Being a former So. Cal resident and G/C, I suggest the following:
#1. You do need proper Buyer representation from a Local Realtor to guide you through all areas of any purchase.
#2. You and your Agent need to talk to the City of Glendale codes dept. about your specific situation... With regards to information, there are TOO many unanswered questions within your question. i.e. is it a finished room that was actually part of the orig. structure? Or did they frame it w/o a permit? Plumbimg? Electrical? Min. window requirement? Proper egress? Stair tread and riser sizes, etc, etc...
#3. Your offer should NOT include this sq. ft.. At this time, this is NOT Legal code approved living space and must NOT be considered in the comparables or the Appraisal. Not to mention, the total Cost to you to rectify this addition to meet city codes requirements...
Best of Luck to you in your endeavor!