Hi Kelvin,
I am an architect in LA that has worked on a number of ADUs, some involving tricky garage issues.
-the fire affected your garage?
-you discovered that the garage itself was not permitted?
Depending on your local zoning, you may be required to provide a certain amount of covered parking. If your garage is not currently permitted you may be in violation of your local parking requirements. That being said, your non-compliance may be grandfathered in, in which case you should be able to convert the un-permitted accessory structure to an ADU as long as it fits within the existing footprint.
I would contact the folks at the Fairfield Department of Building Safety to ask them about this particular issue. Their contact info is here: https://www.fairfield.ca.gov/g...
Generally speaking, an architect would be able to help identify and resolve any zoning issues, as well as aid you in the design. That being said, an architect's stamp is not required for an ADU, and you might be able to find a local contractor to help you with the entire process. It's also good to call around various contractors and design/build companies to get a sense of construction costs in your area. Good luck!