Unfortunately we don’t currently have the means to directly model the impact of attached garages (with of the floor of a conditioned space above a garage). It is relatively high on our list of future improvements.
The walls and doors of the garage itself (i.e. the boundaries between the inside of the garage and the outdoors) should never be modeled in OptiMiser, unless the garage is directly conditioned. In this case, it should be treated as part of the home.
The walls and doors of the main home that are adjacent to the garage will effectively have a lower U-factor. The impact would vary significantly with climate and configuration, but I think 30% reduction would be reasonable. For a door this would have negligible impact on overall building loads, but for an uninsulated wall it could be significant. You can view the U-factor for the wall on the ||Path popup. For a typical frame wall it would be about equivalent to adding an R-2 batt.