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1.5 story farmhouse plans offer flexibility and a separation of spaces. Many 1.5 story layouts allow homeowners to use the 1/2 story only as needed. So, if the day comes when you can no longer safely climb stairs, you should be able to stop using the upper level with minimal aggravation, as the majority of the living space is, and has always been, on the main level.
You'll often find optional bonus rooms upstairs, which can be finished down the road should extra space be required. Lofts are another common feature of the 1/2 story. They often act as home offices or playrooms, and help keep the primary living space clear of everyday clutter. You may also discover one or two secondary bedrooms on the 1/2 story, as they typically house children or teenagers, i.e. people who generally don't have trouble maneuvering stairs.
Every once in a while, you'll see a plan that features only the primary suite on the upper (1/2) story. This type of layout is great for people who are not discouraged by stairs and who desire an extra private space to retreat to each night.