Additional effort for floor slab insulation in KfW 40 instead of 55

Hello everyone, we are now faced with the decision between a KfW55 and KfW40 standard. Could an expert roughly estimate how much more expensive in % or € the additional insulation measures for the slab foundation execution for a KfW40 would be compared to KfW50? Maybe this can also be neglected. We have no idea about the effort and, of course, no one can tell us in advance what a slab foundation for our project would even cost. The house will have a footprint of about 10 x 10 meters and will be built slightly elevated due to the flood protection area. I am grateful for any helpful opinion.

Insulating the floor slab alone is certainly not enough, is it?

No, of course not. Exterior walls and the roof are also better insulated. However, we already know the additional cost for that. The only question is how much more the floor slab will then cost - that is, compared to KFW 55.

Then ask the person who is to construct the base plate, after they have found out which better insulation they have to/should install.

I would always choose a [Klimabodenplatte], better thermal bridge-free insulation, quickly constructed, construction time shortened as no screed layer is needed below anymore, no additional water (with screed) in the already sealed building, and in terms of cost anyway cheaper than a comparable conventional floor slab with under-slab insulation which still is not continuously insulated

"Klimabodenplatte" = "marketing buzzword creation"