Lowering the supply temperature in underfloor heating more complex than expected?

Addition: When you google "Baurecht Recht auf Pläne," for example, you will find an entry by Mr. Heide, who cites several court rulings. It concerns § 650 n of the Building Code. It should be possible to organize all energy-relevant plans. Because: "The heat insulation certificate and calculation according to the Energy Saving Ordinance 2002" is also owed according to OLG DRESDEN, JUDGMENT OF 12.05.2016 - 8 U 438/15. To check this, I need the heating plans.

With 2 AW in the bathroom and controlled residential ventilation, maintaining 24° in the bathroom at 20° in all other rooms becomes a real challenge???

Of course, that entirely depends on the planning. If, as is often the case, it is already planned with a deficit, it will at least not get any easier to achieve the temperature. If, as is often the case, planning/building is done "by rule of thumb" because that’s how it has always been done,...
The question is whether proper calculations are being made or whether the floor heating pipe manufacturer provides standard planning with standard values. The latter is usually the case with general contractors.

The right to control construction execution probably cannot be legally taken away from you.

You are always allowed to check the execution. But if the executed solution is 1. in the construction description and 2. legal and 3. according to the recognized rules of technology, you have no claim to change it. Even if another execution would be better. This is one reason why I would no longer build with a general contractor, but only with individual contracts.

You are always allowed to check the execution. But if the executed solution is 1. in the construction description and 2. legal and 3. according to the recognized rules of technology, you have no claim to changes. Even if another execution would be better. That’s one reason why I would no longer build with a general contractor, but only with individual contracts

With today’s knowledge, I would do the same, but just a year ago I had zero knowledge of most topics. Only a second house build will probably take place in the next life; I won’t put my family through that again. ;-)

If it were financially possible, I would build three to five more houses or, if I had bigger balls, become a developer / [GÜ]

You are always allowed to check the execution. But if the executed solution 1. is in the construction description and 2. legal and 3. according to the recognized rules of technology, you have no claim for changes. Even if another execution would be better. This is one reason why I would no longer build with a general contractor, but only with individual contracts.

If you had built your construction project with individual contracts, you might have just celebrated the topping-out ceremony with luck after the chaos of recent years and would already have been 30% more expensive.

Therefore, this is more of an academic discussion.
The contract is signed and changes cost what the provider demands for them.
If he calls for 30,000 EUR for the new heating system, then you can sign or not.
And that is not unusual either. VW earns almost nothing on the Golf Trendline, the money is made with optional extras, for example wheels for 2,500 EUR, which they get directly from the supplier for 1,000.

By the way, this already existed at gas stations. If they hardly have fuel of one type left, they set the price to 9.99 EUR, the display does not show more. In case someone actually HAS TO refuel, for whatever reason. That way, they basically secure a small reserve for emergencies.

And whoever refuels then, out of stupidity or whatever, actually pays 9.99 EUR, not a cent less. There are rulings about this.
The seller sets the price and the buyer can accept or not.