Is it sensible to plan the DG as a self-contained residential unit?

Hello reabsi,
I would agree with your friends and plan for the current situation. But that is my absolutely subjective opinion. Personally, I would never want to live together with strangers in my house (assuming the children have moved out), because I already know the situation from experience. Naked on the terrace and the "neighbors" from the upstairs apartment watching :eek: (just an example).
Better to sell the house if it no longer fits. That’s what we do with all other items of use (I consider a house no different – just like a suit or a car, if it’s too small --> then new one).
But please don’t let my opinion deter you from your path, that is not my intention. I just want to present a somewhat different perspective.

Of course, you should plan for the NOW, so you should also be able to live with having a subtenant now. (it’s not everyone’s thing) Whether you also rent out the attic in 10 or 20 years is written in the stars. You can plan for it without sacrificing living comfort. For example, I dream of a bungalow with ground-level bedroom and adjoining bathroom. But since we have to build 1.5 stories, that’s what we will do. Still, a guest/work/bedroom will be on the ground floor. Short distances are great, and I see it as a quality of life to quickly disappear into the "private quarters" in the evening without having to climb stairs. The only downside for me would be a "stairwell," which can be a bit uncomfortable.

According to your measurements, your "living" area is generously planned. Additionally, there is the attitude that sleeping and the bathroom (Wellness) are also part of living. Whether any of your children even want to move in with the parents after they have left the nest is also questionable. But I have experienced that many opinions (from friends) are often either standard or, on the contrary, thought in megalomania. If I were to implement all the advice from acquaintances, I would have had to build an oversized villa worth 500000 ;)

First of all, thank you for the answers. We have of course already had the planning done in a way that suits us now, with a view to the future. I also think that a bedroom and bathroom on the ground floor is an advantage for old age. What I find disadvantageous about this is that as long as our children are small, they have to sleep one floor above us. Renting out the granny flat doesn't bother us, as we were both used to it from our parents. We have also deliberately planned a separate entrance on the other side for the granny flat, so that we don't have much contact with the tenants. I am aware that one should not build their house according to the needs of friends, I was just unsettled because most of their opinions were negative.

The downside I see is that as long as our children are small, they have to sleep one floor above us.

Then just switch the marital bed with the desk in the office in the first years ... you can swap furniture and repurpose rooms.
That way you'd have a bathroom and a large dressing room downstairs, but be close to the children :)