Single-family house planning with a granny flat - ~230m² - basement - gable roof

@ ALL
Thank you very much for the many constructive comments! They really helped. As said, we are not approaching the whole topic of building naively but are already very familiar with the subject. Maybe to put a lid on the topic of the granny flat. You have made many correct points regarding taxes, which need to be considered; KFW-wise, everything has been taken into account and actually discussed with a KfW advisor. So again, the topic of investment fraud should not be quickly accused of anyone here, especially not if one is not 100% sure. (small note) I would gladly welcome it if we open a post "What needs to be considered with a granny flat." However, this should not be the focus in this post.

I will now take the many constructive criticisms and discuss them with our architect and general contractor.

P.S. Can someone recommend a site where I can see some floor plans with a granny flat of about 230 sqm?

Building always means a compromise - I can only partly understand your opinion there. It may be that you have a comfortable position, but very few people do...

Not really a comfortable position, just pragmatic ;). But that would be off-topic.

P.S. Can someone recommend a site where I can see some floor plans with a granny flat of about 230 sqm?

Hello,
I believe that with a sensible plan by an expert (architect) you will definitely find a solution that will meet your requirements with significantly fewer square meters.

Originally, we also considered having a granny flat since we also lived for a while in my father-in-law’s granny flat. In the end, we abandoned the idea because in 20-25 years – when our as-yet non-existent children eventually move out – we would no longer want tenants living in our house. That also has something to do with "luxury."
The alternative for us was planning with reasonably large children's rooms (about 18 sqm) and a separate bathroom for the children. This should allow them to stay at home as long as necessary and possible. The children's rooms are now – and probably also after they move out – used as an ironing or guest room. Overall, it’s just over 180 sqm that we hopefully can keep clean on our own for a long time :)
If you’re unlucky, you plan now so your son can live with you in the house for a long time. But after graduating high school, he goes to university 500 km away when he's 18. Then you look for a tenant, and he turns the whole place upside down, stops paying rent, and you have to evict him first...:mad:

Edit: What I meant to say is: you shouldn’t stubbornly stick to the granny flat. There are other ways to accommodate the children well.

I don't understand why the topic of [Einliegerwohnung] is always talked down. Do you watch too much RTL2? There are quite a few examples where it works well together. For example, when children are the same age. If you offer it in a high-demand area, then raise the rent. Then you won't have 150 people applying, maybe 10 interested parties to choose from.

Has little to do with RTL2 - but I don't build to have other people hanging around me. At least that's often the view that leads to disliking ELWs. Something like that wouldn't "come into my house" either - although it would have been possible in terms of space and architecture.

: You are certainly right with your food for thought! In our first draft, we also have a single-family house without a granny flat. We will evaluate everything again and discuss it with our architect and the general contractor. That's why I see all your thoughts only in a positive way! After all, I don't want to build again so soon... ;-) even if that should be typically German now. We currently live in a 170sqm terraced house and already know the advantages and disadvantages of certain things.