I wouldn’t do that, going to be very difficult to make the exterior siding look right. You will have to remove a lot of siding. Why don’t you replace one of the windows with a new fixed window?

 

Why would I have to remove siding? More than just the existing brickmold? I agree, it wouldn't look ideal. Looking forward to a door as it's so much more functional than the windows.

 

Is the window sitting on the floor or is it up 8 or 10 inches?
Remove the inside trim and measure from stud to stud for the correct measure
A double 36" exterior door is about 75" rough opening.
Hard to plan for maybe measurements.

 

I appreciate the idea, I'll think on that more. I'm not even sure there is much of an existing mullion that could be used to attach a fixed window in place of 1/2 of the existing window.

 

Maybe framing in to handle the door width, and adding 5/4 or 6/4 x 6 PVC trim around the door. Not sure if it would be practical, so It's just spit balling.

 

In this case, would you stack ~4 studs to each side of the existing rough opening? Seems like a pretty poor thermal bridge. Make a box for each side with ~3" of insulation in between 2 single vertical studs?

Either way won't make any noticeable difference in your heating or cooling bill, so do whatever is easiest. The important thing with either option is to seal it so there are no air leaks. That will make a difference.

 

The sill is a couple inches above the finished floor, but I think that is on top of a frame which sits on the subfloor.

The door that I have is 71 1/2" wide by 79 1/2". The rough opening width is about 83".

 

 

Your new rough should be 1/2" bigger than the jam , and doors want a double stud on each side.

 

It will look fine if you can finish the top with the same 6" trim. So leave the siding on the sides alone and build up to it. Then remove an equal amount of siding from the top and do your trim.

 

I hear you Half-fast Eddie - I was thinking I'd just slap a small sliver of sheathing on both sides and use extra wide trim. Not sure there's a better solution. I think, like Jeekinz, it'll look okay if it gets the wide trim all the way around and I can even keep it green so it doesn't stick out so much.

 

Wide trim is okay if consistent. But if you have a giant opening you could consider adding two sidelights - perhaps resulting in making the RO wider and replacing the header/King/Jacks. I'm thinking getting two matching 6" sidelights...