Bungalow is more expensive, but you are building now and will eventually need two barrier-free living units. Of course, your father can still walk up at 80 or at 70 the stairs become a torment. Bungalows can be planned individually.
I noticed the kitchen upstairs because we have the problem in our rental apartment. The sink is under the roof window, otherwise, at my height of 1.70, I can’t even do the dishes without hitting my head.
Maybe a bungalow with an expanded attic would be something for you and the children upstairs.
For example something like this - mirrored and instead of storage a utility room, instead of technology stairs or vice versa. Instead of dressing room a study.
Or even with a closed kitchen:
Instead of office a stairwell and that's a nice private area. The utility room is a bit small here, but maybe you can, for example, put the heating upstairs under the attic and then only need a washer/dryer downstairs...
Of course, a stairlift is an option. Depending on the stairs, it varies in price. I am also an advocate of the stairlift solution. But here, it’s not a 30-year-old, healthy person building. The property is large, putting aside what the building envelope says.
Whether it’s one or two levels. Pay attention to turning circles, also for the apartment in the attic. Grab bars in the bathroom, etc. Sufficient space next to the bed, and so on. That’s why I recommended the site to you yesterday.