Could the financing work?

Please provide constructive criticism without any reservations

So I also think it would work. However, the amount would be too high for me!
Also keep in mind that you should secure the amount accordingly (e.g. risk life insurance). For the loan amount, this also results in a higher insurance premium.

The idea was to build a home:
Plot 400m², KfW 40 Generation 5.0 200 house with a basement for 4,500,000 including everything turnkey with 50k ancillary construction costs.

Here I see the starting point: Does it really have to be KfW 40? I too initially had the illusion that this would be the cheapest option in the long run. But I believe with the investment costs it does not pay off. The basement is also not necessarily required anymore today. But that wasn't the question.

So I also think it would work. However, the amount is too high for me!
Also keep in mind that you should secure the amount accordingly (e.g. Risiko-LV). With the loan amount, that also results in a higher insurance premium.

I am aware of Risiko-LV, I definitely need that.

Here I see the starting point: Does it really have to be KfW40? At first, I also had the illusion that it would be the cheapest option in the long run. But I believe the investment costs do not justify it. Nowadays, a basement isn’t necessarily required either. But that wasn’t the question.

I haven’t really dealt much with KFW40 or other variations. Is it not worthwhile? I would be thrilled to hear about experiences and insights here. However, we definitely want a basement.

So from my point of view, clearly no: get different offers made and then you can calculate how long you can heat a "normal" Kfw 70 house for the extra cost. With this standard, you normally have a good price/efficiency ratio.

From my point of view, clearly no: get different offers and then you can calculate how long you can heat a "normal" KfW 70 house for the extra cost. With this standard, you normally have a good price/efficiency ratio.

At Weberhaus, KfW 55 is already included; for KFW 40, there would be additional costs of 7500 euros. Even after a 2500 bonus from the L-Bank or KFW, it is 5000€.
With an additional consumption of 15-20 kWh/(m²·a) and an air heat pump with a coefficient of performance of 3, that would be a saving of 1000 kWh of electricity per year.
So about 25€ per month -> 300 per year -> 16 years until it would pay off...

So yes, thanks a lot for the tip.

Maybe you can get the basement cheaper if you currently have a [Weberith-Keller] in it, otherwise think again whether the basement is really necessary. What heating system have you planned?

Maybe you can get the basement cheaper if you currently have a Weberith basement in it, otherwise reconsider whether the basement is really necessary.

It is the Weberith basement. As a functional basement.

What heating system have you planned?

the Proxom Wohlfühlwärme technology -> doesn’t really convince me either.
According to a blog about Weberhaus heating technology, you can probably remove it (save about €40,000) and assign a new one yourself... more stress but potentially something better. Geothermal energy with underfloor heating.