Planning single-family house with bicycle workshop

Ah, nordanney, if you still distinguish there: washbasin and toilet downstairs (+ bathroom with shower/bathtub upstairs) are a "must" for us - an additional shower is not necessary, but would be a "nice to have" that we would appreciate, especially after sports or when I come from the stable.

There is a supplementary statute that regulates the development, but it only states regarding parking spaces that paths, parking spaces, and driveways require a water- and air-permeable structure. Further down, where permissible roof design is discussed, garages, carports, and outbuildings are listed as exceptions to the regulations concerning roof shape. As far as I can see, there are no further regulations, but if necessary I would try to call the municipality again.

right after sports, or when I come from the stable.

We manage well ourselves with three riders (at least 20 hours per person/week in the stable) and a "family bathroom". That works – more is of course always better. But then it also has to be paid for and costs space on the ground floor and thus also on the upper floor (or vice versa).

Is there a planned driveway to the NE on the street that is 3 m away from the property?

I had overlooked that point in my answer. Yes, there is a driveway to the NE. This is explained by the fact that in 2025 a footpath will be built along the street, and in doing so, driveways to the properties will also be created. Two properties always "share" a 6 m driveway, which is located either on the right or left at the "corner" of the property depending on the property orientation.

I find a family bathroom on the upper floor as the only option unfavorable, especially since when I come home - the first thing I do after taking off my shoes is to wash my hands. The bathroom is currently always a point of contention for us when we are getting ready in the morning. We keep getting in each other’s way. Or when we’re both working from home. Everything is doable, or possibly not a problem in a slightly larger bathroom, but the problem could be eliminated. I keep it in mind, honestly I was not aware that this is already so unusual, because actually almost every existing house (sometimes even smaller bungalows) and the typical floor plans all have a small bathroom, a "guest WC", on the ground floor.

also "guest WC" on the ground floor.

Toilet + sink on 1.5*2m is standard, yes.

to plan a "guest room" on the ground floor that gets an outside door/patio door, through which the bicycle can be brought in directly (It would also be conceivable to start directly with the racing bike from there and bring it back in through this side door.

Workshop room

Do I understand correctly that you want to use the guest room on the ground floor basically as a heated workshop room?
Honestly, I don't understand the reason. Is it supposed to be used commercially? Or does one of you just enjoy tinkering with his racing bike?
Wouldn't it then be more sensible to tinker in the garage, where you could put a radiator if it's too cold? If necessary, it could also be a fixed shed? At least that is cheaper than a room in the house, which hits you with 3000€/sqm?!
Or if it is commercial, is that even allowed in a residential area?

No, not commercial. Initially, we planned with a cellar, which was then basically "replanned" as an additional room. We didn't even include a garage in the planning because we don't necessarily consider it necessary for the car; we can also build a carport ourselves.

As long as the room is warm and the moisture doesn't completely seep into your bones, it's okay. Unfortunately, the cold, wet season is the time when the bike needs the most maintenance, or at least the everyday bikes that are also ridden to work. We will definitely take a closer look at the idea of the garage. Planned directly at the house, immediately with electricity, etc., it could be a good option. I also like the idea of possibly planning a parking space in front and, if it makes sense, covering it with a carport; this way, you still have the option to "build up." But I am not the user of the bike workshop either; my partner will have to assess what is manageable.