I'm working on a project now that is similar. A friend of mine took the Sobon/Carlon workshop last fall at Hancock, and brought his own timbers, and bought the finished frame. It is a 14'x16' Dutch house frame. He is putting a shed addition on it as well. The rafters will lap onto the top of the main rafters, and be mechanically fastened. In the past, I think that would be just a big spike, but we'll probably use a Headlok type screw, unless he wants me to forge up some spikes. The addition plate will be tied to the posts with a small tie-beam, much like the outshot ties in a Dutch barn. You may want to make a scaled drawing of the shed to see where you can put the additional tie beams. If you continue the 12/12 pitch roof, you will loose headroom fairly fast in the shed. In the frame I'm working on, we are going from 12/12 to a 6/12 on the shed.