I have purchased Pella new construction windows with a frame size of 3'9" x 2'11". Pella lists an RO that is 3/4" greater than the frame size for both height and width. There are existing studs in the location where I want to install the window that are 3'10" apart. I already did some electrical and plumbing work below the location of the new window such that I would need to remove the electrical/plumbing work to put in any new studs all the way to the bottom of the wall. One of the studs I'm hoping to use has a 2-1/2" diameter hole through it to accomodate a drain pipe.
I'm hoping to use the existing studs but do I need to reinforce them and will it matter that the RO is 1/4" larger than the maufacturer's spec? Should I add a 1/4" plywood to fill the space adjacent to the window?
Add the 1/4" if it makes you feel better, it doesn't make that much of a differance. The RO is larger for a couple of reasons, one if it's out of square, it also allows for expansion and contraction. It also allows for any further movement of the framing in the future due to settling.