Energy Saving Certificate KfW55 - Target KfW40 - Question: Steps & Costs

Hello everyone,

I have a structural and energy question:
We are currently planning a house in timber frame construction with a basement. A ventilation system is also planned but has not been considered in this energy-saving verification. According to the energy-saving verification, it is currently a KfW55 house. See excerpts in the PDF. If further information is needed, please write.
When I asked the energy consultant what is necessary to achieve KfW40, he said that the basement insulation must be 16cm instead of 12cm thick. This resulted in the timber frame wall having to be thicker (also for structural reasons). When I asked if it would be possible to make the wood fiber insulation on the outer wall thicker (120mm instead of 60mm) to achieve a U-value around 0.11, he said this is not possible because the timber studs have to rest two-thirds on the floor slab or basement ceiling. This again would lead to a thicker stud. Overall, it would be about 30,000-40,000€, so it would never pay off. How these high costs arise was not explained to me in detail.

My questions:
1.) What do you think of the additional costs; could they be realistic?
2.) The wood fiber insulation boards do not have to be flush with the outer edge of the concrete, can they protrude? If the wood fiber insulation boards become thicker, they simply protrude a bit (6cm) further out without anything changing in the timber frame. And if the wood fiber insulation protrudes further, the basement insulation can also be thicker.
3.) What other possibilities do I have to achieve KfW40 cost-effectively?

I hope you can help me a bit so that I understand it better. Thank you very much

A classic example that KfW funding in itself is usually not desirable, but rather a nice gimmick if the desired house also meets the KfW requirements.

Whether the high costs are justified in your case cannot be answered; if you were to start planning from scratch to build a KfW 40, it would probably be cheaper than trying to upgrade from KfW 55...

Surcharge in my case €6200

Addition if I remember correctly:

€3000 for additional insulation of the floor slab
€3200 for additional insulation of the house

€3000 for additional insulation of the floor slab €3200 for additional insulation of the house

I find such an amount quite acceptable, which you almost completely get back through the additional repayment subsidy. You save heating costs and the house gains added value

definitely, especially when you have 2 living units