Mid century modern ranch house plans with substantial but undeveloped attic space, you can create usable space by adding dormers. They can be shed dormers (a single flat roof at a flatter angle than the main roof) or gable roof dormers (with a top that is perpendicular to the main roof top). Shed roof dormers tend to be less expensive to have built, but gable dormers see usually better, especially on the front of a house. An important factor is if you create the living space of the former attic to put the stairs. This can be a problem for your first mid century modern ranch house plans, if the house was originally built without a staircase.
A more comprehensive alternative to a dormer raise the roof and add a second story. This is obviously a major renovation, and you probably want to live somewhere else while it is being made. It wills double the usable area in which part of the house can add the second story. You should make careful plans and perspective drawings before an addendum of this kind. Planned properly, it can improve the appearance of the house that expands its interior. Planned bad, it can make the mid century modern ranch house plans look awkward, bloated, and proportions.