Ongoing additional costs - Realistic?

It's not about the price suddenly changing, but about what is included at the stated price.

By the way, we have quite a few with HUK24, which is not even included in the relevant comparison portals. I also always get information directly from the insurer and read up on what makes sense for us and what I really need. I don't take out insurance with the motto "as long as it's cheap." Instead, it's more like: How much do I really need and which useless "stuff" do I not need for us and therefore do not want to pay for.

For example, included in our insurance is a premium waiver in case of unemployment and a currentness guarantee.
My experience with such comparison portals: the real problem comes when you actually want comparable benefits to the supposedly more expensive insurances.

It's just a question of whether you need everything. You pay more every month so that you can suspend the premium for a few months of unemployment. For most people, this would be a pure subsidy business and even if you have a gap of 2 months, stable financing should be able to handle it. If you become unemployed long-term, insurance should be the least of your problems.

I really can’t make sense of why your costs for residential building insurance are so high.

Sure, the location plays a role among other things.
I just quickly entered the current situation on a comparison portal and got about €130-150 as an annual premium for my 100 m2 house including glass breakage. With an excellent tariff rating at €150. The same situation transferred to my previous address in Dresden makes no difference. If I change the living area to 150 sqm, the value goes up by €50–60. Each time including drainage pipes on or outside the property. The waiver of underinsurance is also included each time.
I’ve also never heard that you have to explicitly book that if you have the insurance sum calculation done automatically.

That was already mentioned earlier in this thread: the older the house gets, the more expensive the residential building insurance becomes. I just checked again: 2014 €155, 2019 €369. Without changing anything. Comparisons elsewhere show similar results for our house.
It also always depends on how you feed the comparison calculators. By now you know that an apple is not always an apple, even if you think so.

Also consider that 100 sqm is not 160 sqm. And if someone has nothing for household insurance, they simply don’t need to insure anything.
It’s also a very individual matter: if someone has to be hospitalized frequently, they really need to choose the premium variant so that the phone and wallet are insured too. The comparison calculators won’t prompt you to select that at all. And then there are many things where there are large percentage differences.

It’s the same as when someone tells me they only pay €40 for the outbound flight with Lufthansa/Condor, but I pay €100. They have nothing included; I have the train to the flight, seat selection, 23 kg luggage, and something to eat. And possibly cancellation up to 14 days before. The upgrade of hand luggage from 6 to 8 kg is included for both. Anyone who doesn’t need all that can gladly pay less.

I believe that mainly depends on the condition and size. Our house is from 1921, renovated inside in 2017 and outside in 1993. I always have to specify when what was renovated for how much money, and our premiums are not higher than yours with the new buildings.

It is also a very individual matter: whoever frequently needs inpatient hospital care must choose the premium version to have the mobile phone and wallet insured as well. The comparison calculators do not at all prompt you to select that. And then there are so many other things where there are large percentage differences.

I agree with you that of course everything is individual. But even if you cannot select it directly, the insurance terms and conditions do clarify the matter. Attached is your mentioned example. ;)

So everything is solvable. And relating to my information in the example, that would be €60 per year. So in my opinion, not necessarily the premium version.