Welcome Scott...just done similar, siamese'd two sheds together. One was pitched the other lean to, both 6' wide (nominally) x 8'. Due to logistical reasons, I ended up with just under 14' in length.
My set up was not normal, seldom is, but the simple way to go about it is, measure the studs and runners of the existing, cut/buy enough for the extension inclusive of the bits for the pitched roof. make up a suitable floor, finished height as per existing, remove gable from existing, leaving the rest in place (I advise you to put a prop in place before you take out the gable end, just in case) I'm assuming the existing shed is put together with coach bolts? and nailed, heads still proud of the wood, at the bottom runners??
Now you need a hand, place the removed gable on the floor, where it will go finally, prop or have your assistant hold it perpendicular and mark the bottom runners inside edge on the floor and remove. Now cut both top and bottom runners to fit between old shed and this mark, cut your studs the same size as existing and nail these up to form two frames. if you stitch the two frames in position and offer up the gable, these should fit up nicely if you've done it right.
Next you need to build the extension pitched roof. If the existing shed is standard "shed built" the roof will be of two side sections and the ridge boards will be two thinner sections angled to form a perpendicular centre spine that bolt together when lifted into place??? If that is the case, you'll again need to copy these for the extension section. Build these on the ground and lift into position from inside the shed extension (once both are in place, waggle them about until everything fits hunky dory). Fix matching weather boards (but jointed) if the existing are ok, otherwise strip off the old and buy new ones long enough, and thick enough to suit you sir. Fit ply roof boards and re-felt.
If you want a metaphorical hand, PM me and I will try and help from a distance...bosshogg
I never teach my pupils. I only attempt to provide the conditions in which they can learn.
Albert Einstein 8)
