Floor plan discussion: Single-family house + guest apartment as a multi-generational house on the northern slope

And what about the windows facing south in the basement? Above them is the balcony, so no light ever gets in there.
You also massively underestimate the effort a hillside location entails, just in terms of sealing/drainage for the house. Let it rain heavily sometime, and then water will run into the house through closed windows.

Why must a technical room be accessible to the tenant? I can neither get into the heating room nor to my electricity meter. Locked. What business do I as a tenant have there?

You want to give up at most 10m².
This is not going to work.

PS:
Normally one builds to have peace and quiet. And then you want to bring tenants with a child into the house?
Are they only allowed to use the terrace? Or the entire garden?
All very poorly thought out.

I would also plan a separate technical room. Because no landlord is probably interested in the following. "If the electricity meter belonging to the apartment is located in a locked basement room to which the tenant does not have a key, the landlord must grant the tenant free access once a month for the purpose of reading the meter (AG Cologne, judgment 201 C 464/12)."

I would also plan an extra technical room. Because no landlord wants the following. "If the electricity meter belonging to the apartment is located in a locked basement room to which the tenant does not have a key, the landlord must grant the tenant free access once a month for the purpose of reading (AG Cologne, judgment 201 C 464/12)."

Yes. I would also spend 30,000 EUR for 10m² instead of letting the tenant into my utility room for 5 minutes once a month. Especially since no tenant does it; I have not read my electricity meter once in 2.5 years. Why would I?

Oh, one more thing:
Don't fool yourself into thinking you'll have more financial leeway once you've moved in. In the first years, so much money will go into furnishing, making the place nice, etc.
Doing the garden yourself is fine, but the materials cost money too. And you have to get them at the hardware store, the landscaper gets them cheaper from the wholesaler. Machines are expensive too... compact loaders or mini excavators 200 EUR/day. Vibratory plate 80 EUR/day. And so on.

Neighboring buildings on the south and west sides Slope: slight northeast slope, the terrain rises from the northeast corner of the property to the southwest corner (approx. 34m) by about 2.2m Floor area ratio: Not explicitly mentioned in the development plan Floor space index: Not explicitly mentioned in the development plan

If not explicitly mentioned, one must probably assume that the building authority will use the surrounding buildings as a reference. I am not sure, but it is very likely (in case you missed it somewhere ;))

Number of floors: basement level + 2 full floors

I also assume that the building authority is aware that there is a slope there. And therefore, in addition to the 2 full floors, a basement is allowed which is built into the slope. However, this basement must not be a full floor, otherwise the development plan would state 3 full floors and no mention of a basement level. Your basement level, however, probably protrudes an average of over 140cm above ground level?

Especially since, from my point of view, the slope does not pose a problem,

… and I completely disagree with you, because for you it would be a problem to plan with 3 full floors.

I have not posted our project here to discuss the price already (that makes little sense as long as we do not have any concrete offers.) As mentioned in the initial post, I would first like to discuss the floor plan, especially the entrance, hallway, stair area. Are there any criticisms or suggestions?

Dream houses cannot be discussed realistically and constructively… otherwise one could advise you to plan another 10% for tight spots ;) No, seriously, what should a proposal or criticism be if you have to reduce 50 - 80 sqm from the planned 270 sqm anyway because you are over quota with the budget?! In principle, I like the floor plans (open, of course, flexible) except for a few things… (apart from the fact that the hallway in the granny flat is too narrow for barrier-free design) and the main apartment has no workspace in the kitchen and no storage space anywhere either (unless you want to store your suitcases and garden cushions in the pantry)… but I am of the opinion that a 3-floor building is not allowed anyway.