Financing single-family house with granny flat for parents

Due to moves, there are now 5-10 different officials in four different cities. Therefore, no goodwill, but a fundamental intention to generate profit. If advertising expenses come from the maintenance reserve due to renovations / refurbishments / repairs, I cannot influence that - my intention to generate profit exists without special factors. (...)

However, with a new basement apartment, he has no special factors such as renovation, refurbishment, or repairs for the next 10-15 years...

However, with a new-build granny flat, there are no special factors such as renovation, refurbishment, or repairs over the next 10-15 years...

That makes it even easier to document the intention to generate profit. Besides that, you have no idea how many advertising costs can arise with an apartment rented to family members. ;).

Get into trouble with the wrong tax officer and you'll never enjoy your rented apartment again...

Get on the wrong tax officer's bad side with that and you'll never enjoy your rented apartment again...

What do you mean by that? In tax law, there are so many legal possibilities – you just have to take advantage of them.

Creates a financing gap of about €450k which would have to be compensated by (1) reducing our claims, (2) building up more equity, or (3) a gift from the parents (TBD); probably a combination of all three.

Questions:
- Does this all make sense roughly calculated and is it realistically estimated (on this very rough basis)?

So €450k are missing from your own calculation and you have mentioned three options:
1) Reducing the claims might be possible: for example, a smaller plot with less garden.
2) More equity is a matter of time: how many years do you want to wait? Your wife's return to work would help a lot though.
3) Yes, that should be clarified first and foremost: can and do the parents want to give €450,000 as a gift? At least the rough magnitude should be discussed with them, how they imagine it. Giving away €450,000 and still paying €1100 rent, would they find that appealing?

Before number 3 is clarified, all this makes no sense at all.

What do you mean by that? In tax law, there are so many legal options - you just have to take advantage of them.

Then you need a tax advisor again... all that for a few hundred euros per year?