Detailed questions on floor plan design renovation with extension

Thanks for your feedback. The granny flat basically already exists. Only, in the last 60 years, someone from the family has always lived there, so it was never officially divided. We discussed the option last year to put our living spaces in the basement. However, we prefer the option upstairs for various reasons (among others, higher ceilings on the ground floor, direct level access to the kitchen) despite only indirect access to the garden. Since the plot is relatively long and slopes further downhill, the way back into the house is long anyway. The roof needs to be redone. Currently, the top floor is finished very badly as a do-it-yourself project. Due to the lack of a knee wall, there are two narrow chambers and one large room. We don’t like that at all. The extension basement has existed for about 30 years. Nobody can really reconstruct why it was built then. The electrical distribution box is already in the extension basement; we have to plan a new heating system anyway. The oil heating is simply too old. What remains is the routing of the pipes. The effort is worth it to us. Almost everyone who sees the house finds the arrangement of the technical room right in the middle of the living spaces strange. That would bother me too if we were to move the living room downstairs after all.

@Katja see answer above.

Recently, the ground floor and basement were lived in separately, which is why there are two kitchens, living rooms, etc.

We like the rooms downstairs, among other things because of the lower ceiling height, no option for underfloor heating, having to carry all groceries down, and more impractical daily routines.

But maybe we are also thinking too much in the structures as the house has been used so far.

I have read my name. I am for a residential UG and direct garden access.
especially garden to rental apartment there is no privacy at all.

Children love fresh air until they eventually go to school. Then it becomes less. First you have the problem they can't walk, then they become more mobile but still cannot stay alone for a long time, so you carry everything including children up and down.
Water, sunscreen, diaper stuff, toys and the child. Then child 1 runs off and you still have another child by the hand. It keeps you fit and is impractical. I had that for 3 years. It is really true that you have to carry everything up and down with a child, quickly grabbing something is not possible.
Eventually the children walk safely and are a bit bigger. Then it is enough to open the door, kids go outside and from the kitchen you have everything in view. With you only the tenant, not you. So it takes time for you until the kids. Children's birthday parties, bigger celebrations all up and down, even if you just want to read a book in the shade you always have to go up and down and the tenant gets to enjoy your view. I would even feel uncomfortable there.
Friends of ours live in an older house - back then the garden was a utility area and deck chair, eating outside or even a pool for fine people - you can only say every walk keeps you slim and finally the youngest is so old that no one has to jump because it wants to come in or out anymore.

The separation of living and garden is no longer common in most new buildings in our area.
Many new buildings have building technology on the living level - we do too. What should bother anyone about that?

lower ceiling height,

What exactly are we talking about here?

Can be on the same level as the kitchen

Experience shows that the garden connection of the living areas surpasses all other wishes. This discussion has come up here quite often. Everyone, but really everyone from my circle of relatives and acquaintances who has it differently, eventually moves the kitchen or part of it downstairs. Now you apparently already live there. That is why it is hard to understand why it is not the case for you. How is it with you now? Do you use the garden at all? Does someone live downstairs?

Experience shows that garden access from the living areas outranks all other wishes

I can only underline that. A good example from the residential area where I used to live (new development). One party loved having the sleeping area with garden access and cooking upstairs and looking out to the garden from the living room. Then unfortunately one person died far too soon (before that, with illness, it was already a struggle that the living room was upstairs) and the sale was imminent. The interested buyers successively withdrew because no one wanted to pay a high price for a house only a few years old and then take on a complicated renovation from downstairs to upstairs. Access to the garden is - especially with children - the be-all and end-all.

What exactly are we talking about?

I would be interested too. In the end, it was enough for an official living area.