The total assessed value of Island County decreased in 2009 for the first time in at least two decades.

Island County Assessor Dave Mattens said the county’s total assessed value dipped by about 2 percent to $14,629,093,831 in 2009. That assessment will be used for the tax year 2010.

“There’s been a steady climb until now,” Mattens said, referring to a graph of assessed value that goes back to 1989.

Mattens expects that the county’s total assessment will decrease much more dramatically next year. He explained that the total assessment roughly corresponds to real estate sales prices — the price real estate actually sells for — though new construction and improvements slightly bolster total assessed value each year.

“Sales data is a good indication of what assessed values will do,” he said.

Real estate sales prices declined by about 13 percent in 2009, Mattens said, which will likely translate to a 10 to 15 percent drop in assessed values in 2010. One year’s assessed value is based on assessment work done the year before.

But don’t necessarily expect property taxes to go down. If the total assessed value decreases, then the average tax rate in the county will likely increase. Mattens said his office is in the process of calculating tax rates for all the county’s taxing districts.

He’s interested to see how the county’s rate will compare to San Juan County’s rate. The two counties have battled for years for the honor of having the lowest property tax rate in the state, with Island County coming out on the losing end. San Juan’s “total tax rate,” which includes regular and special levies, is historically the lowest.

Mattens said assessors have a “rough rule of thumb” that homeowners can use to predict their 2010 property tax bill. Historically, if the assessed value of a person’s house increased at the same rate that the county’s total assessed value increased, then his or her tax bill will likely remain the same.

So for this year, if the value of a house in Island County decreased by 2 percent, then the tax bill on that house will probably remain the same. If the value of the house didn’t change or increased, then the tax bill will likely increase. And if the value decreased by more than 2 percent, then less property taxes will probably be due.

Mattens said the county had the greatest increase in total assessed value from 2005 to 2006. The value went from $9.16 billion to $12.45 billion. That’s a whopping 35-percent jump.

“That was the bubble,” the assessor said.