Well, a few years ago I rented an office space with a friend of mine, just the two of us in a very spacious (100m2), light and well-ventilated office. We received no phone calls, there were no people walking in, or cats and girlfriends asking for attention. I had less distractions than I had at home, and much less than in a traditional office. And it was a dedicated area for "work". A comfortable desk and chair, two monitors and a fast pc.
All these factors made it so much easier to concentrate and get into the flow... If I had to put a number on it I'd say I would be at least 1.5x more effective working in an area like that compared to a traditional office or at my desk at home.
I will NEVER again say that I am someone who has trouble concentrating, having worked in a place like that.
I wonder if (and how much) the lack of a proper workplace explains the decrease in output when working from home in this study. I can imagine a decrease in productivity if you have to share your home and workplace with other people or even children... But when your children and partner are at work, or if you have a seperate workspace, I wonder how the two compare then.