The boom in sales of single-family homes continued in the metropolitan Washington area last month, exceeding the most optimistic predictions made earlier this year by Realtors and builders.

Sales in Northern Virginia hit the highest point since 1978, 44 percent above last July's total, according to Joseph Hayden of the Northern Virginia Board of Realtors (NVBR).

July sales totaled 2,332 units in Prince William, Loudoun, Fauquier, Fairfax and Arlington counties and Alexandria. Those jurisdictions are all included in figures supplied by the NVBR.

In addition, sales in Northern Virginia for the year to date are 20 percent ahead of this point last year.

Montgomery County sales were up 30 percent over July 1984, according to the Montgomery County Board of Realtors. "The dollar volume is up 35 percent," said Hans Nessler of the board. "Sales this year to date are up 12 percent."

He said he thinks the strong market will continue "as long as interest rates are good."

Interest for 30-year fixed-rate mortgages is hovering in the 11 to 12 percent range, depending on the total package, industry officials said. Adjustable-rate mortgages are available at about 9 percent, depending on the lender.

According to Rusty Lobban, head of the Prince George's County Board of Realtors' multiple listing service, sales of single-family homes were up 34 percent in July over the same month last year. He said that sales of single-family homes in the county, like those in other area jurisdictions, exceed last year's.

Condominium sales in Prince George's are up 13 percent this year. Northern Virginia condo sales also are up, according to the NVBR.

Hayden said that home prices are up 8 percent for July compared with the same period last year but cautioned not "to read too much into those numbersbecause a lot of factors can influence prices.

Although figures for the District for July are not yet available, James H. Molloy Jr., senior vice president of Rufus Lusk and Sons Inc., predicted sales would reflect a continuing strong market.

Several District agents said sales are brisk and traffic at open houses is extremely heavy.

Molloy also predicted that statistics his company is compiling for July "will be higher than those the boards have."

William Laughlin, a vice president of the NVBR, said the market will remain strong in the Washington area for months, especially "as long as interest rates are attractive to buyers." His company, Laughlin Realty Co., said July's sales were the strongest in its 53-year history.

Jane E. Coates, chairman of the NVBR's finance committee and an Arlington-area agent for more than 16 years, said she is "very optimistic things are going just great."

She said the recent "tightening of the Federal National Mortgage Association guidelines will have a ripple effect, not a tidal-wave impact," on the area market. She said she wished Fannie Mae had restricted its new rules to areas where foreclosure rates are high instead of applying them to all mortgages.

Several builders and agents said the new rules may hurt first-time buyers who cannot make the larger down payments that will be required after Oct. 15.

Tony Ahuja of the Northern Virginia Builders Association said that, although many professional couples in the Washington area earn sufficient combined salaries to qualify for large loans, most do not save for the down payment they need to buy a house.

"We are not necessarily savers" in this area, Ahuja said.

Gary Garczynski, president of the Northern Virginia Builders Association and head of Signature Communities, said that "sales in June and July had been even stronger than even I anticipated."

"Home buying usually goes strong from January to April and then there is a slowdown until mid-October," he said. But the summer slump didn't come this year. "It is surprising that the July market stayed strong," Garczynski said. He said there has been a slight slump in sales in the first days in August. Industry sources attribute that to people being on vacation.

Garczynski said sales would remain strong throughout this year, and he predicted "stability of interest rates throughout the Reagan administration."

He also said Fannie Mae's new underwriting guidelines may hurt first-time buyers. "The buyer is going to be really scrutinized before he sees if he is ever going to realize his dream of home ownership," he said.

Many builders agreed, but said they don't think changes announced this week will hurt the Washington sales market immediately. Several agents said they were glad to see the announcement made publicly rather than having to tell clients about the new regulations.