Title 24 - Multifamily occupancy vs single family

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I don't have a definitive answer since it may depend on a number of elements, but based on what you've described, this is a building that does not contain more than two dwelling units (See CBC 310.4), and therefore it's R-3. Title 24 part 6 defines this under the single-family category (see 100.1 definitions) and should be modeled in CBECC-Res or other approved single-family compliance software.