How to convert a detached garage into an ADU? Here are the details from LA on garage conversions. Take it as a sample of what you might need to do if you are facing a similar challenge – even if you are not living in LA.
You will need to cover issues like foundation, framing, fire protection and fire-resistant rated construction measures (if any of the walls are too close to either the main house or property line), means of egress and emergency escape, certain requirements for the interior environment (cooking, sleeping, heating, ventilation facilities, as well as ensure natural lighting).
And, naturally, the new ADU shall comply with California Building Energy Efficiency Standards (Title 24).
A garage conversion might be the least expensive option for adding living space if one can confirm the structural integrity of the foundation and walls. Typical budget is between $100,000-140,000. Remember, that you are changing the use of the structure from non-residential to residential, thus it must comply with the current building code.
Useful tips: address the moisture barrier issue for the slab (often done with the help of a sealant); budget for electrical panel upgrade to accommodate the increased usage; use mini-split for heating and cooling.