Prefabricated house vs. solid house - price difference?

Ok, then I will have to speak with him about this again. Thanks for the hint!

oh yes - but even without KFW (I'm still not sure if it pays off) I could apply for the investment grant for a ground-source heat pump at BAFA, couldn't I? (Provided, of course, that I meet the annual performance factor).

It is not mathematically impossible, but in this case the wall contributes little to improvements compared to the Energy Saving Ordinance. This could then be compensated with extremely good passive house windows, extreme roof insulation, and an exact thermal bridge calculation (and of course a thermal bridge-reduced construction). However, this would be very uneconomical, so with lambda 0.09 bricks in the thickness of 36.5 cm, I assume that it is not realistically possible to construct a KFW55 house with them.

My client is supposedly extremely skilled in thermal bridge calculations and construction (heard from third parties). Nevertheless, I will now listen to everything calmly and then decide what to do. I will also ask about the additional cost for the more expensive bricks with a lambda of 0.07. Let's see what difference that makes.

I just looked for and found a price list from Ytong.

Ytong PP 2-0.35 costs 176.70€/m³
Ytong PP 1.6-0.25 costs 291.30€/m³

That will already add up to a considerable amount for the whole house - I hardly think it will pay off with the KFW funding, and I won't significantly save on heating performance with it either. So maybe build without KFW?

A KFW loan is not always worthwhile. You definitely have to do the math beforehand.

Our GÜ now builds 55' standard only with 42.5' Ytong and triple glazing......together with a brine-water heat pump.

You especially have to consider whether you want to build a “Fertighaus“ meaning timber frame construction or monolithic.
Of course, the financial framework available to you also comes into play.

Everything else will follow.

Timber frame construction or monolithic as the 2 alternatives? Yes, and what about KS+WDVS? KFW55 can generally also be achieved with a 42.5 cm wall thickness and the "normal" bricks (Lambda 0.09). That could be more cost-effective. And living here in an old Gründerzeit building, I can only say that 42.5 is by no means too much wall thickness. But everyone has to know their priorities.

The aerated concrete price list is still, as far as I know, without VAT, right? Another 19 percent on top.