With our older home stock, it's always been the case that the County Assessor's record of our homes is sometimes different from what's on the ground. Sometimes this is because of illegal construction (naughty, naughty! Unfortunately a past owner's illegal construction is now your problem--). But sometimes the inconsistency is more innocuous.
If you know your house has four bedrooms and two baths rather than three and one, and the construction was all permitted, go to this form at the Assessor's Office website. Fill it out, be prepared for the possibility of an Assessor visiting your home, and wait for the record to be corrected. Assuming you purchased the home as a 4/2, there would be no change in your assessment.
In contrast, if you improved the home with permits, and the Assessor's office has not caught up with the changes, you can fill out the form and you'll eventually see a bit of an increase in the assessed value, but not a wholesale gross-up to current market value--Prop 13 still "protects" the assessment of the pre-existing elements of the home.
Why go through this rigamarole? When you go to sell your home, your buyers will expect all permits to be "finalled" and all space to be legal. If your rumpus room was informally drywalled in the '50s, your windows were replaced without permits with aluminum ones in the '60s, or a bathroom was added without permits in the 20s, a smart buyer and agent will notice, and you'll likely have to negotiate the issue through. While maybe 15% of sales were affected by these issues in years past, it seems like 50% are seeing them these days.
Call me if you want to discuss your situation further--better now than a day after you accepted a great offer for your home!
Click here for access to the current assessment record on your home.