When the one bedroom was converted into a kitchen upstairs, they took the two windows and shortened them. The one I can understand due to the counter being installed, but the other...???? Plus they hacked the original sill flush with the sheathing so they could put siding over it. It was real quality work too! They used ceiling tiles to insulate the section of the old window opening they enclosed - top notch craftsmanship.
So first thing was dismantling what they did. Of course this wasn't too hard to do. Then in order to repair the sill I had to remove it. This involved removing all of the original window construction down to the wall framing.
Once the sill was out (a real beast at over 3 inches thick) I ran it on a table saw to get a nice clean edge to join a new piece to it. It just happened that there is a 3/8 drop from where the old window closed against the sill and what was the outside part of the sill. So this made a perfect point to joint the new piece. I used a piece of salvaged pine for the replacement piece which will hold up to the elements compared to any new piece of pine. I glued and countersunk 4 inch drywall screws to join the two sections. All stripped and sanded it was time to put it back in place.
There is a cavity between the wall framing and the side pieces that will frame the window opening. This is where the old window weights hung. The original pieces were beat up and split, so I installed new sides. They seat in notches cut into the sill and header piece. Then the boards that cover these weight openings install on the inside and out. Finally install a stop trim and ready for a new Pella window.
This will all have to be done on the other window in this room too... but the counter is still in the way. So this will wait till the kitchen gets dismantled.