ᐅ How do you properly balance the requirements of the Building Energy Act against the standards of KfW 55, 40, and 40 Plus?

GSGaucho schrieb:

I think I’m still the best judge of how much money I waste with which compromises.

You can think so. But most people have nothing to waste ;-)
Most just want to live comfortably with reasonable costs and as few compromises as possible.

GSGaucho schrieb:

The fact is, tens of millions of square meters of living space in Germany are vacant precisely because of this situation. That is economically nonsensical.

That is only partially true. Vacancy exists, but for other reasons. A good example is the Ruhr area. High vacancy and still high housing demand. Vacancy is wasted space, while reasonable (and sought-after) housing is in short supply.
And in rural areas, there is plenty of vacancy. But no one wants to move there. I’m talking about declining villages with no younger generations, often in the eastern regions. This is also reflected in the prices.

nordanney schrieb:

You can do that. But most people are not giving things away ;-)
Most just want to live comfortably with manageable costs and as few compromises as possible.

That is only partly true. Vacancy, yes, but for other reasons. A good example is the Ruhr area. High vacancy rates and yet a severe housing shortage. Vacancy is wasted space, good (and sought-after) housing is scarce.
And in rural areas, there is plenty of vacancy. But no one wants to move there anymore. I am talking about dying villages with no younger generation, especially in the East. This is also reflected in the prices.

That fits well. I live in a rural area. And in a community of 2,000 residents, land for building is already at €250/m2 (about $29/sf). Families with three children can’t get land, as it is allocated by lottery. Reality in 2021, thanks to Euro preservation policies.

GSGaucho schrieb:

When building a new house, it is definitely advisable to design it so that parts can be separated when the children move out.

With your described 3 children's bedrooms + large bathroom + extra room, this is of course recommended, otherwise it becomes hopelessly too large.
With about 150m² (1,615 sq ft), you might have a guest room (for children/grandchildren visiting?) and a hobby room. That’s how I would estimate it, at least.

GSGaucho schrieb:

When building a new house, it is definitely advisable to design it so that it can be divided after the children move out.
Either one of the children uses the separate apartment or it can be rented out externally. Those who can afford it in old age might still leave it empty. However, that is not ecological.

It depends on the size and design of the house. I see it with my parents right now—at 120 sqm (1,290 sq ft), there’s not much space to separate, and the former kids’ rooms upstairs are now doubling as an office and guest room. Selling and moving into an apartment is not a medium-term option; the ground floor is now accessible and barrier-free, and with such a nice garden, no apartment can compete. As long as you’re still fit, it works well, and after that, it hardly makes a difference.
I also considered the option of separation, but that requires a staircase directly at the entrance, which ruins most floor plans. Suddenly, you end up with unnecessarily long hallways upstairs, bathrooms with lots of roof slope, etc., and downstairs the open space is much smaller if you still want other rooms. Ultimately, I decided to build just for us first. If necessary, an external staircase can be added later, a partition wall installed on the ground floor, and the original stairwell replaced with an added ceiling—if that situation ever becomes relevant at all.