Here is a good thread on the subject.

how-to-model-underground-floors-in-openstudio

@nfonner the link above refers to a measure that sets boundary condition by story, but you could also have surfaces that fall below a global Z value changed to ground. While surface geometry is defined relative to the space origin, you can get transformation that gets global values taking account for space origins and rotation.

While in the SketchUp Plugin and switching to "Render by Boundary Condition" (to see changes) you can select individual surfaces and change the "Outside Boundary Condition" from "Outdoors" to "Ground", or any Ground+ processing option.

The surfaces reference a local coordinate system as far as I can tell while in a space block, so I would not know how tackle this from a measure perspective unless the surfaces also reference a global coordinate system somewhere. By this I mean some measure that would look for all surfaces with global Z-coordinates < 0 (below grade) and change the Outside Boundary Condition to Ground or whatever automatically. I believe this is the case in some programs where a space/ zone below grade automatically is converted to below grade constructions.

If the project has below grade surfaces that form a basement or at grade surfaces that form a slab, the following steps should be taken in the Open Studio Plugin to prepare the model for CBECC-Com:

1. In OpenStudio using the Inspector tool, under Constructions, create a new CfactorUndergroundWall and/or a new FfactorGroundFloor (there is no need to fill in the Construction details for these because they will need to be entered into CBECC-Com later)
2. Double click on the model / space that has the below/at grade surfaces
3. Select one of the below grade or at grade surfaces
4. In the Inspector tool the OS:Surface dialogue box for the surface you have selected will come up on the right panel
5. Set the Outside Boundary Condition to Ground, Sun Exposure to NoSun, Wind Exposure to NoWind, and assign the appropriate Cfactor or Ffactor construction that you just created to the the surface's Construction Name
6. Repeat steps 3-6 for the remaining below / at grade surfaces.