Despite official predictions of a 10.5 percent increase in property assessments, Arlington County said yesterday the average home rose in value by 13 percent last year and in some cases shot up by as much as 25 percent.

The unexpected increases, contained in notices sent to residents on Wednesday, brought cries of anguish from some county homeowners who got the news yesterday.

"They're just trying to tax the property owners out of here so they can turn it all into commercial," said retired government worker Lester Thacker, whose two-story frame house in the Addison Heights area near Crystal City was assessed at $61,000, a 24 percent rise over a year ago.

Besides Crystal City, the greatest increases occurred along the Potomac River and in the Metro subway corridor, according to James Vinson, director of the county's Department of Real Estate Assessment.

Vinson said the increases pushed the average cost of a home in Arlington to $78,000, from $69,000 a year ago.

Vinson yesterday described Addison Heights as "one of the hot areas" of the county. "There was a 20 percent increase in the whole Addison Heights area. Those homes are really old, but they're behind Pentagon City and there's a substantial amount of redevelopment happening there," he said.

"The older areas of the county, Brandon Village, Cherrydale, Willett Heights, Ballston and Clarendon, are undergoing phenomenal rehabilitation. All of a sudden there is a tremendous demand for the property. People are buying delapidated houses, putting in new kitchens, bathrooms, putting on siding. Everyone wants to move in close."

For the Arlington County Board, which last month vowed to try to offset rising assessments with a corresponding cut in the property tax rate, the surprise increases may make that promise difficult to keep.

"We've got to get the bill for all the snow removal first," said Board Chairman Dorothy Grotos yesterday.

The board, which tentatively cut the tax rate by eight cents in January, must decide the new rate by March 24. Tax bills reflecting the new assessments and tax rate will be sent out May 15.

Grotos said she spent all day fielding calls from angry homeowners.

"I feel real bad," she said, "There are a lot of old faithful out there who are telling me they've got to move out of the county now. They can't afford to live here anymore."

Assessments on commercial office space, apartments and in strip commercial centers increased an average of 16 percent, mostly due to new developments and property now under construction in Rosslyn and Pentagon City, an official said.

Vinson also said that some less affluent areas suffered the greater assessment raises. In Ballston, where the average rate of increase was 13 percent, Metro, I-66 and several rezonings helped to promote an increase in property value. "That neighborhood is in the eye of the hurricane," Vinson said. "In five to ten years you won't even recognize it. It is in the middle of a complete reversal."