Can I add a lean-to to a large shed?
If (as I infer) you are in an area where an "IBC Shed" is not requiring a permit, and larger things are, you either need to separate the  new space (so you have two "sheds" that are not actually connected) or you need to pull a permit; unless your area restricts you to one "shed" and then you're simply going to have to pull a permit.

If memory serves, the IBC definition is based on "projected roof area" of the "shed" - so two conjoined maximum-sized  "sheds" are no longer a "shed."