I don’t think a granny flat is so bad if it is intended for the family. But I wouldn’t do that if the children’s future is still so uncertain. Depending on how the entrance is designed, you might later be able to separate a part (for the offspring or tenants), but I would actually live in the here and now. Besides, you say you don’t want to maintain the large area and therefore will rent out 50m². Why does your son need 50m²? 20m² is more than enough and maybe a small bathroom added. In most cases in my acquaintance it is like this:
Option 1: The son studies/does an apprenticeship after school and still lives at home (costs, convenience, etc.). These children usually manage with the available space.
Option 2: Children move out to study, after/during the apprenticeship, and then basically never come back to the house.
As soon as the children want something “of their own,” they are 90% gone. In the parental home (even a granny flat) the parents are simply permanently nearby and therefore it is nothing of their own. That also includes own kitchen, own washing machine, etc., and many granny flats do not offer that.
50 years ago the world looked different. I have also lived with my parents again temporarily (once during my studies in my old childhood room, once in an 85m² rental apartment). Ultimately that brought me 250km further away to my own home now. My two siblings also have their own houses and my parents a house with 500m² of living space and 3 rental apartments inside. These were never built for us, but of course it was also their wish that someone of the offspring would move in there. Now they have 3 rental units in a house that was previously fully occupied by the family. The current tenants are great, but this situation was and is a huge adjustment for my parents (strangers in the house).
This is how it goes for many acquaintances and their parents. For example, from my school class only one out of 24 still lives in their parental home. Many of them have newly built in their parents’ hometown. This is (unfortunately) currently the rule and not the exception, and that is why I would never plan for how my children might possibly be accommodated with me in the house. I have been through these scenarios for 20 years.
I don’t want strangers in the house now or later. That is one of the main reasons for a home of one’s own. I need distance from other people and want to be able to decide for myself in my own four walls (volume, structural changes, lifestyle habits, etc.). Everyone sees that differently, but even the best tenants are not invisible.
First see what you actually need in living space. So besides bedroom, [KZ], kitchen, utility room, bathroom and living room: what is important to you? Hobby room, office, dressing room, fireplace room, conservatory, sauna, fitness room, etc. For most rooms there are rough standard sizes, e.g. bathroom=10-15m², children’s and bedrooms=14-18m², kitchen=10-15m², utility room=10m². You add these together + the size you would like for the living room, 20m² for circulation, and then you have the approximately required size. And I am sure you will find plenty of matching floor plan suggestions on the internet.