The easiest way to tie in the new shingles with the old is with a cut valley see four ways to shingle a valley.
How to tie existing roof shingles to shingles.
After framing and sheathing the new addition s roof snap a line over the existing roof shingles.
Up from the valley center and cut through the shingles and underlayment.
I will be creating a new valley on one side and tieing into the existing shingles on the other.
You want to be about two inches above the very center of the valley itself.
I am about to put an addition on my house and the only thing that is keeping me from doing the shingles myself is the tie in to the existing.
After the addition roof is framed and sheathed snap a line on the existing roof shingles 2 in.
It really just depends on how the existing roof shingles ended.
I e how big the last ones are i would recommend that you remove enough of the old shingles so that you don t leave the plywood seam the tar paper seam and the shingle seam all stacked on top of one another.