Cost estimate single-family house with granny flat in Hessen

800k incl. incidental construction costs I also have the "contacts" for materials, but in the end you save less than you think

Ouch, that sounds expensive. Did you have to go through such an experience? I am currently desperately waiting for the planning from the architect so that I can create a more accurate calculation

In the discussions with the GU, it was priced at about 90-100k

Even if you calculate conservatively with 2500€/m², you can double your number. Additionally, with a granny flat, you lose the big advantage of having a single-family house!

Who says that we can’t afford the construction and are therefore planning with a granny flat?

You say that indirectly yourself:

Our budget for the project is about 500k.

At some point it would also be conceivable that my parents move in when they are older.

Then a staircase would be counterproductive

Ouch that sounds expensive. Have you had such an experience? I am currently desperately waiting for the architect’s plans so I can make a more accurate calculation.

Well, it’s stated earlier that a living sqm costs €3000 for a rough calculation.
Are you building a two-family house or a single-family house with a granny flat? The former is more expensive.
I’ll put it briefly: if you rent out, then for the first 20 years without profit. But with obligations as a landlord. That’s not bad or a nuisance. Only when you have to get the money elsewhere (loan) to be able to afford it at all (i.e. second housing unit), there will hardly be any profit calculation for the landlord, i.e., you guys. Usually, you get in less than you put in—even with subsidies.
At least that applies to a two-family house… the first thing that comes to mind is limited privacy. You no longer have that with rental income. But that brings us to your question

Who says that we can’t afford the construction and are therefore planning with a granny flat?

You don’t do it because you don’t want a stranger in your property.

This no longer applies to rental income,

So, for days I have been struggling here with the - is it still called Thesaurus? - fencing writing program … spell checker. No idea, I’m writing as before, but the program considers everything wrong. I don’t even want to correct myself, at least not in this post, so that you can see what artificial intelligence does, namely nothing. If I correct while writing, the little program doesn’t respond.. so whatever… live with it.
The same applies to liking. Is it because of my iPad or what? o_O

The 500k are not generous despite your own contributions, possible but not generous. So leave out the [Einliegerwohnung], it costs 200k+ and thus half of your budget. Whoever says it costs 90k is talking nonsense. Planning back to zero, plan a nice single-family house with ~150sqm and save yourselves a financial fiasco.

To reduce the second "ouch" we also considered reducing the granny flat to one floor. With two floors, we would only have the advantage in old age of living barrier-free on the ground floor and possibly renting out the upper floor.

I don't understand ... it would rather be the case that you plan the granny flat on the ground floor and the 160 sqm as two stories. And in old age you move into the granny flat yourself. Apart from that, I also think that 240 sqm of living space cannot be realized with 500,000 euros.