When adding a straw bale addition to an existing straw bale house, you have to be sure to create a connection between the two walls (old and new). One way to do this is with dowels or rebar embedded into the old wall and laid in place in between courses of new wall. This allows you to leave the existing plaster in place and still accomplish a connection.
Another way is remove the plaster from the existing wall and then use plaster lath on the top of each new course that is bent at 90 degrees and pined to the old wall surface. Use landscape pins on the old and new walls to attach the plaster lath. You will want to offset the new walls in either application so they are not in line with each other. In other words, add a right angle turn from the old wall and offset the new wall by at least one foot.
This will help when you plaster because it gives you a stopping point (the corner). Without it, you will see a large plaster scar or will be required to plaster the entire wall (both old and new) to a corner. Keep these points in mind while creating a straw bale addition and you’ll be in great shape!