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

Rental income is presumably tax-free for the first few decades

Nope. Income is always taxable.

You are of course right about the rest. However, with new construction, everything should be separated, including financing as well as planning, etc.

But I don't see this as important for the discussion here, since the granny flat is absolutely poorly located. It takes the prime side of the property and deprives the house of privacy, complicates the entrance situation, and removes the house residents' connection to the garden.

In addition, a lot of [EL] is supposed to be provided, and these are simply not deductible.

Oh, so that's how it is, if you question why a granny flat is being built at all and point out possible related problems, you're a blabbermouth? Great attitude .

But I want to emphasize again: without the granny flat and without the second bathroom door, I find the floor plan good, or more precisely, I especially like the upper floor. All comments on the justification and "must" of this granny flat are important and correct, so far the OP has not at all stated how these remarks were received and questioned by him, a casual "I was also a student in a granny flat before, it's fine" is not really helpful. And that there is no reaction to "this costs over 500k, but your architect talks about 350k, is he aware of that?" is at least surprising. No one here wants to "ruin" anything but to help, even if sometimes you only realize that at second glance. I had to learn that myself two years ago. But once you recognize it, you are grateful.

Oh, so that's how it is: if you question why a granny flat is even being built and point out possible related problems, you're considered a blabbermouth? Great attitude .

Yes. And especially when: the user asks if the floor plan is good. And the worrywarts of the nation unite to say that renting it out isn't worth it, that students will constantly be drunk loitering under the terrace, that after 10 years it will have to be completely renovated anyway, that you have to pay so much tax, that the architect is useless and can't even add up square meters correctly...

yes.

No.

No. Income is always taxable. You are of course right about the rest. However, when building new, everything should be separated, including financing as well as planning, etc. But I don't see this as important for the discussion here, since the granny flat is absolutely badly located. It has the premium side of the property and takes privacy away from the house, makes the entrance situation complicated, and takes away the residents' connection to the garden. Moreover, a lot of EL must be worked, and they are simply not deductible.

yes. let's assume the house costs 400,000€. and let's assume about 25% of the total area is for the granny flat. let's further assume the OT can depreciate 2% annually. that gets me to 2000€, which would be tax-free from the rent each year. subtracting interest, land costs, etc... roughly speaking then half of the rental income is tax-free. yes, you're right. you have to tax it. but you can depreciate so much that it is "virtually" tax-free.

And just because it’s practically tax-free, one should do it? Bold thesis...

My parents used such a model 30 years ago, just a bit bigger. They immediately built a second house with 3 apartments and 1 flat. But not in their own house.
That really worked well, also financially. Especially because you also drastically minimize the concentration risk compared to a single housing unit.

I initially also absolutely wanted to realize an accessory apartment, but then decided against it for the reasons discussed here.
Why build your own house if you then have to bring in neighbors again?

I see only disadvantages arising from the accessory apartment, especially in the house discussed here.