Single-family house without a basement - generate parking space elsewhere

My sister has an uninsulated attic (which is currently being converted). The staircase to the first floor is completely "enclosed". In front of the staircase leading up is a normal wooden door.

Why don't you heat the attic or build a cellar?

I would first check how large the area upstairs actually is. We had also considered what we could sensibly store up there - now I wonder if the consideration and then the hassle for a few boxes is even worth it. The space is really limited... If we had installed a proper staircase for that (there was a brief thought, because it didn’t look so small in the sectional drawings), I would probably have been very annoyed.

In fact, you shouldn’t plan too little storage space in the house and not rely only on cheap makeshift solutions. You’ll just be annoyed afterward. Many things simply cannot be stored in the garage, garden shed, or attic. At least I am a bit cautious there because I wouldn’t want to dig out my nice things after a year with mold or insect infestation.

It is a cold roof, i.e. the ceiling between the upper floor and the attic is insulated with wool with a vapor barrier foil in between

I would always insulate the roof (as well). In our case, both the upper floor ceiling and the roof are insulated. We store all sorts of things there. With a classic cold floor, the stuff gets damaged over the years because in summer it practically stews away.

With a classic cold floor, the stuff breaks down over the years because in summer it practically stews by itself.

Plus cold in winter, plus moisture, moths, spiders... I wouldn't overestimate this storage option.

Asked differently. What would a basement cost more compared to the larger garage and the garden house? If the additional costs can be managed (through own effort or so), I would build a basement.

In the basement, you can accommodate the heating and washing machine, thereby having more space upstairs. You can also wonderfully store a chest freezer and supplies there since a constant temperature prevails throughout the year. In our case, the basement temperature is always about 12 degrees C.

And the undeveloped garden area can then be used for useful plants like apple or peach trees, but also for currants or other plants.