When Howard Homes Inc. opened a sales trailer in a temporarily vacant lot in Columbia recently, 114 families were waiting in line to sign sales contracts for houses. Some had been waiting for five days - in tents, vans and station wagons.

The first 57 signers were given firm contracts. The others put deposits on lots - without price commitments from the builder. By the end of the first day of sales, 149 of the 199 lots had been sold. The buyers gave similar reasons for being there.

"This may well be the last chance to buy a house in Columbia of this quality at this price," said Gary Saks, the first in line. Saks came to buy a town house in a section where prices range from $55,800 to $61,600.

Seymour Raphael, an official of Howard Homes, says the crowd at the trailer was not a surprise for his company. He says the firm is "building for the young people. Where else are they going to live?"

Of 21 new developments listed on the most recent model home tour in Columbia only one other model - a town house substantially smaller than that of Howard Homes - was priced in the mid $50,000s. No other model was cheaper.

Beverly Nearney, a realtor in the Columbia office of Russell T. Baker and Co., says that buyers can find some detached resale houses in the low $60,000s in Columbia, but adds that in the rest of Howard County there's "virtually nothing" in that price range.

Joan G. Bounds, president of the Howard County Board of Realtors, said that land prices and the cost of financing are hurting builders all over the Baltimore-Washington region, a situation she says is "just knocking out the $40,000 market."

Bounds also looks at the problem in terms of young, potential buyers. She predicted that Columbia, which has zoning permitting a wide variety of construction, will be "one of the few places young people can still afford, about the only place to break into home ownership is this area."

All of Columbia is zoned as a "new town" district. This allows for several different types of land usage within one zoning district.

In order to fulfill Columbia's goal for a social and economic mix within its boundaries, 50 to 60 percent of the residential building there over the next 10 years will be town houses or other attached units.

This percentage relfects the building that has been completed to date, according to Douglas A. McGregor Sr., vice president of the Howard Research and Development Corp. (developer of the new town), and general manager of Columbia. And it gives all of Howard County a total housing mix that reflects the Baltimore-Washington corridor, he believes.

Columbia has about 14,000 acres, about 12 percent of Howard County's land. About 60 percent of the residential building in the county is in the new town. The central and western thirds of the county, according to county planning director Thomas G. Harris, are zoned for lots with a minimum of three acres. Lots in the other third of the county, which includes Columbia, Ellicott City, Elkridge, Savage and North Laurel, average a third to a half of an acre.

While there is some multiple-family zoning throughout the eastern part of the county, the bulk of the moderately priced housing is in Columbia.

"Outside of Columbia," says realtor Bounds, "Howard County is becoming a very very expensive place to live."

Although McGregor describes Columbia as a "microcosm of the whole region," in terms of housing costs. he adds that "if anything, Columbia can build less expensively because of the scale on which we're developing." There are currently about two dozen builders active there, construction everything from subsidized housing projects to single-family detached homes costing $250,000 or more.

Columbia, he believes, sustains a "more diverse builder community" than the region in general because "they've found the new town to be a very steady market. . . ." CAPTION: Picture, Buyers camped out for five days recently in Columbia to sign sales contracts for medium-priced houses. By Tom Allen - The Washington Post