KfW40+ with a basement inside or outside the thermal envelope?

I want to build a KfW40+ prefabricated house with a basement, two full floors, 12m by 8m. In the basement, I have technical equipment as well as a music room, with underfloor heating and connection to [Kontrollierte-Wohnraumlüftung]. The plot is flat.

Provider 1 said calculating the basement outside the thermal envelope is "better and cheaper," therefore there will be insulation between the basement ceiling and the ground floor, as well as insulation under the music room.

Provider 2 said to calculate the basement within the thermal envelope.

I tend to prefer Provider 2, as having the basement within the thermal envelope makes more sense to me. Also, I save on the extra insulation between the basement and the ground floor.

What do you think?

Thank you in advance.

If there should be underfloor heating and controlled residential ventilation in the basement, and it is also "inhabited," you can only include the basement in the thermal envelope.

I tend to prefer provider 2, since including the basement in the thermal envelope makes more sense to me. In addition, I also save the extra insulation between the basement and the ground floor.

However, the extra costs for insulation under the slab (or on the slab under the screed – but then a higher basement is necessary) and around the basement still apply. This becomes more expensive than option 1.

What does the energy consultant say, who has to calculate KfW 40? Option 1 is simpler.

The extra costs for the insulation under the floor slab (or on the floor slab under the screed – but then a higher basement is necessary) and around the basement still have to be added. That will be more expensive than option 1.

Provider 1 wants 8,000 euros for insulation between the basement ceiling and the ground floor, but also for insulation under the basement slab, about 18 cm below the underfloor heating. In addition, insulation in the interior door and under the basement stairs. That also seems somewhat elaborate to me.

What does the energy consultant say, who has to calculate KfW 40? Option 1 is simpler.

I don’t have an energy consultant; I rely on the provider because they have to do the calculation. At the moment, I’m only talking to salespeople to get an offer. But the KfW calculation only comes after the contract signing.

The basement with excavation will already cost quite a bit; I don’t know if it makes sense to save some costs by planning the basement outside the thermal envelope.

If there is underfloor heating and controlled residential ventilation in the basement, and it is to be "inhabited," you can only include the basement within the thermal envelope.

Provider 1 knows that I plan a music room in the basement with underfloor heating and controlled residential ventilation. That is not necessarily a "living basement" but a "hobby basement."

I just have some concerns that the energy consultant might say after the contract is signed, "that's not possible" :)

Thanks again.

The conditions for KfW40+ are very detailed, though. There must be an energy consultant; KfW40+ only works with one. If basements are designated as "living space," they automatically belong to the thermal envelope. Therefore, Option 1 is eliminated. There you go, life can be that simple.

There must be an energy consultant; only with them does KfW40+ work.

I understand that too, it was not questioned :) They only get involved later after contract conclusion, just like the architect. This is about offers before contract conclusion.

If the basement is declared as "living space" it automatically belongs to the thermal envelope.

I did not say that the basement is declared as "living space."

Provider 1 (seller) might want to declare the basement as a "hobby room" and therefore a utility basement.
Nobody sleeps or "lives" in the basement. The music room doesn't even have windows, so it cannot be declared as living space, right?

Here you go, life can be that simple.

Sorry, but it doesn’t seem that simple after all.

The basement has no habitable rooms, the "hobby room" has no windows, but it does have underfloor heating and controlled living space ventilation. That means it’s "pleasant" to spend time in the music room/studio, but it’s not "lived in" there.

Therefore, both options seem "legitimate." My question is what makes more sense for me.
I tend to go with provider 2 (within the thermal envelope) but maybe I’m missing something.

It is also clear to me, it was not questioned :) He only gets involved later after the contract is signed, like the architect. This is about offers before the contract is signed.

But you cannot sign a contract if it is not at all clear that your wish can actually be implemented. I also don't buy a car and then find out after the purchase that the all-wheel drive I wanted unfortunately cannot be installed in my model.