ᐅ Additional insulation work for the foundation slab when building to KfW 40 standard instead of KfW 55

Hello everyone, we are now deciding between a KfW55 and KfW40 standard. Could an expert roughly estimate how much more expensive, in percentage or euros, the additional insulation measures for the foundation slab would be for a KfW40 compared to a KfW50? Maybe the difference is negligible. We have no idea about the effort involved, and of course, no one can tell us in advance what a foundation slab would cost for our project. The house will have a footprint of about 10 x 10 meters (33 x 33 feet) and will be built slightly elevated due to flood protection requirements. I appreciate any helpful opinions.

Insulating the concrete slab alone certainly isn’t enough, is it?

No, of course not. Exterior walls and the roof will also be better insulated. We already know the additional cost for that. The only question is how much more the foundation slab will cost—compared to KFW 55.

Then ask the person who will be constructing the concrete slab after they have found out which better insulation they need to install.

I would always choose an insulated slab-on-ground foundation. It offers better thermal bridge-free insulation, can be constructed quickly, reduces construction time since no screed installer is needed underneath, avoids adding extra water (from the screed) inside an already sealed building, and is generally more cost-effective than a comparable conventional slab-on-ground foundation with a sub-base insulation that still isn’t continuously insulated.

"Climate slab" = "Marketing buzzword invention"