Double KFW 297 funding through two residential units?

And you think that's okay? 2x funding, but only one effective housing unit? I call that subsidy fraud.

It would only be subsidy fraud if the legal requirements were not followed. Fraud always involves an act of deception that causes an error in the other party.

Since - contrary to common belief - many outstanding lawyers are employed in ministries dealing with processes such as funding guidelines, opportunities for creative design (within the regulations!) are certainly desired.
If the guidelines are now different, that only shows how simple a change is when subsidies are to be distributed differently.

This interpretation of the politically desired subsidy fraud on demand is not so original after all. In any case, I do not consider a tea kitchen and a separate water meter for the children's bathroom to be keine "second residential unit" within the meaning of the funding purpose.

But I already see that it probably only makes sense in the basement

It actually makes less sense in the basement, since you need to create living space. For that, the basement often has to be partially "dug out" so that you have adequate window areas for lighting and ventilation as well as an entrance, which in turn restricts you in the garden. A second emergency exit is also a requirement. Then heating and so on, which makes the basement, which nowadays is already hard to finance even as just a utility basement, even more expensive. A parking space also needs to be accounted for. In return, you might then have a tenant below you who has the right to express their personality.

But I can already see that it would probably only make sense in the basement and that I would then have to discuss it with the architect once the soil survey is available.

No. "He who digs a pit for others falls into it himself" - this saying could have been invented for A-L-L forms of granny flats, where the builders' imagination has been stimulated by greed for subsidies. There is N-O second dwelling unit whose creation really pays off. At whichever point the additional living space associated with extra costs is generated, it makes no difference. In the long run, without exception in my forty years of experience, it is a Pyrrhic victory to go to great lengths to create a second dwelling unit. The only positive effect can be a financing booster, but even that is overrated, and the long-term effects are neglected to be offset against it. When the model was a tax advisor's favorite half a century ago, tenants still did not require a second controlled ventilation system and so on. Playing landlord is fundamentally not profitable on the scale of "1 dwelling unit". A granny flat tenant is someone you have on your hands. Several apartments out of sight of the main house (and the comparability of the standards of main and granny flat!) are a completely different world - that is just for the dreamers who think they can also be "a little bit" landlords here. You cannot be "a little bit pregnant," it’s all or nothing. In individual cases the idea may be tempting to let the offspring take a small step out of hotel mama - but apart from the case of , I cannot recall any functioning example (and the latter requires a comparable four-building plot). So: dreamers of all countries, unite for the common cause of pushing aside the pipe dream of the self-paying granny flat!

And in Hampshire, they were really separate living units that could easily be rented out.