Sales of single-family homes are soaring in Northern Virginia, setting the stage for a stronger spring sales season than real estate professionals had predicted just a few months ago.

According to brokers throughout the region, sales of houses and town houses haven't been this good in the last six or eight years.

"We can sell homes faster than we can get the home inspections done," said Libby Ross, an agent in Arlington for more than 15 years. Home sales are often contingent on inspections, which generally take five days. Ross is affiliated with Town and Country Realty in Arlington.

According to the Northern Virginia Board of Realtors (NVBR), 1,768 single-family dwelling units sold in February, a 5 percent increase over the same month last year.

Although those statistics do not sound earthshaking, most Realtors see the increase as an indication of a continuing good sales market that reflects reasonable interest rates, increasing confidence by buyers in adjustable-rate mortgages and confidence in the stability of the local real estate market.

Several agents said that measurements of February sales have to take into account the fact that the month has fewer selling days than other months.

Total sales this year are outpacing sales last year by 16 percent, according to Joseph Hayden of the Northern Virginia Board of Realtors.

The 1985 February figures are the best February figures since the board started keeping records in 1978.

Ross said single-family detached home sales in Arlington have never been better, with more and more young couples buying their first house with a yard.

Dorrie Jackson, president of Jackson Temple Realtors in Vienna, said his company, which normally does about $2 million in business monthly, "did a shade under $3 million in February."

He said homes in the $100,000 to $125,000 range are the best sellers because there are more potential buyers who can qualify to buy in that price range.

The average price of units sold in February, excluding condominiums, was $115,800, according to NVBR statistics.

William L. Laughlin, vice president of the NVBR and president of his family-owned real estate company in McLean, said sales are booming throughout the region. "We have agents who are selling a million dollars' worth of property per month. It used to be good to sell that much a year," Laughlin said.

He said March sales in Northern Virginia are even better than February, a fact that leads him to agree with other brokers and home builders that spring will be an excellent sales time.

Sales of new homes in February "surpassed our expectations," said Tony Ahuja, an official of the Northern Virginia Builders Association. "Sales are even better than we thought. Interest rates are staying stable. That is important to keep buyers confident in the market," Ahuja said.

New homes are generally outselling existing homes, according to brokers, a fact that encourages local builders who predicted months ago that the spring sales season would be excellent.

"I believe there are repeatable sales cycles in the housing industry that repeat themselves every four or five years. It is my belief that we are at the peak of a cycle at the present time," Laughlin said.

Jackson predicted "a really good spring unless the doggone [federal budget] deficit gets completely out of hand.

"As long as interest rates hover at 12 or 13 percent, we will have a strong market," he said.

He has a warning for those who may be waiting to buy: "Those waiting for the 30-year fixed-rate mortgages to go from 13 to 11.5 percent are making a mistake."

Hayden said there are some interesting trends developing in the market and in the industry as sales increase.

"There has been an increase of more than 100 new agents joining the Board of Realtors every month," Hayden said. "Not all are staying. Some are falling by the wayside, but most are making it," Hayden said.

Others in the business agreed that there may be some leveling off of prices as sellers are more realistically analyzing the market before they put their homes up for sale.

Two-story colonials have become an even hotter commodity than they usually are in neighborhoods inside the Beltway as long as the price is under $250,000, said one agent. She ran the Multiple Listing Service computer looking for a two-story colonial several days ago and found only one inside the Beltway in that price range in the McLean area.

Houses in the Arlington area with price tags under $100,000 sell overnight unless they are total wrecks, an Arlington agent said.

Ahuja said, "More and more of the baby boom generation is coming to the age where they represent two-income households here." That means more families are qualified to buy. A real estate agent echoed Ahuja's feelings and predicted that many of those buyers would be out looking for homes this spring.

Based on statistics for the best "February in the past eight years, Realtors believe that their February record will sustain earlier predictions made for a most successful 1985," an NVBR spokesman said.