Nearly 20% of Orange County homes are over-assessed, a San Diego-based property tax advisory firm estimated. Thursday is the deadline to file a request for an informal property tax review. Adam Berkson, founder of EasyTaxFix.com, estimated that 115,000 Orange County homeowners should file since they could be eligible for a tax cut averaging $1,149, or nearly a third off the average tax bill. In Placentia, more than half the homes have assessments that exceed the homes’ current market values, Berkson estimated. Nearly 43% of Portola Hills homes and 35% of Aliso Viejo homes are over-assessed.

South County beach cities stand to get the biggest tax cuts, the analysis shows. The average tax savings in Laguna Beach was estimated at $4,808, or 61% of the average tax bill there. Monarch Beach taxpayers may be eligible for an average cut of 57%, while Capistrano Beach and Corona del Mar both could be eligible for more than half off their taxes. “What I tell people is they should have some say in how their property is assessed,” Berkson said. “You know your neighborhood best.”

The assessed value of your home is the basis for determining how much you pay in property taxes. Thanks to Prop. 13 — which limits tax hikes — most homeowners are ineligible for tax cuts. But if you purchased it in 2003 or later, chances are it’s worth less than what you paid — and you could be eligible for a tax cut.

The one-page form needed to request an informal tax review can be downloaded from the Internet at www.ocgov.com/assessor. O.C. Assessor Webster Guillory said you don’t even have to fill in the part requesting three comparable sales, although it’s obviously much better to do so.

Miss the deadline for an informal review, you still can file a formal tax appeal between July and mid-September, but that process takes much longer.

In his analysis, Berkson also determined that:

•Higher-priced homes have a greater tendency to be over-assessed. For homes assessed at $500,000 or more, 45% of the properties are over-assessed, he said.

•The more recent a home’s purchase, the higher the proportion of over-assessed homes (see chart above).

•For example, just 9% of the homes purchased from 2000 to 2002 were over-assessed, the analysis shows.

•That rises to 14% of homes purchased in 2003, 36% of those purchased 2004, and more than 60% for homes purchase from 2005 through 2007.

Guillory, the county assessor, said it’s likely that more than 115,000 homes will be eligible for tax breaks this year. Last year, his office lowered taxes for 147,000 homes. His office plans to review assessments for more than 300,000 properties for the current tax year.