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

Anbieter1 can also calculate for unheated spaces. In residential construction, a 2-zone model can also be used. However, this is rather uncommon and may apply in the case of apartment buildings and stairwells.

With waterproof concrete 28 cm with XPS insulation, it is only feasible with 180 mm or 200 mm perimeter insulation.

Ok, is there a big price difference between 200 mm and 140 mm? The current basement offer has waterproof concrete 24 cm.

What is also possible, of course, is core insulation made of PUR/PIR between element walls plus XPS

Ah, ok. I will talk to the basement builder about that. Thanks.

The decisive factor here is how the thermal bridge surcharge is proven.
For this, the KfW provides 3 methods.
Strictly speaking, only then can the concepts/specifications and costs be compared.

Hmm, sounds complicated :(

Correct, the heat pump MUST be within the thermal envelope. So you still need a technical room above ground.

If that is the case, then provider 1's offer cannot be implemented. The heat pump is in the basement (air-water heat pump + hot water + controlled residential ventilation combined unit). In the technical room upstairs (ground floor) there is space for washing machine + dryer, nothing else.

Thanks again to everyone for the advice and [n guten Rutsch ins neue Jahr] :)

Can't you just have provider 2 write in the contract that you are building a KFW40+ house? Then it's his problem how thick he has to insulate afterward. I wouldn't sign any other contract, because then everything is open for addenda. What if he tells you afterward that the calculation resulted in 35 cm insulation thickness and that costs 275 euros per square meter?

Can’t you just have provider 2 write in the contract that you are building a KFW40+ house?

That is in the contract, but "from the basement ceiling"... I can ask for an offer where provider 2 also sells the basement, everything from one source.

Then I can also ask right away about the cost difference between KfW55 and KfW40+.

At first, I didn't know whether I could afford a basement. But after the soil survey, chemical analysis, offer from the excavation contractor for earthworks, and basement offer with 12cm/14cm perimeter insulation, I found the costs still acceptable. That’s why I thought the prefabricated house manufacturer could then build the house based on that. But it’s not that simple after all.

Now I know what to do, thanks again.

Correct, the heat pump MUST be within the thermal envelope. So you still need a technical room above ground.

That is not correct (see our house construction thread with an external technical room where the heat pump is located and which is not within the thermal envelope). This required separate phone calls between the energy consultant and KfW to clarify. The corresponding clause is apparently not formulated quite clearly.