Large office or a separate apartment?

2 parking spaces per housing unit. We want a double garage anyway. I think the driveway (at least 5m paved area) could be called a double parking space ;).

Parking spaces arranged one behind the other are often not recognized because the rear one is not accessible when someone is parked in front.

But is it allowed to be additionally directly connected to the main apartment (door)?

That should not be a problem. For two of my aunts, the option was a granny flat in the basement with a separate entrance from outside, but with a door inside the stairwell so the grandma could come up and see the grandchildren without leaving the house.
Basically, I see little advantage in adding the granny flat on the ground floor. You then need a kitchen and bathroom in the separated unit, which costs space and thus money. Overall, your ground floor will then be large, and that affects both floors. We are currently planning with a study downstairs and a work/guest/spare kids’ room upstairs and already have the problem that our ground floor is half a room too small and the upper floor half a room too large. A complete second housing unit requires even more space.
Oh yes, and about sharing an office—I wouldn’t do that. You do quite a lot of phone calls when working from home, sometimes for a long time. Having quiet is definitely pleasant.

A few of them have a slight slope (I’d say 1-1.5m difference in height in the building plot).

To quote the basement rule by : For every 20cm difference in height, you pay 10% of the basement not to build it. At 1.5m, a slab foundation usually no longer makes sense, and at 1m it’s questionable. Especially in view of your plans to squeeze a granny flat into the ground floor.

Don’t bend yourselves. Often the space requirement on the ground floor is larger than on the upper floor. Rarely does the plot allow for a corresponding extension. If the granny flat is to be suitable for disabled seniors, you have a different space requirement. Check what is possible once you have your plot. I wouldn’t compromise and take flexibility just because it might be nice someday. Regardless, I would plan for 2 offices. Rather smaller. With a large one, you might disturb each other. No idea if the driveway can be approved as a parking space. Actually, the garage would no longer be accessible because of it.

To quote the basement rule by @11ant: For every 20cm height difference, you pay 10% of the basement cost to not build it.

It is known (very briefly put) that "with two meters of height difference in the building window, an unbuilt basement costs practically the same as a built one and merely offers no value for these costs."

So: "to not build it" is wrong, it must be: "whether built or not."

I don't think it's fundamentally bad (but rather good) to create a separate unit for the offices. However, I must also say right away that I don't think it's good if this is done only because of bonuses or subsidies. I or we as taxpayers have to support it, and if no immediate rental housing is created, I actually don't see why I should help finance it. A separate unit is wise in itself, to gain distance from the domestic environment. But how is it if children have to be cared for in the main apartment? As a teacher, do you want to unlock two front doors in the evening when you just want to quickly correct dictations? And my predecessors say it: it is already difficult to realize your own needs on one property – so what about the sensible space of a granny flat? For you two, an expanded attic would actually suffice, which hardly affects your finances noticeably. So why tie yourself to the burden of a granny flat?

Good morning,
thank you for the numerous replies.

Parking spaces one behind the other are often not accepted because the rear one is not accessible when the front one is parked.

Yes... that would not be a knockout for the 2nd residential unit.
In our apartment building (same municipality, built in 2012) there are double parkers in the underground garage. The cars (owned by the same apartment owner) are parked one behind the other and block each other’s exit. Wouldn't that be a similar situation?

Regardless, I would plan 2 workrooms. Better smaller. With one large one you might disturb each other.

Since I don’t have mandatory presence times in the office, I can organize my time myself and decide when I work. Usually it’s in the evening when my husband no longer works and can take care of the children.
Also, my husband only needs the home office sporadically. Most of the time he works on site at the company office.

However, I must also say right away that I don’t think it’s good if you do it only because of premiums or subsidies. I or we as taxpayers have to support that, and if living space is not created for rental, I actually don’t see why I should help finance it.

Nowhere in the KfW guidelines does it say that the 2nd residential unit must be rented out. I think that was also intended by the legislator. Some people purposely rent small apartments to be able to work in peace. We don’t want to set up children’s rooms there, but private offices.
In old age, one would still be more flexible and could, for example, offer a separate living space to a caregiver (instead of moving directly to a nursing home).

But how is it when children have to be cared for in the main apartment? As a teacher, do you want to unlock two front doors in the evening if you want to quickly correct dictations?

As I wrote above: my husband and I never work together. One of us always has the task of taking care of the children.

As already written above: my husband and I never work together.

Well, then one room is enough.