The value of a single-family house in Alexandria rose an average of 4 percent to $131,600 this year, but house values in the Old Town district, already high, climbed significantly higher, rising about 7 percent, according to city Assessor David Chitlik.

The value of commercial property rose 7 percent and the overall value of residential plus commercial buildings, boosted by about $200 million in construction, increased by about 8 percent to $5 billion, according to figures released yesterday.

Low interest rates, which made higher-priced houses more affordable, and the increasing scarcity of land in the expensive downtown area contributed to the sharp increase in Old Town house values, Chitlik said.

The assessor said he did not yet have figures showing the average value of a house in the oldest part of the city, but Thomas M. Wharton, an Old Town real estate agent, estimated that the average house there costs $200,000, with select properties valued at more than $1 million.

"Our major concern is that very few people will be able to afford living here," said Andrea Dimond, president of the Old Town Civic Association. Dimond said that as the larger downtown houses increase in value, they are more likely to be divided and sold as condominiums.

"The large garden homes are like a breath of fresh air in summer," she said, "but with the land around them becoming so expensive, there's a good chance someone will build on it."

As a result of the increased tax assessment, most of the 32,000 Alexandria property owners will receive higher tax bills, even though the tax rate is expected to remain $1.41 per $100 of assessed value. Tax bills will be mailed March 29.

An example cited by Chitlik was a house valued at $140,500 last year that will be worth about $146,120 in 1985 and the owner's tax bill will be $79 higher, or $2,060 compared with $1,981.

Chitlik said there is an oversupply of condominiums and office space in the city, and those property values may increase only about 3 percent.

Alexandria's tax assessment increase is greater than Fairfax County's 3 percent increase, but lower than Arlington's 6.6 percent rise. In the District of Columbia, single-family house values increased 4.3 percent this year.

Chitlik predicted that the planned opening of the Van Dorn Metro Station in 1990 will cause a tremendous increase this year in the value of the adjacent land now that funding for the station seems assured. Real estate values rose around the King Street and Braddock Road Metro stations in the past.