Hampshire,
I like it, you lateral thinker!
The play corridor was the very first paper floor plan. Cool idea! Every children's room with access to the garden. But then we preferred to sleep downstairs and have the children's rooms upstairs. In your argument, that would also be the case, just later, when the children have moved out again.
The basement costs at least 90k, the garage driveway with an appropriate slope about 40k more. Plus the fancy stuff with the elevator or freight lift. Values are estimates from the planner of the construction company.
We had actually already discussed that idea as well
But in the end, it’s just not worth it to us or probably exceeds our affordable limit...