But yes, I also have mixed feelings, I’m increasingly looking into it for my parents. It seems that things are also moving forward with heat pumps, so that up to about 50VL it doesn’t have to be incredibly inefficient. That’s still less than what you usually have with an oil heating system, but the leap isn’t so huge and you’re no longer necessarily forced into “we have to switch to underfloor heating now.”

Otherwise, there are still BAFA subsidies for heating replacement between 25 and 40%. I don’t know how it looks right now, last year heat pump prices exploded and nothing was really available anyway, but maybe it’s not that much more expensive than replacing with a new burner? On the other hand, if everyone is forced and there is good funding for existing buildings, prices probably stay high?

I don’t really know how much I think of the obligation. Just the total number of systems alone is really not insignificant. But I don’t want to see it all that bleak yet either.

By the way, in this context: what happens if you could only cover the costs through a loan, but you don't get a loan, for whatever reasons?

I assume there is no regulation for that. Obligated means obligated. And yes, anyone who bought an unrenovated house from the 70s 2-3 years ago and didn't plan for renovation could now have a rude awakening. Although it doesn't necessarily have to be the heat pump. Pellets or wood chips are also allowed and could experience another upswing due to the regulation. Whether that is really environmentally friendly, remains to be seen.

That's something completely different. It was only about replacing a boiler. Now it's about installing a heating system where you practically have to renovate the whole house at the same time.

You absolutely don’t have to. But if you piece together your knowledge about, for example, heat pumps from headlines and fundamental rejection, unfortunately all that remains is hysteria.

Apart from that, we have already had "renovate the whole house" in the past decades as well, namely when converting from storage heaters to mostly gas.