Energy consultant for a KfW 70 house costs 2,500€?

Besides, it is now more or less mandatory due to the ventilation issue.
Otherwise, the architect would have to be released from liability in writing if mold develops. Very few architects agree to that.

Then a cheap window rebate ventilation is installed and with that the architect is off the hook as well.
Whether that is sensible or not, I don't want to argue about.

Again. Why doesn't the architect calculate with the Ytong? At 36.5 he is even slightly thinner than your clay brick sandwich, attaching mailboxes is also easier, he is relatively inexpensive as a stone and also saves labor due to the large format. You gain the 5000 repayment subsidy and lose zero sqm inside. What is the sacrifice? The Ytong may have worse sound insulation than your sandwich stone. If you don’t want that, deselect KFW 55, that would be my advice. It’s simply not worth it if the subsidy completely seeps into the pockets of the companies. Karsten

Window rebate ventilation makes sense for me, Max, in rental properties, possibly....

Then an inexpensive window rebate ventilation is installed and that way the architect is out of the picture. Whether that is sensible or not sensible, I don’t even want to argue about that

But basically, one should be aware... Just because an architect is fixed on a certain type of brick doesn’t mean that this is the "stone of white."
And just because someone says, you will only save 100€ in energy costs. Then I would like to see the calculation.? That’s short-sighted, especially if a ventilation system is already installed....
Nowadays, air heat pumps are often sold because they are so cheap.
Unfortunately, they are only cheap at the purchase price for the tradesman. With better insulation or better wall construction, you can easily use a smaller heat pump. That alone saves a few euros.
If you then also look at the BAFA subsidies, you quickly realize that a much more efficient ground source heat pump is not significantly more expensive than an air heat pump.
Depending on the source, you get between 4000-4500€ subsidy for a ground source heat pump in new buildings.
If you provide your own work, you can create a source with earth collectors, flat collectors, or trench collectors for easily 1000-2000€ — without needing to take a drilling.

Then you additionally save on energy costs annually by switching from an air heat pump to a ground source heat pump.

Example:
Required heat energy + hot water 13,000 kWh.
Air heat pump with a seasonal performance factor of 3.5.
Ground source heat pump with a seasonal performance factor of 5.
13,000 kWh : 3.5 = 3714 kWh electricity demand
13,000 kWh : 5 = 2600 kWh electricity demand.
At 0.25€ electricity cost per kWh, you save nearly 280€ annually just by switching the type of heat pump.

In your case, I would really calculate more precisely. Whether the combination of a better brick + switching the type of heat pump wouldn’t ultimately be the significantly better solution.
You are still flexible. You just have to make that very clear to your architect.

For the architect, it is clearly easier to plan according to a fixed scheme and apply his fee regulations accordingly.
But you want the perfect solution for yourself and not a fixed scheme.

This is a special topic, but no bashing on [Fensterfalzlüfter], please. We have that, with exhaust rotor, and it works perfectly. Perfectly! I emphasize. You could also bash [Kontrollierte-Wohnraumlüftung], because it is absurd to move into an airtight plastic bag for energy-saving reasons, only to say, help, I'm suffocating, I'll build a forced ventilation that runs on electricity.

I cannot understand the statement from your energy consultant/architect. The exterior walls have a U-value of 0.20 in the 37.5cm version.

If you have not made any planning errors regarding the windows, the insulation of the ground slab (in relation to the insulation of the basement/basement floor), then a single thermal bridge verification should "easily" be sufficient.

Of course, it may be that you have a "non-economical" building shape. But for a normal rectangular building form, it should be enough.

By the way, we also build KfW 55 with a 36.5cm brick (T8 unfilled) that also has a U-value of 0.20. For us, the decisive factor is also the single thermal bridge verification.

Is the single thermal bridge verification included in the offer? Was this offered to you? If not, get it! And don’t fall for nonsense like 500€ per thermal bridge, but all inclusive.

Ventilation is merely a matter of comfort.
Furthermore, it has now become more or less mandatory due to the ventilation issue.

Which regulation forces you to have comfort?

Otherwise, the architect would have to be released from liability in writing if mold forms.

The architect is certainly not to be held responsible for the user's behavior. Therefore, there is no point in trying, because regular airing by opening windows is quite (legally) reasonable.

But otherwise, I agree with you, controlled residential ventilation is a comfortable way to save manual airing and always have fresh air. For allergy sufferers, it is also filtered.
From an energy standpoint, I would never have controlled residential ventilation installed, but definitely for comfort or to prevent mold.