What does “overassessed” mean?

A home is overassessed when the value the county assessor has on file for it — its assessed value — is higher than what the home was actually worth on January 1 of that year. Because your property tax bill is a percentage of assessed value, an overassessed home is an overtaxed home.

Compare two numbers. First, the assessed value on your property tax bill or your annual Notice of Assessed Value. Second, what your home would have sold for on January 1 — the best evidence is three to five nearby homes like yours that sold close to that date. If those sales point to a value below your assessed value, you're likely overassessed. A quick way to get a first read is a free automated check that compares your assessed value to a market estimate; a gap of more than a few percent is worth investigating with real comparable sales.

Yes — overassessment is the condition, overassessed is the property. Some counties and appraisers write it as “over-assessed” or “over assessed”; assessor offices usually call the fix a “decline-in-value review” or a “Prop 8 reduction.” They all describe the same thing: an assessed value above market value.

Because market value moves and your assessed value only moves one way on its own. Under Proposition 13 the county carries your purchase price forward, growing it by up to 2% a year. If the market softens after you buy — even slightly — that carried-forward number can end up above what your home is worth on January 1. California law says the assessor must enroll the lower of the two numbers, but the assessor only knows to lower yours if their automated review catches it or you ask.

Roughly 1.1–1.25% of the overassessment, every year it persists — that's California's effective property tax rate in most counties. A home assessed $100,000 above its market value is overpaying about $1,100–$1,250 a year. The amount compounds in the sense that each year the overassessment stays on the roll is another year of overpayment; it doesn't come back retroactively unless you appeal within that year's window.

Ask your county assessor for a free informal review (called a decline-in-value or Prop 8 review): you submit your comparable sales, a county appraiser reviews them, and your assessed value goes down or stays the same — it cannot go up through that process. If the informal route doesn't resolve it, every county has a formal appeal to its Assessment Appeals Board, with a filing deadline of either September 15 or November 30 depending on the county. You can do all of this yourself; paid services exist because assembling credible comparable-sales evidence is the hard part, not because filing costs money.

Yes, and in California it's common — Proposition 13 keeps long-held homes assessed far below market value, because the assessed value can only grow about 2% a year no matter what the market does. An underassessed home isn't something you need to fix; the assessor cannot raise your value above your Prop 13 ceiling just because your neighbor's sale was high. Overassessment is the opposite case, and it's the one the appeal process exists for.