The plan is for a study/guest room in the attic, but that could also be pushed back. However, it won't save huge sums. The basement is supposed to house the utility room, workshop, technology/heating, and party room.
Party basements are so eighties! Going down to the basement for a laugh.... preferably with the basement bar in Baroque oak.... phew. Your wife should free herself from these inherited ideas from her parents.
If you make the study smaller and move the technology upstairs, maybe a small utility room will still fit in the floor plan, and voilà, the basement is no longer necessary. The huge garage can easily fit a workbench and two cabinets – and if you also add a heating connection, you can leave the cars outside sometimes and have nice parties on 8x7 meters – definitely nicer than sitting in a bunker. A large window from the garage to the garden would definitely provide a nicer view!
Basements only for sloped sites or very small plots, then they create great living space. Otherwise, I would also recommend, due to the costs, to rather enlarge the rest of the house a bit.
Your wife seems to be an aesthete. Now, after having read that your plot is 300 sqm in size, I am of the opinion that it is too small for two masonry structures (i.e. house and double garage). On this plot, I would prefer a carport entwined with plants, which has a tool room/workshop at the back. But presumably, this is really a matter of taste; a masonry garage may look tidier from the outside. Maybe you can argue with aesthetics instead of money, if you can stand by it?