DALLAS HAS SLIPPED AHEAD of Houston in the amount of vacant office space and now trails only Los Angeles as the nation's leader in empty buildings, according to a national survey.

Houston, which until recently topped the survey by Office Network Inc., now has 37.6 million square feet vacant, while Dallas has 38.3 million and Los Angeles 39 million.

"Houston is on the upswing," according to David Weekley, head of the Greater Houston Builders Association. "Dallas may have peaked." Weekley builds homes in both the Houston and Dallas markets and says "some of the bloom is coming off the rose" in Dallas.

Charles F. Anderson, Dallas' city manager, terms the growing office and apartment vacancies a temporary problem, however. "We are watching the situation with caution, but we are not alarmed," Anderson said.

But Russell Sampson, Coldwell Banker Commercial Real Estate vice president in Houston, says Dallas "is in serious trouble but doesn't realize it as yet."

Sampson, who was transferred to Houston after working as his company's resident manager for downtown Dallas, said developers in Dallas are cutting rental rates and offering incentives like the ones Houston developers began about three years ago when a space glut hit.

Sampson says Houston is doing the best job in the country of reducing its vacancy rates and cutting back in new construction.

"Dallas, on the other hand, is continuing to overbuild," he said.

Linda Kerr of Office Network said the firm's latest quarterly survey showed 22.7 million square feet of office space under construction in Dallas and 12.7 million being built in Los Angeles. Houston has just 2.6 million square feet under construction, she said.

THE FAIRFAX COUNTY planning commission has deferred a public hearing scheduled May 28 to June 11 on the proposed giant Kingstowne mixed-use development in Southeast Fairfax. The public hearing will be held at 8:15 p.m. in the Massey Building in Fairfax City.

The delay is supposed to give county planning staff members and developer Greendale Development, owned by Miller and Smith, a McLean-based development company, time to work out final details of the complicated proposal. The application for the $500 million, 1,100-acre new community has been in the works for more than a year. Plans call for construction of more than 5,600 housing units and office and retail space.

Also, a planning commission hearing on a proposed Centennial Development Corp. application for a hotel and office park at South Van Dorn Street near Franconia Road and the Capital Beltway has been changed from May 28 to May 30 at 8:15 p.m. The May 28 meeting has been canceled.

YOU CAN BUY ALMOST three houses in Buffalo for what it costs to buy one in Orange County, Calif.

That statistic comes from a survey of median prices of previously owned homes, regardless of size, released by the National Association of Realtors this week.

Half of all homes in Greater Orange County -- the Anaheim-Santa Ana area of California -- cost more than $132,000, down 1.1 percent from a year ago but up slightly from three months ago.

By comparison, half of the homes sold in the first quarter of 1985 in the Buffalo-Niagara Falls area of upstate New York went for less than $46,900.

In a list of 43 major metropolitan areas, it was the only place where the median was below $50,000.

On the high end of the scale, the New York City area was at $125,400, greater Los Angeles at $114,300, and Boston with a median of $108,600 joined Orange County in the six-figure range.

The median in Washington was $94,900, up 3.9 percent, compared with the national average increase of 3.4 percent.

Home prices in the Boston area jumped 21.5 percent from the first quarter a year ago, when the median price stood at $89,400.

In New York City, northern New Jersey and Long Island, where home prices moderated during 1984, the lid popped off again in the first quarter. A home that might have sold for $107,700 before Christmas sold for $125,400 by Easter. That's an increase of 24.65 percent compared with the first quarter of 1984, when prices were near the $100,000 mark.

Prices declined in Orange County, Calif.; Birmingham, Ala.; Columbus, Ohio; Milwaukee; Salt Lake City-Ogden, Utah; and the Tampa-St. Petersburg-Clearwater, Fla., area, but in all cases the drop was less than 5 percent.