So many Fairfax County home buyers are complaining about flaws in their new homes and broken builder promises that these problems now make up the largest category of calls to the county's consumer office, surpassing even automobiles.

And in Montgomery County, real estate difficulties trail only cars in numbers of people involved in complaints to the consumer agency.

New-home construction problems accounted for 25 percent of the Fairfax consumer office workload in September, continuing the pattern of the last few months, said Gloria Kornasiewicz, chief of investigations and licensing for the consumer agency.

In Montgomery, which keeps records differently, the figure for real estate was 14.2 percent of all complaints, a consumer office spokesman said.

A home is one of the most expensive purchases most people make, "and many things can go wrong with it," Kornasiewicz said. "Because of the cost, consumers will take time to complain."

Government officials noted that Fairfax and Montgomery are two of the area's busiest jurisdictions in terms of home construction, so it is not unexpected that they should get the lion's share of complaints.

Construction is also brisk in Prince George's County, but the level of complaints there could not be determined because, according to a Prince George's Consumer Protection Commission official, the agency's charter forbids it from handling any real estate complaints.

The official referred a reporter to the state Real Estate Commission, but that agency also said that it doesn't handle new-home-construction complaints and referred the reporter to the state builders association.

As it turns out, it is the consumer affairs office of the Maryland Attorney General's office that handles real estate complaints.

Alexandria and Arlington County consumer office spokeswomen said they receive few new-home construction complaints, noting that very little homebuilding is going on in their jurisdictions.

Construction has boomed in Fairfax since the recession, which hit the construction industry hard, began easing two years ago. Builders rushed to build homes for people who could not afford to buy when interest rates reached as high as 20 percent.

The construction rise explains the increase in dissatisfied buyers to some degree. But in Fairfax the number of complaints has increased much more rapidly than the number of building permits issued, according to the Department of Environmental Management.

As a percentage of permits issued, Fairfax complaints jumped from 13 percent during the last six months of 1983 to 32 percent in the period from July 1 to Sept. 28 this year, according to figures provided by the Department of Environmental Management.

These figures represent total complaints received about building code and site violations, said Brian Smith, enforcement coordinator for the department. "Some are justified, and some are not," he said.

"At this point we're still being run ragged" and have not had time to figure out why this is happening, he said. "We're bringing in people out of retirement" to handle complaints and hiring college students to help with clerical work, he added.

About 10 percent -- or 150 -- of the complaints received so far this year have been taken to court, Smith said.

"Whenever building activity is up . . . there is a bigger population to complain," said Samuel A. Finz, executive director of the Northern Virginia Builders Association. "Many of these complaints may not have substance. Many may be the kind that builders intend to get to but haven't been able to." He said he believes "we're building better-quality houses today than ever before."

Montgomery County records its new-home construction cases differently than Fairfax, keeping figures of complaints that are resolved, said Eric Friedman, an investigator with the office of consumer affairs. During fiscal year 1984, which ended June 30, the office "closed" 251 cases. "We believe these complaints affected 601 consumers," he said. "If we get a complaint from one person . . . complaining about a situation that affects everyone in a subdivision or a block, it would be recorded as one complaint, but the impact is far greater." About 100 home complaints are being investigated now.

Problems concern either sales practices, such as contract disputes in which buyers believe they don't get what they contracted for, or warranty complaints, in which new owners believe builders don't make repairs they should.

"There was not one area of the home that received more complaints than another, and no area that did not receive complaints," he said.

It is difficult to determine how much home construction is under way in the county because "lots of agencies have different numbers," Friedman said.

When a large builder gets a block of several hundred permits, he may not use them all in one year, and may never use some, he added. "A very rough estimate" is that 8 percent of new construction will result in a complaint to the consumer office, Friedman said.

While the Alexandria consumer affairs office has not received a new-home complaint in the last year, home improvement problems are the third-largest category of complaints received, said director Rose Boyd.

A similar situation exists in Arlington, where housing complaints represented 10.8 percent of the total received in fiscal year 1983, said consumer affairs director Jean Galloway. Most of these concerned problems with sales contracts and home improvements, she said