Go to the San Gabriel City Hall (See links below) Building Dept.
Just give one of the city clerk the "address" of the subject property than they will provide you ALL the permits available for the subject property. Good luck...
You can go to San Gabriel City Hall on 425 S. Mission Drive, San Gabriel, CA 91776. As soon as you walk in, you'll see the planning and development department right in front of you. Just give them the address and ask to see all the permits for this property.
The City Hall Building Department is always the right place to go to get any information on a propertyâ€™s history of permitted work. If permits DO NOT show up, then the odds are 99.999999999 that permits were not pulled. Properties with unpermitted work are a substantial risk.
Your local building department will have the information on all the permits that have been opened and completed on your property. Depending on when the house was built they should also have the approved plans (possibly on film) that the house was built with. You can then determine if the addition was permitted. If this is a house that you bought the information about the addition should have been in the disclosures.