Optimization of floor plan for detached single-family house 155 sqm

I would, in your place, get information from the online providers and maybe clarify in advance whether and to what extent you could retrofit it later. Then at least the necessary structural preparations could already be made.
As I said, this is just an idea and probably rather cumbersome, but the price seems completely exaggerated to me. Our Zehnder Comfoair 350 currently costs online €2500 to €3000 (just the bare device). I could imagine that these dealers could also provide you with a contact for the later installation. Maybe the heating engineer is just a bit inflexible, the general contractor might not care or could implement the required preparations you will then demand. Zehnder, for example, also does the complete sizing calculation for you. That was the case for us, and the general contractor only wanted to install Vaillant, while we wanted Zehnder. Allegedly "not deliverable," just like with roof tiles, etc., until I named him several dealers with stock, and he was also happy to add a reasonable markup; so that's how it happened. Maybe you just have to stay a bit "stubborn" there.
But I'm sure there are people in the forum who have more and better information on that.
I also remember photos where the controlled residential ventilation was installed in the attic; I basically see no problem there—but—you still need it in the ground floor as well, and for that, the respective openings in the concrete ceiling or space for the pipes, wherever they will be in the planning. So it doesn't help, better have space in the utility room and preparations in the walls/ceilings/floors (depending on needs).

I don't understand. That seems a bit naive to me. What prevents you from buying the device later? What does the contract say? That you are only allowed to buy from this provider forever and ever? That would be ridiculous.

The question would also be whether you can take the installer completely out and give the contract freely. I would definitely want what I want inside, and something I don’t want for such a high price would not be my solution.
I'm curious what other people with such experiences will say. You certainly don’t need a heating device.
I also think it should be possible for the construction company to make the preparations structurally and according to specifications, e.g. from Zehnder, against payment.

I don't understand. That seems a bit naive to me. What is stopping you from buying the device later? What does the contract say? That you are only allowed to buy from this provider forever and ever? That would be ridiculous.

I assume he is bound to the construction contract: either he accepts the heating offer from the general contractor or he doesn't. According to the description of services, x is installed; on request, he is offered y xy, and the client cannot come up with z.
However, one could! Just ask for an alternative to the combination device . If it's about the space in the freezer, a ventilation unit can also be hung from the ceiling.

According to the construction service description, x is installed, he is offered xy by y on request, and the builder cannot come back with z.

I rather understood it as the GU cannot deliver what was agreed. So he wants to remove this element from the contract. But that doesn't mean you now have to take something else much more expensive from him. I would also, like , look at what is still feasible and compatible with a device from an alternative supplier in the contract. Everything else just gets removed.

I don't understand. That seems a bit naive to me. What is stopping you from buying the device later? What does the contract say? That you are only allowed to buy from this provider forever and ever? That would be ridiculous.

We have a contract with the developer and they are also providing the described heat pump.

Of course, we can also install a ventilation system from another provider in addition, but in the end, the walls also have to be prepared accordingly.

And time is running out a bit as well. The house is to be erected on March 3rd.

What does your theory actually look like? That will never work.

Install the ventilation system and ventilation ducts in the attic to the respective rooms and drill a hole through the ceiling of the upper floor accordingly. Fresh air and exhaust air through the gable.

I also think that it should be possible on the part of the construction company to carry out the preparations structurally and according to specifications, e.g., from Zehnder, for a fee.

That would also be my preferred solution. I will see if I can find someone for that on short notice.