When adding onto an existing structure, we suggest a totally detached new structure, foundation and roof line. Maybe 10'-0" away from the existing home. Once the new structure is 80% complete, then connect to the existing home by bridging the floors, roof and walls. This connector can result in a sunroom, music room, or covered breezeway. Doing this minimizes the existing home from being exposed to the weather during drawn out construction, and avoids undermining the existing foundation. Off-setting the connector walls and roof also avoids trying to align the existing walls and roof thickness, which is rarely "square, straight, level and plumb".
This total detachment is essential with log home construction, where some shrinkage and settling is inevitable and easy to manage. The end result is a blend of architecture, over time and looks natural.