Opinions on the single-family house with a granny flat floor plans

I know how practical it is to quickly dash over without a jacket.

The door has nothing to do with privacy. That depends on the residents.
In our case, no one has entered another person's apartment without knocking and waiting to be invited in. (This is also the case with friends. Multigenerational households are not uncommon here.)
Opposite my parental home lives a large family spread over 3 separate houses. Privacy is zero.
Everyone has the key code, everyone stomps into everyone else's house, whether someone is there or not.

Knocking... but many don't want that. Also, the focus of a granny flat for the parents is not necessarily care or contact. Rather simply an economic reason, combined with the possibility, in the worst case. But that also works with a 10 meter sidewalk in the storm.

the granny flat is almost 90 our house about 180 sqm

yes with parents :) but that’s also because both parents live very far away and with children, living nearby has many advantages. Of course also disadvantages... no question.

Going out of the house every morning in storm, snow, rain is not comfortable. Ok, one can just fulfill their clearing and gritting duty right away.