KFW 55 / Decentralized Ventilation / Exterior Wall

What happens if I hire a KFW expert who calculates everything for me with central controlled residential ventilation and then I do not make use of KFW 55? The costs are lost, right?

If you hire an expert, you of course have to pay them. KfW 55 is not rocket science compared to kfW70 in terms of what needs to be built. Plan for more money for walls, controlled residential ventilation, and a heat pump as heating, and you usually have 55.

Ok. Since I already have an air heat pump included in the house price, already 36 cm walls, maybe only roof insulation and the ZWL are still coming my way, that should not exceed 18,000 by much. I will tackle it... let's see. Thanks for now.

My developer has probably never installed such a system either, at least it never came up in the preliminary talks. I think that's a bit of a shame

A bit of a shame that you're just realizing that now.
If you deal with house construction, you also deal with the Energy Saving Ordinance. Then with KfW, then with controlled residential ventilation.
New construction: always with central controlled residential ventilation, decentralized is just an alternative.

What happens if I commission a KFW expert who calculates everything for me with central controlled residential ventilation and then I don't use the KFW 55?
The costs are lost, right?

You don't need a controlled residential ventilation expert. You need the offer for a KFW 55 house. The rest is done by the developer by having it calculated.

Don't forget: You buy what you want. Also with house construction. It's not the developer who decides, but you.
If in doubt, use your 14-day withdrawal period and find someone who will build you a technically up-to-date house. Nowadays, if you can afford it, always with controlled residential ventilation.