Floor plan design of a basement bungalow with a granny flat

It's always nice to include your parents in the plans, but did you actually ask them if they want that? And if yes, then plan that really firmly and they can contribute to the financing. Then you save yourselves a bungalow and basically make a 2.5-story house. Parents in the basement and you in the upper floors. The garden access then runs via stairs. Oh, and of course no garage, but a carport. It's more convenient anyway, as it blocks less light from the entrance area.

Personally, I would plan without a granny flat. Just a normal 1.5-family house. Then rather look for a well-located condominium that is nearby.
Consider this: You are going heavily into debt now (the apartment is not great, it won’t be snapped up. Maybe suitable for commuters) and later the parents will have to go into a nursing home.

I know my parents' thoughts. 1. stay in the house as long as possible. Possibly hire a caregiver from Poland who lives in the house 2. if that doesn't work, rent out the house and rent a barrier-free apartment next to the local nursing home, which is still in planning. Same street, same faces, lots of freedom and help whenever needed. 3. never be a burden to the children In-laws similar. Mother and mother-in-law said that when they are widows, they will come to us if my husband is not there, so that I am relieved. They only want 1 guest room then.

Mother and mother-in-law said that when they are widows, they will come to us when my husband is not there so that I can get relief. They only want one guest room.

Both in one?

We have a similar project planned right now, but we have planned the use of the EWL in terms of the office or later use by our own child. Later rental is also an option. However, we also have experience with it. We have slightly spaced out our private area with a buffer zone (dressing room). Honestly, I only realized from this discussion here that we solved it so positively to separate ourselves a bit from the open plan living area. Or rather, that was the idea of our architect. What bothers me about you guys is the entrance through the basement. Due to the topology of our property, we were able to solve both the EWL and bungalow in a barrier-free way / will solve when it finally starts. For that, our EWL has a north orientation. As inspiration, my plan here: https://www.hausbau-forum.de/threads/Bungalow-geplant-auf-vorhandenem-Keller-Ideen.33057/page-3 Apart from these points, I think your plan is good.

No I can't stand both together. Then they even mother me. Acquaintances have solved it like this, grandma has her own room, and if it gets too much for everyone, she goes home. Win-win for everyone

I find both apartments terrible. The granny flat might be generous for a single person. For an elderly couple, absolutely insufficient. You don't have to sugarcoat it by saying something about adjusting bedroom windows or so. The living space, which is also meant for sleeping, becomes extremely dark. Although a slope often means living in the basement, which I always prefer, here you still step into a cellar space because the terrace of the main floor of the granny flat takes away a lot of living quality. I have seen much better basements with granny flats or living spaces. Regarding the main apartment: I am a fan of few walls and open living. For that, floor plans can also be different sometimes. As someone without children, I can afford somewhat freer living, as you only have to consider your partner. But here there is absolutely no privacy for the parents. Where does the partner stay during a skat, football, or girls’ night? Yes, you could build a wall. Then at least you have a TV wall. But also a huge hallway. The way to the bathroom always leads through the kitchen. Great, if you sometimes don’t feel well or want to do body care, but the children have visitors and are occupying the dining table. For the granny flat, you are sacrificing a nice, common, and spacious floor plan yourselves. Instead of an optimum, you are planning two living units full of flaws. My advice: instead of four parking spaces, plan only two, build smaller, and give the kids a nice children’s floor in the basement. When they have moved out, you can always let one or two seniors live with you.