Our old house is 10m wide, has a 45-degree roof pitch with a clipped gable roof and a knee wall of 90cm above the finished floor level. We reach the attic via a relatively narrow but otherwise fully functional staircase. There are built-in cupboards along the entire length on both sides. Below are always a huge drawer with a depth of 1m and a width of 1m, above that a cupboard with a height of 1m and 2 shelves. On top of that, there is still 30cm of storage space. We have 12 of these on each side. Most basements offer less storage space. We can hardly fill it ourselves. Additionally, there are built-in wardrobes with 3 doors and 1.5m width on the gable sides. We also dry our clothes there.
The roof is insulated, but the attic is not heated. Still, it is never cold there, even in winter.
The floor is just OSB. This attic has, at least for me, an enormous utility value. The new house is only 9m wide and now has a higher ceiling height on the upper floor. That is a huge difference.
Edit: Just insulate the floor slab and then you do the roof yourself. Then your heating load on the upper floor will not increase. That’s how it is with us as well.