Additional costs for residential units

Is it documented anywhere what one must have done to be eligible for subsidies for a housing unit with KfW40+?

Yes, on the KFW pages

Basically, are connections, insulation, and air filters (and the other structural standards) sufficient?

No, as already said. No opinion, facts

Or does the thing have to be completely finished, and do I have to open the door to the first tenant on the first day after construction ends?

Yes, facts. To be read on the KFW pages

that is why the prerequisite of construction completion!!!

Already written here on page 2

According to my (of course amateurish) calculations, I come to about 9.70 m ridge height. Up to 10.5 m should not be a problem on the property.

It will certainly be higher. That is an opinion. Reason: the staircase must be passable upwards in the pitched roof and thus a clear room height must be present, not only at the staircase but also in the living area (housing unit). In addition, habitable rooms, including in a granny flat and especially when a rental unit, i.e., an approved apartment, is supposed to be created, require a minimum room height which depends on the state building code. But your consultants have surely already told you that. I don’t even mind the rest ;)

by the way, we don’t know whether you intend to build it with rafters or trusses

I just read into it a bit. It might be that I don’t fully get it yet, but to me it seems that a truss roof is not an option because there are tons of beams crisscrossing through the roof structure. Converting it would then be out of the question.
That would leave the rafter roof or the purlin roof. There is relatively little information overall about hip roofs, especially with a pitch of 30-35 degrees.
I will discuss that with my architect.
Do you have any advice on the roof construction and which type is basically most suitable for a conversion?

Yes, facts. Can be read on the KFW pages

I honestly could not find a list of exactly what must be present there, even after some searching. Neither in the descriptions of the funding nor in the FAQs. Interestingly, only what other people/companies do is eligible for funding, not one’s own labor. But that does not tell me yet what exact level of conversion must be achieved so that it counts as a legitimate third residential unit for the KfW.
So I simply wrote to the KfW customer service. Let’s see what they specifically expect.

Moreover, living spaces, also in a granny flat and especially when a rental unit—i.e., an approved apartment—is to be created, require a minimum ceiling height, which depends on the state building code.

The possible ceiling height in the hip roof honestly surprised me very positively. I roughly sketched it with 35 degrees. The house currently has a floor area of 10.5 x 10.5 meters and a 1-meter roof overhang. At a 35-degree slope, I reach a ceiling height of 1.50 meters within a few meters (from there I would want to put in the walls, i.e., start the room). One meter further I’m already at about 2 meters ceiling height. At the peak/middle that means ceiling heights of about 4 meters (insulation and floor are not included here, so this is all just rough estimation for now). As far as I calculated, this creates a living area of 8 x 8 meters (or about 7 x 7 meters if you only count the area with ceiling height rising around 2 meters) in the attic.
Of course, it might be that I am overlooking something crucial. But so far it looks pretty good and would be too much space for me to just give away unused (whether it becomes a subsidized official residential unit or I simply convert it into a residential unit without subsidy).

35 degrees

10.5x10.5

Room height of 1.50

As far as I have calculated, another living space of 8x8 meters (or approx. 7x7 meters) is created

I sketched your data, which you should also do. These data were still missing. Since I don’t have a protractor at hand, I approximated the DN from 30 and 45 degrees. But it roughly comes out as I thought: You are making a conceptual mistake. With a room height of 2 meters, you get about 20 sqm. Of these, less (about 12 sqm) have a height of 2.30 meters. The stairwell would have to be here. And the shower. And the recreation room, if I assume a minimum height of 2.30 meters without knowing otherwise. So, if you want to stack waste pipes and drains from bathrooms and kitchen, then in each floor within these central 12 sqm. And the stairs. The knee wall of 1.50 then results in where the windows in the attic? Where would the emergency exit be in your opinion? Regarding the subordinate apartment itself (which ironically dominates both other floors): how much rent should the chargeable 16 sqm bring? Who would be the target group for this “apartment”? As far as I know, it must have at least 25 sqm. I don’t have a source. Is it "cost optimized" for you? I am somewhat surprised that, according to your statement, something is already in planning, but the roof topic apparently not at all, not even the concept of the section exists in your mind.

I have a list of what exactly must be present, but honestly couldn’t find it there even after some searching. Neither in the descriptions of the funding nor in the FAQ.

Because it is already integrated into other principles that funding is only paid out after the construction is completed. Common rooms are also defined in the state building code. Apartments as well. Almost everything is defined. You just have to put it together. The KFW assumes that an architect informs his clients. There are simply so many who can only afford a house with various theoretical subsidies. Many come up with ideas, just like you, suddenly see a too expensive plot, start googling for funding, talk about cost optimization, and don’t even realize how this idea of the granny flat dominates the house. The questions no longer revolve around how to build a nice, fancy little single-family house; the questions only revolve around how to best realize such a granny flat thing (regardless of whether one even wants to rent the unit out, not to mention the construction costs and rental taxation). That one’s own apartment or house is completely botched in planning is not something we necessarily see three times a week here, but the trend is increasing.

That one’s own apartment or house is totally poorly planned is not something we necessarily see 3 times a week here, but the tendency is increasing.

Anyway. I can only admire your tirelessness and must sadly admit for myself that I have to capitulate before the genius of the OP and probably can do nothing more than wish him all the best in this case. (Günni, no Doornkaat today, some days I am just too old for this world, here’s to a Dujardin).

I sketched your data, which you should also do. These data were still missing.

I have already done that for some time. With a roof overhang of 1 meter, you get a roof surface of 12.5 x 12.5 m. With a slope of 35 degrees, you roughly reach a room height of 1.5 meters after about 2 meters of roof slope. A room height of 2 meters inside the room is reached after about 2.7 meters of roof slope. If you now subtract the 2 or 2.7 meters from the 12.5 meters on both sides, you get an internal dimension of the attic of about 8 x 8 (or 7 x 7) meters.
This is of course calculated very kindly, you still have to see how much space the insulation additionally takes up, but basically it goes in this direction. It may be that I am overlooking something very big here, but I wouldn’t know what. At least from your statements I cannot read exactly what is supposed to be wrong with my estimate.

Many come up with ideas, just like you, suddenly see an overpriced plot, start googling for subsidies, talk about cost optimization, etc., and don’t even realize how this idea of the granny flat dominates the house.

To be honest: gladly judge others, but please leave me out of it. I can decide well for myself when something dominates an idea and which frogs I am willing to swallow. I have to say that the 3 residential units and the kitchens/bathrooms stacked on top of each other have not significantly limited me so far. I was able to reconcile that quite well with my ideas. If it were not the case, I would not pursue this plan further.

that subsidies are only paid out after completion of construction.

No one doubts that. The question is the definition of completion of construction for the KfW. I am very curious about their answer. Until then, I will just wait before I judge or plan more concretely in the matter.
Honestly, the subsidy for the third residential unit is not a matter close to my heart. If it is possible, I think that’s good. Since the attic is supposed to be used that way, I would personally also consider that fair.
If it should not be possible because something prevents the classification as a residential unit, I can live with that too. Then I will simply convert it accordingly without this classification/subsidy. Since it is used within the family, a legal classification as a residential unit is not necessarily required.