ᐅ Single-Family Home Construction: Planning a Granny Flat / Accessory Dwelling Unit for Future Family Expansion?

I have a general question in the hope that you either completely dismantle the idea or that it might be workable. It’s more of a rough idea than a concrete plan:

We (a couple planning to have children but currently without any) would like to build a single-family house on a relatively large plot of land (1000sqm (10,764 sq ft)). Since we plan to have two children in the future, we would like to design two children's bedrooms accordingly. A home office and possibly a guest room would also be nice to have. A friend who is an architect suggested incorporating a separate apartment within the single-family house.

So we came up with the idea to possibly plan the house with the master bedroom, bathroom, home office, and guest room on the first floor, and to place a separate apartment in the basement with two rooms, a small kitchen, and a bathroom, which could also be rented out. If there is a later need for two children's bedrooms, the separate apartment could be used as the home office and guest room, and the freed-up rooms on the first floor could be used as children’s bedrooms. In the more distant future, the separate apartment could potentially be used again as a separate living unit for teenage/adult children or as an accessible living space for aging occupants.

This way, there would be no unused rooms in the new build, no risk of the house being too small in the long term, and as a side benefit, there might be additional subsidies and, of course, rental income.

Completely crazy idea or possibly reasonable?