Architects design windows to be above the 2' floor to sill threshold... since they have no idea whether you will use the window manufacturer as prescribed in the plan. So unless someone renovates the sill heights... they should all meet the 2' threshold. This keeps designs and grades from being altered in the future in such a way as to violate the 6' rule.
The usual situation is a residential home that is then being used as a commercial... with a vinyl, the manufacturers that I have used have a field application option, while the others require the WOCD to be factory installed.
Historically, a 3' height was used to allow furnishing to be placed below a window without blocking it.
I've quoted a lot of windows for a lot of houses, and the WOCD option is seldom needed. That being because the 2' rule was initial followed in the framing.
In fact, I will not name the well known builder that had a customer request the WOCD and because the windows were all the same size, accidently placed almost all the WOCD units on the first floor... that is how I learned the little trick on the vinyl windows.
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