Construction costs are currently skyrocketing

I have to honestly admit that I haven’t seen a concrete heating load calculation for our house yet, but according to the general contractor, the heating energy demand should be around 20kWh/sqm p.a. Roughly estimated, that’s about 4,700kWh p.a. I already asked him whether the ALM 4-12 isn't a bit "over the top." You can guess the answer. To my shame, I have to admit that I didn’t read deeply enough into the topic in time. Meanwhile, the heat pump is ordered and on its way. There’s not much I can do about it now. :-/ In hindsight, I should have taken the smaller unit, the ALM 2-8.

The heating load isn’t about the annual consumption but the value of heating energy you need at the design temperature. I don’t know that value now, but based on your other data and unless you live at the North Pole, the unit is obviously much too big. Your heating load will surely be below 6 kW, maybe even significantly lower. The problem isn’t so much that it has too much power "on top" (i.e., the 12 kW), but that it cannot modulate down. In other words, you only need 6 kW from my example if it’s -12 degrees for 3 days straight. The other 98% of the year you would get by with 2-3 kW. Yours can’t do that, so it will turn on and off more often. Presumably, this is why the installer gave you a nice big buffer. But if the damage is already done, that’s of course unfortunate.

It's always a question of what is included in the construction costs per sqm. Only the costs from the general contractor, also incidental construction costs, also the landscaping? The range for us also varies from 2,200-3,200/sqm

I always calculate the house including any possible own work (painting...), base slab/cellar, earthworks necessary for the house, but without kitchen, furniture, utility connections, landscaping, notary,...

Since we are building on a 300sqm plot, the cellar is actually indispensable with 2 kids in the house. If we subtract the area of the house, the garage, and the driveway (mandatory, parking space regulations), then maybe 130sqm of unsealed area remains. After deducting the terrace, light well, and paths/splash protection strip, even less than 90sqm.
The cellar is therefore the cheapest area for us. Building a third floor upwards was not possible because of the setback distances.

To be honest, I can't imagine how you build detached on 300sqm. There's really only the setback strip left?

What I also found interesting... both with the tiles and the sanitary equipment, we basically always stayed within the cheapest range that the dealer had in the showroom. Still, I find the almost €9000 for both bathrooms quite expensive and €38/m² for 30x60 tiles not exactly cheap. At least with the tiles, the dealer can quite well keep up with internet prices, so that's probably not the reason

Tiles are fired, gas is involved, it has all become really ridiculously expensive. We recently went to choose tiles as well, and took almost the same combination for the main bathroom that we had in the first bathroom. And tiles that were below the €35 gross standard back then are now at €37 plus VAT. You can still get the house furnished up to the €35 standard, but the selection has really become very limited. We're adding a bit more this time.

Wiesbaden (dts News Agency) - In August 2022, the construction of 28,180 apartments was approved in Germany. This was 9.4 percent or 2,934 building permits fewer than in August 2021, the Federal Statistical Office (Destatis) announced on Tuesday.

From January to August 2022, a total of 244,605 building permits for apartments were granted - this was 3.0 percent or 7,624 fewer than in the previous year period (January to August 2021: 252,229). The results include both building permits for apartments in new buildings as well as for new apartments in existing buildings. In newly constructed residential buildings, a total of 211,636 apartments were approved from January to August 2022.

This was 3.0 percent or 6,622 apartments fewer than in the previous year period. The number of building permits for single-family houses decreased by 15.8 percent (-10,374) to 55,260. However, a base effect due to the expiration of the [Baukindergeld] in the previous year must be taken into account, according to the statisticians.

Accordingly, families with children could apply for the government subsidy if they had a building permit for owner-occupied residential property by the end of March 2021. The [Baukindergeld] contributed to the fact that almost 7,400 more single-family houses were approved in the period from January to March 2021 alone than in the first three months of 2022. For two-family houses, the number of approved apartments from January to August 2022 decreased by 2.8 percent (-612) to 21,176 apartments.

I have to be honest, I can't really imagine how someone builds on 300sqm detached. The only thing left is basically the buffer strip, right?

Yes, pretty much. But the surroundings are worth a lot (central location, parents next door, etc.). So the kids, for example, have the swing in the parents' garden :-)

The heating load is not about the annual consumption, but about the value of heating energy you need at the design temperature. I don't know that now, but based on your other data and unless you live at the North Pole, the unit is obviously much too big. Your heating load will certainly be below 6 kW, maybe even significantly lower.
The problem is less that it has too much power "on top" (so the 12) but that it cannot modulate down. In other words, you only need the 6 kW from my example when it's been -12 degrees for three days. For the other 98% of the year, you would get by with 2-3 kW. Yours can't do that, which is why it will turn on and off more often.
Presumably, that's why the installer put a nice large buffer tank in for you.
But if the damage is already done, of course that's annoying.

Thank you for your explanations, that certainly makes me think.

I have to add that the buffer tank has a modulating electric heater and the plan is that the photovoltaic system will mainly take over the heating function in the transitional period/summer, so that the heat pump won't turn on and off so often. But that would actually argue for sizing the heat pump differently. Hmm, damn it, but that's just how it is now :-/