Have a time frame in mind when you buy a home, and let it guide your purchase. If you plan to start (or expand) a family in the near future, a studio is not a good idea. If you want a home in which you can age in place, or want to preserve that option, consider a ranch-style dwelling on one level, rather than a multilevel dwelling with stairs.
Attic doors are a major source of heat loss in many homes. To stop airflow, weatherstrip the edges and insulate the back side of the attic door. Fold-down stairs can be covered with a lightweight box made of rigid insulation.