Subfloor or partition walls first in a cottage attic?
You'll want to put in your floor joists, lay down the sub floor, then build partition walls on top of that.
It's unlikely that all your partition walls will fall exactly on a joist, so without subflooring you'll be going through all sorts of gymnastics to support the wall.  Since none of your partition walls will be load bearing, you won't have to worry about walls bearing on joists - just use standard dimensional lumber with stud spacing to match your national standards.
Also, I believe that &quot;chipboard&quot; means different things in different parts of the world. So long as the stuff you're using is rated for flooring use, you should be fine. (Here in the States, what I think of when I hear &quot;chipboard&quot; would not be suitable for flooring.)