Springfield and Lorton led the county in real estate assessment increases this year, with an average rise in assessed value of more than 18 percent over 2002, according to county tax officials.

The lowest average increases were in Clifton, Fairfax Station and Oakton, where appraisers estimated home values about 10 percent higher for 2003.

Nearly all of the real estate assessment notices that the county is mailing to 319,383 taxpayers went up for the fourth straight year. Appraisers boosted single-family home assessment values an average of 14 percent. Townhouse values rose 17 percent over last year, on average, and condominiums were up 20 percent.

The assessment jumps reflect strong appreciation in real estate prices and high sales volume.

Translating the higher values into residential real estate taxes, the average bill countywide will go up $424 over last year.

The mean assessed value of single-family homes, townhouses and condominiums is $317,240, compared with $276,945 last year. Based on the proposed tax rate of $1.19 for each $100 of assessed value, the real estate tax on the average home would be $3,775 this year, up from $3,351 a year ago.

County supervisors, who received the assessment figures Monday, said they would probably go along with County Executive Anthony H. Griffin's proposal to trim the tax rate by at least 2 cents, although that would still result in a substantial tax increase. The current tax rate is $1.21 per $100 of assessed value.