How to add space types to midrise building type?

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For the DOE mid-rise and high-rise apartment building types, "Apartment" refers to an entire apartment unit which has loads normalized per sqft. This assumes some mix of rooms you would see in an apartment like kitchen, bathroom, living room, bedroom etc. If you just model each unit as a zone you can use this. If you model the rooms in each unit as their own zone and assign this space type, because it is normalized by sqft, it would still give the same results. If instead the load were per unit, then having too many zones would change the results.

To get back to the second part of your question, you can make new space types and add any loads and schedules you would like. So you can make bed room, closet, bathroom, kitchen, etc. It could be good to get up and running with the existing space type and then add more fidelity once that is running. When you are ready to make space types see the OpenStudio Application documentation on space types.