Air-to-air or air-to-water heat pump?

Do you have both offers in writing and with comparable items?

Do you have both offers in writing and are the items comparable?

No, unfortunately not.

I have one offer with all items completely available.

The other basement I have as a "basic basement." Various items for usage class A are then added to this, resulting in an additional cost. However, it is far from that. This was communicated to me by phone.

Unfortunately, I do not have a concrete basement construction and service description or similar.

That is exactly why I have some concerns and why the basement topic doesn't really sit well with me...

Are there things one should pay special attention to?

Except for a good basement builder who knows his craft?! In addition to the WU concrete, we also had metal fibers mixed into the concrete, which our architect insisted on

Except for a good basement builder who knows his craft?! We had metal fibers mixed into the concrete in addition to the WU concrete; our architect insisted on that.

In principle, one should probably trust the prefab house provider when it comes to basement construction and not hand it over to others?

But if I know which basement builder my prefab house company works with, can't I just casually get a quote from one of the largest prefab basement providers in Germany and outsource this externally?

That's what it's about... I think you could save certain commissions and surcharges here if you inquire directly with a basement builder.

I think you can just make it easy and ultimately have everything done by the prefab house provider... sure, that's the easiest way but probably also the most expensive, right?

Am I going in the wrong direction here, or is something like this feasible?
How did you handle this with your prefab house providers? Everything completely from one source (basement, garage, outdoor facilities, photovoltaics, etc.) or did you also hire external specialist companies for different things?

We have a brick house. Planned with the architect up to service phase 4 and had our private construction coordinator and the architect obtain offers. After that, our construction coordinator took over and coordinated the individual trades. Basically built the way it was done before prefab houses/general contractors existed.

We awarded demolition, garage, foundation slab, retaining wall, and earthworks ourselves, partly to companies recommended by the general contractor (GU). (And even there, there were price differences. Demolition was almost three times what we actually paid. Earthworks, garage, retaining wall/house wall on the hillside side had a 200% difference between the cheapest and the most expensive.) The architectural service and construction supervision were taken over by the GU. Certainly not for free, but included somewhere in the price. But mistakes can become really expensive afterwards.

Friends planned with another GU, it went similarly there.
Basement, earthworks, etc. were awarded to a company recommended by the GU, and the GU took over the architectural service and construction supervision.

Consider what work you want and can take on yourself as a layperson. Perhaps a shell construction house is an option for you.
Removing individual trades often does not pay off. On the one hand, not 100% is deducted; on the other hand, it can cost you the completion date. And what hopefully never comes into play, in the case of warranty, it was not the case.

For the basement, pay attention to what is included in the earthworks in the offers.

LLWP No underfloor heating that has to be installed = cheaper construction price. But very expensive in operation.
Air-to-water heat pump Underfloor heating has to be installed = more expensive construction price. Cheaper and more efficient in operation.