KFW70 with gas-solar heating

As far as I remember, the house construction must be accompanied by an expert who certifies the construction according to Kfw 70. Afterwards, it is no longer possible, certainly not after a longer period of time. Then there would be many builders who want to have the certificate afterwards.

KFW doesn’t say anything except that you achieve a certain insulation standard (HT) and a certain primary energy demand (kWh/m²a). I believe you don’t get a sticker there, only the energy traffic light to stick in or something ^^ It’s more of a sales argument for the "now" or for the next few years. Eventually, you have to say "KFW 55 according to the Energy Saving Ordinance 2009" or something similar.

We are facing the same question of whether to build according to kfw 70 or "only" according to the Energy Saving Ordinance. The price premium for additional insulation and solar thermal energy amounts to almost €8,000! The interest rate fixed for only 10 years puts me off a bit. On top of that, there is the 300l tank in the small 9sqm utility room! Is it really worth it only in terms of a possible resale? Because for €8,000 I can get a lot of gas...

What is supposed to cost 8 T€ there - why do you need a 300 l tank?! Just so you can use the ST? A 300 l tank is not that big. The ST rarely pays off economically, as you use too little hot water.
What do you get from KFW70? About 2 T€ interest relief over 10 years and then, of course, the gain from saved energy consumption - whether you can save the 6 T€ here is uncertain... but it won't be a huge loss either.
If you now artificially add measures that don't actually fit the house concept, I would question that too - especially with such strange heating concepts. A little more insulation never hurts.
What does your KFW70 depend on? The HT (i.e., the insulation) or the primary energy demand?

What does the wall structure look like without the package? How is it afterwards? What else changes? Roof insulation, base slab, etc.? The 300 l: I assume you really mean ST and not photovoltaics?