Prefabricated mass timber modular ADU prototype adaptable to the steep hillsides of Southern California. The carbon storage in contrast to an ADU built from a light-frame wood structure, as well as the health benefits of living in a space built from solid wood, air quality, and acoustics, are all good reasons to migrate from light-frame wood structures to prefabricated mass timber structures. The prototype can take advantage of the wood sourced from sustainably managed forests in the country, alleviating the housing crisis as well as playing a part in fighting climate change.