Federal funding for efficient buildings (BEG) from Q1 2021

The start of the project is considered to be the conclusion of a delivery or service contract attributable to the execution.

Does that mean the start is the day I agree with my first craftsman (structural builder) that I want to build with him?
How is that supposed to be verified in practice?

From that, I would conclude that the date of the building application or the building permit is not decisive, right?

I understood it the same way, that neither the building application nor the building permit play a role in this.

I will probably just call and ask in the next few days.

Good question... just call the KfW hotline. It's free and they will answer your question. You can then write what they said.

Do I understand correctly that for a new building during the transitional period until 01.07.2021, one basically gets nothing regarding the funding of a heat pump (+ controlled residential ventilation etc.)? The old BAFA program has been discontinued and I can only use the old KfW subsidies. The new BEG would compensate for the discontinued BAFA funding, but that will only be possible from summer onwards.

Yes, during the transition period you are somewhat out of luck if you have not already submitted an application to the BAFA as a precaution in December. Especially the point project start = contract conclusion causes a bit more problems than it seems at first glance. In case of doubt, build normally and continue without funding instead of risking a price increase in half a year.

Could I then, for example, start with earthworks and the foundation slab in May / June? Paying with equity

And everything else from 01.07.? So signing the contract (house construction), applying for funding, ...

That means you could only finalize his financing in July... It's a gamble how the interest rate will develop...