Since you want to build with KFW, you need an energy consultant anyway. Only he can tell you the exact requirements, everything else is mostly groping in the dark. I would probably also recommend the "better" brick, and what I have read so far as an extra charge sounds pretty good. Of course, the building envelope is an important point in this calculation, but just as much the issue of thermal bridges, windows, roof, etc. The best shell is useless if other components are mediocre, or from a certain thickness on, insulation, measured against the application, no longer has such a strong effect. If you "save" on the brick, then you just pay for it elsewhere to achieve the desired value. This "playing" with the values is done by the energy consultant once he has recorded your house. After that, you quickly have your result or see the possible/sensible ways. One should also be aware that achieving such a standard often costs more than the subsidy provides. It’s supposed to be a subsidy and not a complete government payment takeover (I'm not referring to you here!). Sometimes I have the feeling that the subsidy is only seen as a grant from which something should still be left over in the end. I think it can cost more if necessary, because I also wanted a better-insulated house, Kfw or no Kfw. Isn’t there also the T8 brick? I had read a bit about it at the beginning for our area?