New construction - How to build economically sensibly?

Heating. The popular air-to-water heat pump has the advantage that no gas connection is needed and the technology is now mature. On the downside, it may heat purely with electricity in severe winters, it always requires more electricity than the gas heating system, it has a component through the compressor that may not last 10 years without repair, but it otherwise requires neither maintenance nor a chimney sweep. Economically, it is a bet that the gas price rises more than the electricity price. With the current price structure, it is inferior to the condensing boiler. Insulation. If you build solidly, insulation is an additional step. If you take thicker stones right away and save this step, you do have to pay more for the stone but save labor. It pays off. Therefore, better monolithic than ETICS. The highest quality is certainly double-shell with mineral fiber insulation, air layer, clinker or sand-lime brick facade. You get what you pay for. Forced ventilation systems with heat recovery certainly provide the comfort of constantly good air without opening windows. Economically, these systems are not worthwhile, the extra cost is too high. The be-all and end-all is good walls, careful roof insulation and good windows. Karsten

With 200,000 for the whole house, it is and will be cheap, either like with 36 cm Ytong, but otherwise with Poroton and polystyrene insulation. Gas with solar is the cheapest heating method. With your budget, you have hardly any room to maneuver

That is tendentious in language. Cheap always has the connotation of being inferior. Let's say affordable. Good value. A Ytong is not inferior, a white window is not functionally worse than a gray one. A 400,000 house can have the same concrete in the foundation slab as a 200,000 house. A Tegalit roof does not seal better than Harzer tiles. A 25 cm tile does not shower differently than the 50 euro per sqm variant. KNX does not light differently than a light switch. A plastic front door is not energetically worse than aluminum or wood. A fingerprint sensor is not more secure than a good lock. Softwood roof soffits protected with brackets are not worse than those made from plastic panels. Etc.

First of all, thank you for your quick answers. I did not know REGEL-air yet. By "efficient" I mean that I will only make additional investments that do not pay off only after 50 years and are not too high. Is a prefabricated house not an alternative? (just saying)

What does efficient refer to? Primary energy or money? What about "pointless" – saving CO2?

As long as the question is not answered, it is actually difficult to help

Edit: You beat me to your answer Do you really mean 50 years? Over such a long period, you can calculate economically a lot, or not. I just have to change small assumptions (electricity price increase by about one percent up or down) and already you have huge effects over this time span... It is crystal-ball gazing.

Another example: Over a 50-year perspective, you can practically economically calculate any photovoltaic system. Does that mean now you want to have a maximally large photovoltaic system on the roof?

that I will only make additional investments that do not pay off until after 50 years

Then you must not build, it will never pay off.
It is a luxury that one must be able and willing to afford.

And anyone who wants to make all kinds of compromises should simply consider what they are doing it for. In the end, some rental apartments will probably be better equipped than their own house.
You can save on everything that can be changed later. However, everything that lies under plaster and screed should be done right from the start.

That is tendentious in language. Cheap always has the connotation of inferior quality. Let's say affordable. Good value. A Ytong is not inferior, a white window is not functionally worse than a gray one. A 400 t house can have the same concrete in the foundation slab as a 200 t house. A Tegalit roof does not seal better than a Harzer tile. A 25 cm tile does not behave differently from the 50 euro per sqm variant. KNX makes the light no different than a light switch. A plastic front door is not energetically worse than aluminum or wood. A fingerprint sensor is not more secure than a good lock. Softwood roof sheathing protected with consoles is not worse than that made from plastic panels. Etc.

We are rather spartan as well, for example triple-glazed plastic windows or good-looking standard fittings suffice for us. However, I invest more in things like the roof tiles (glazed clay roof tiles) because I prefer to have something robust there.