Single-family house with 2 residential units for KFW funding

I am still in the middle of planning a single-family house (approximate base dimensions 10x8.80m).

Not "in the middle" at all. You are still wandering around several times at the edge of the pool, simply due to lack of willingness for a systematic approach (as I had thought so far) ...

The KFW consultant said that we might go for two residential units so that we could get a total of €200,000 under KFW conditions...

... and now an obscure consultant is involved who risks his listing if he incites circumvention. If the KfW rules orient even halfway on the AO, then structuring eligibility for funding must not be the main focus or even be suitable to provoke criminal liability.

What might such a floor plan look like so that it passes as two residential units for the KFW, but can ultimately be used as a single-family house?

It will not work with a ten-meter ridge length because it will be most economically feasible if the granny flat adjoins the ground floor of the parent apartment: that means placing the children's rooms together with the children's bathroom and at least a kitchenette for independent household management next to it. Do you have near-adult offspring for whom a gradual "wilding out" (independent living) would make sense? Everything that can be said about the efficiency of upgrading the efficiency house tier (quite generally, not just from "maximizing funding" – a zero-sum game or fool’s errand) and about "fake/sham instead of real granny flats" has already been said.

One should not forget the 5 repayment-free years at the beginning.

But that only benefits a few, for whatever reason. I only had one or two years repayment-free, which I would have declined in hindsight, because I would have finished the financing two years earlier.

I would just look online with the search terms: "How can I cheat without getting caught." Besides that, this idea of the "advisor" (instigator) is simply also economic/planning nonsense; was he ever active in the coffee trip industry?

If two residential units are approved, then he is also entitled to funding for two residential units. Whether in the 2nd unit in the end the dog, the cat, or silverfish live is of no concern to the funding provider.

This has nothing to do with fraud at all. It has already been mentioned that the 298 conditions are not profitable.

But that only benefits a few, for whatever reason. I only had one or two years without repayments, which I would have declined in hindsight, because I would have finished the financing two years earlier.

It depends. In our case, it happens at a time when our wife is only working part-time or is on parental leave. If we still have money left at the end of the year, we can make special repayments with the difference at the bank.

For us, that amounts to a "saving" of 380 euros per month for the first 5 years.

But that only benefits a few, for whatever reason. I only had one or two years without repayment, which I would have declined in hindsight, because I would have finished the financing two years earlier.

With the KfW 300, the topic could be worthwhile if you put part of the saved repayment into the main loan with 3.X% interest. You would have to calculate that.

Whether at the end of the 2nd weekend it is dogs, cats, or silverfish living there is of no concern to the funding provider.

Sorry, that’s nonsense – I would write exactly that in the application according to reality and see whether it really is all the same and whether this does not contradict the actual purpose of the funding. I once knew someone who also always based his decisions on where he could get the most out for himself, regardless of the community. Terrible, but such things can be treated. So should I also design my house in a way to exploit possible funding? I can’t follow that, because such a floor plan requires adjustments that affect the main layout. Everyone complains about Ukrainians getting citizen’s benefits and whatever else (maybe rightly so, I don’t want to judge that), but most of the time it helps if you just look in the mirror and see someone who doesn’t behave any differently than what he loudly condemns in others, maybe even more reprehensibly. And: just because someone does this as a "German" in their own country doesn’t make it any better. Not everything that is not explicitly forbidden thereby becomes allowed. Cum-Ex also used a loophole.