The main issue you wil face are the minimum "setback" requirements for your property. You can go to the Simi Valley building and safety department and ask right over the counter and they will tell you what the requirements are.
The problem with most garage conversions is that the front of the garage is already using up the minimum setback from the street so you can't put covered parking in front of it. Covered parking is required; so if you convert the garage you will need replacement covered parking.
The side yard set backs vary for single vs two story homes and the minimum distance for the rear property line will determine how much structure you can add on to the back.
If you live in a development with an HOA, then check with the HOA first as their rules may be more restrictive than building and safety.
The big unknown is if you live in a development not controlled by an HOA, but has CC&Rs or deed restrictions that were put in place to preserve the look and uniformity of the neighborhood. City Building and Safety dept may not know about any of those restrictions so even if they issued a permit, you would still be in violation of those restrictions/rules.
Rory Hernandez, owner
ASHI Certified Inspector #244418