Single-family house approx. 155 sqm + separate living unit 40 sqm. First draft. Any suggestions for improvement?

the architect is an idiot who can't even correctly add up square meters...

either the architect or the OP. I didn’t want to say it that directly to him. The fact is that in the main apartment alone, the plans count well over 180 m2, which is 25 more than the "about 155 m2." In the granny flat, it’s another 7 more. So in total, 30 m2 extra, which surely no one will just give you. If you want to call that "nitpicking," go ahead...

I find the floor plan itself already coherent, but this huge dining area also caught my eye, which just seems to be "empty" space (between kitchen and dining table).

Students also often change apartments, so you constantly have to renovate.

Nobody here wants to "ruin" anything but rather help, even if sometimes you only realize that at second glance.

Nobody here even wants to ruin anything but of course I have also noticed small scratches. And it’s not like just because I don’t complain sometimes, nobody else would be allowed to. But I see this as a generally well-rounded matter, although I have to agree with the criticism about the garden access. "Students" by the way are quite a heterogeneous group; besides mechanical engineers, there are also theologians. For a granny flat, other rental applicants should definitely be found than the lower average of those who populate dormitories. Also, a single-family house living environment has an influence. So, everyone: get a grip again, we have already been presented with considerably more hopeless crutches here – this is at least a fairly well-designed house, and the small pimples here can mostly be concealed with concealer.

I also find the view on students here somewhat confusing. Especially since I know from various properties rented to students that when moving in, either the renovated condition is accepted or they redecorate themselves at their own expense, etc. If it is about really necessary things, there is still the deposit... but of course, the topic is actually another one.

I also find the entrance to the granny flat strange. Having no hallway at all for arriving is really inconvenient, especially in winter, but I believe this was already acknowledged as a criticism by the OP.

I would not want a granny flat in my house. My house, otherwise I could have continued living in an apartment building. Once we have landscaped the garden, hardly anyone can look in there anymore - a, MY terrace above the outdoor area of the granny flat... really not... would absolutely not be mine.

But about students as tenants: I also had a condominium in MUC and in retrospect I would ONLY rent to students. With one limitation: guarantee by the parents. But otherwise, I had the least trouble with my students. Money always flowed (mostly paid by dad or mom), additional payments as well, and after at most 3 years they moved out again. So you don’t have long-term tenants. Honestly, with the current tenant protection laws, that is a decisive advantage. An old couple who have been living in your apartment for 20 years or more you almost can’t get out anymore - no matter about personal use. In Munich there was just such a case, where an old man had lived in a three-room flat - forever. Wife passed away, he himself is now over 80. The owner registered (justified!!) personal use and even granted the senior a very generous notice period. But he went to court and won. He is allowed to stay. That is almost expropriation. Therefore, rental agreements that do not last longer than a degree are in any case better for the landlord.

but you can write off so much that it is "virtually" tax-free.

Mixing and offsetting so that costs approach zero is something different than "tax-free."

In 20 years, when the granny flat is supposed to be worthwhile, the house will be too big anyway and sold ...