A single story detached garage is subject to building permits in most California counties and incorporated cities. Where I live, the foundation must have a minimum 12" depth, resulting in a single story footer 6" thick x 12 inches wide with reinforcement steel required. This varies with increased loading caused by load support framing above the garage, and for larger structures that may impose larger foundation loads. This is based on a soil bearing strength of 1000 pounds/square foot minimum for the county, but may be increased to 1500 lbs/sf with a site visit and soils report. Site specific requirements may be different depending on soil conditions.
Throughout California, foundation and building requirements vary due to seismic zone, geology and soils, basic design wind speed, frost, flood plain and other issues. A garage permit here requires a plot plan, foundation plan, framing detail, wet signed engineered roof framing detail, elevation drawings and an electrical / mechanical plan.
Your building department will freely give you a punch list of minimum requirements for submitting plans to obtain a permit to build the garage. In many locations, improvements are exempt from supplemental tax in accordance with section 75.12 CCR.
Without knowing exactly where you are, site characteristics, topography, seismic zone and soil type, your question cannot be answered. Contact your building department. If you are in El Dorado County with me, check this site out: http://www.co.el-dorado.ca.us/building/ResidentialGaragesRequirements.htm
Hopefully this met Moderator Glenn's code reference requirements.