Self-managed construction project Simple single-family house

Can he do that or does he recommend a builder or something, I don't want an architect ... it should be a simple 4-sided house without anything else

Well, you can't just build a house completely without these things (at least in Germany). Structural calculations don't just fall from the sky either...

Because of "Erkenntlich zeigen," take a look at the BG Bau website; it states what effort can be demanded from private construction helpers. Everything beyond that is considered "employment" and must also be taxed normally.

One must also distinguish between an illegal construction and building illegally ;)

Have you already spoken with a certified energy efficiency consultant? You need one anyway to claim the funding.

Not necessarily. For KFW 55 there is a simplified proof procedure. If certain U-values for the roof, walls, windows, etc. are met and certain heating systems are installed, a calculated proof is no longer necessary.

a computational proof is no longer necessary.

And then you don't need an expert anymore?

The KfW says:
"An energy-efficient new building requires sound professional planning and qualified support during construction. As part of the planning, application, and execution of a funded project, an expert is required to support the builder."

I thought even if you adhere to the reference values, you can't confirm yourself that the house has KfW xx.

Does anyone have experience with how to get cheaper building material prices? What use is doing the work myself if I have to buy at expensive prices...