The hot real estate market gave a $74 billion, or 9.5 percent, boost to total assessed valuation in L.A. County during calendar year 2004, according to preliminary figures released Tuesday by the county Assessor’s Office.

Total assessed valuation in L.A. County climbed to $855 billion as of Jan. 1, up from $781 billion one year earlier, according to the assessor. The 9.5 percent increase is the largest year-over-year jump in property valuation since the late 1980s.

“This assessment roll is the product of the strong seller’s market,” County Assessor Rick Auerbach said. “It reflects low interest rates and the short supply of low- to medium-priced housing.

Higher home values translate into roughly $740 million in additional property tax revenues flowing into the coffers of county government, the county’s 88 cities and dozens of school districts.

L.A. County officials had banked on a 9 percent increase when they drew up their 2005-06 budget that took effect July 1, so the assessment means several million dollars in unexpected revenues although not a spectacular amount.

Final assessment figures, including valuations for individual cities, are due out on July 28, spokesman Robert Knowles said.