Roof insulation hip roof new building KfW 40

I just don’t quite understand yet why this variant is better than letting the heat through the insulation directly at the roof go straight outside from the house.

This is called a cold roof and is the most cost-effective construction for the roof. One surface, which is smaller in area than the four sides of the hip roof. The insulation between the rafters is much more complex.
As always, you have the choice between a non-convertible roof (the standard variant) or a high-quality upgrade to a usable variant, where the height or roof pitch will still play a role.
So if it’s just a flat hip roof variant anyway, where there is no standing height, then there is no need to include the roof in the thermal envelope.

Why do you absolutely want the 40Plus? I don't see any advantages in it except for costs

Not quite, for our prioritized home builder the additional costs between KFW 55 and 40 are extremely low (<30,000€). Since we are planning with a granny flat, 80,000€ funding from KfW is already more than lucrative… (We want to have the photovoltaic system anyway to receive the Plus.)

Thank you for the explanation! So it’s only about costs!? Is the insulation in the upper floor ceiling much worse than between the rafters? Removing it doesn’t make sense for us, due to the slope we need a basement and therefore have more storage space than we will ever need. And in addition, because of the limited permitted building height of 8m, we are not allowed to go much higher.

Is the insulation in the upper floor ceiling much worse than between the rafters?

No, it is not. In your case, there is absolutely no reason to consider a different roof construction including different insulation.

That sounds really good already! Thank you!

Would the additional insulation between the rafters improve it a bit? As I said, I think such things could still be negotiated…

Would the additional insulation between the rafters improve it a bit?

You might not quite understand it yet. If the ceiling of the upper floor is insulated, the roof remains air-permeable and uninsulated. If you want to negotiate something, then a thicker insulation of the top floor ceiling would be suggested. The question is whether that still brings any significant advantages given the already good U-value. The marginal benefit of additional insulation is extremely low.