Your word picture is of a gable roof being extended or lenghthened. If that is the case, you will need beams on the front and back sides to support the roof trusses at the same height as the existing ones. Then use the same thickness roof decking, use 15lb felt underlayment and weave the new shingles into the existing ones. Usually you can pull out every other piece that comes to the end of the existing roof, which will be cut back 6" or more and we'll say the alternate existing shingles are full length.
By weaving the shingles to match the existing ones and a flush roof deck there will not be any need for any flashing.