In order to build an enclosed closet within your garage, the garage must maintain a minimum interior clear space of 20'x20' to accommodate a two-car garage. If the garage is less than 20'x20', a walk-in closet is not permitted.
The minimum vertical clearance for all parking areas in the garage must be seven feet. Storage cabinets are permitted within the 20'x20' area when located in an area above the front end of parking spaces, and must be at least 4.5 feet above the ground and no more than 3.5 feet from the front end of the space. Refer to ESMC 15-15-5.
From the Building code perspective- per CRC 2022, Section R302.5.1 Opening Protection- a garage is not permitted to open directly into a room used for sleeping purposes. Additionally, any walls separating the garage from the residence must be constructed with a one-hour fire-resistance rating, and all penetrations or openings must comply with that rating.