When renovating a house or garage or why when it comes to a time to extend a wall or match new with old the first thing you will most likely find is the old wall isn't plumb or the top of it isn't level.
So how do you match up to it if you don't want to pull the existing walls apart?
There is no black and white answer for this situation as it depends how far out it is but how I normally approach it is to have the wall plumb or level by the end of the new area.
Eg if the wall is 10mm out of plumb and the new wall has 5 studs in it I would mark the end of the new wall at plumb and then chalk a line to the old wall and us that as the marking for the new top plate that way the wall getting back to plumb will be done gradually at 2mm per stud and will not be as noticeable as trying to build the new wall dead plumb from the start.
