Newly built single-family house KfW55 standard - How to heat?

For an air-to-water heat pump in a kfw55 house, 175 is also far too small.
We have 300, 2 adults, 2 children – that usually isn’t enough for 24 hours. With gas it wouldn’t matter, it just refills. With an air-to-water heat pump, especially if there’s also photovoltaics, you want to use the midday heat and sun. Besides, a heat pump doesn’t refill the tank in just 15 minutes.

For us, it will probably be a kfw40plus house. And we are not fans of bathtubs. And as far as I have read on the sites, 175 liters are completely sufficient.

You both are talking about hot water tanks. with (gas boiler +) solar thermal you (presumably) with air-to-water heat pump (possibly with photovoltaics - but independent of that). Joedreck explains in the post you quoted why the hot water tank must be adequately sized for solar thermal. This is irrelevant when not using ST.

Thank you for the correction

We have nearly 200l heat pump hot water for 2 people, the children's apartments each have an instantaneous water heater - once the boys have moved out, these apartments will only be used sporadically - as an office, guest or holiday apartment and when we are old as a ground-floor senior-friendly sleeping place or apartment for a "life helper". Meaning: you can also mix. As always, the sizing depends both on the demand and on the technology used, as and write. We found the KfW standards unsuitable for us and have therefore built independently of them.

So we had a 200-liter tank in the old house with 2+2 plus our parents in the granny flat. The old oil heating system probably heated the water significantly faster than a heat pump does today (at least that's what I think), right? However, in 25 years we never had a single hot water problem. I think when one person is bathing or showering, usually no one else showers at the same time and even if they do, 200 liters is still quite a lot; as I said, it very much depends on individual behavior in most cases. In recent years, we then had solar panels for hot water on the roof, but the water tank was not enlarged, it remained 200 liters. In the summer, the solar system always drained some water but that ultimately did no harm. That was at least my experience and information from the heating technician. Of course, it might feel nice to have a buffer, but this buffer has to be reheated completely every time, even though you might only need it once in an extreme exceptional case. In my opinion, that would make little sense.

What are the costs for such a system? Can you manage with €20,000?

All in it was including installation, scaffolding, planning and registration, accessories etc. 18k net.

With an air-to-water heat pump, you often have double the heating costs in a new building compared to gas. Despite correct pipe spacing and heat load calculation. Only through your own optimization can this be equalized. It's harsh, but often the reality.

Actually, the best heating technology is still pellets. Reliable, affordable, and flexible. Tank underground in the garden, done.

With a heat pump, you also have the great electricity price risk. No energy source becomes more expensive as quickly as good electricity.